THE EFFECT OF UREA-AMMONIUM-NITRATE FERTILIZER AMENDED WITH UREASE AND NITRIFICATION INHIBITORS ON NITROGEN CYCLING IN HIGHLY ORGANIC AGRICULTURAL SOIL
Joelene M. Diehl
A thesis submitted to the faculty of the University of North Carolina at Chapel Hill in partial fulfillment of the requirements for the degree of Master of Science in the Department of Environmental Science and Engineering.
Chapel Hill 2007
Approved by: Advisor: Stephen C. Whalen Reader: Michael D. Aitken Reader: Michael F. Piehler
ABSTRACT JOELENE M. DIEHL: The effect of urea-ammonium-nitrate fertilizer amended with urease and nitrification inhibitors on nitrogen cycling in highly organic agricultural soil.
Starting in the mid 20th century the use of nitrogen fertilizers has steadily increased due to the rising demand in food production. Currently, urea-based nitrogen fertilizers are the most commonly used source of nitrogen in agriculture worldwide. Although there are benefits to urea fertilizers (high nitrogen content, high solubility, and low cost), its rapid decomposition to ammonia and carbon dioxide could lead to environmental and human health concerns. Laboratory experimentation was conducted to determine the rate of urea decomposition, ammonia volatilization, influence of soil moisture on urea decomposition, potential for fertilizer loss below the rooting zone, and the response of the microbial community to fertilization of highly organic agricultural soil with urea-ammonium-nitrate (UAN). In addition, urease and nitrification inhibitors were added to UAN fertilizer in order to determine if these amendments altered the rate of N transformations in soil.
TABLE OF CONTENTS
LIST OF FIGURES…………………………………………………………………………..vi LIST OF ABBREVIATIONS……………………………………………………………….vii Chapter I.
INTRODUCTION.............................................................................................1 Increased use of N fertilizer in agriculture........................................................1 Plant uptake of nitrogen.....................................................................................3 Nitrification........................................................................................................3 Denitrification....................................................................................................5 Mineralization and immobilization....................................................................5 Volatilization......................................................................................................6 Nitrogen Fixation...............................................................................................6 Loss of nitrogen to surface waters and leaching................................................7 Nitrogen use efficiency......................................................................................8 Pollution of surface and ground waters with nitrogen.......................................8 Nitrogen loss due to volatilization.....................................................................9 Future fertilizer use..........................................................................................10 Timing of fertilization......................................................................................10 Urease and nitrification inhibitors...................................................................11 iii
Precision agriculture........................................................................................12 II.
LIST OF FIGURES Figure 1. Study site................................................................................................................17 2. Urea hydrolysis experiment nitrogenous concentrations for UAN and UI treatments…………………………………………………………………..….…26 3. Urea hydrolysis experiment ammonia volatilization.............................................27 4. Urea hydrolysis experiment nitrogenous concentrations for UAN and UNI treatments...............................................................................................................29 5. Moisture dependence experiment percent change of nitrogenous nutrients..........31 6. Moisture dependence experiment mass of ammonia volatilized...........................32 7. Nitrate and Ammonia leached during simulated rainfall experiment....................34 8. Simulated rainfall experiment mass of nitrate and ammonia leached...................35 9. Carbon dioxide production....................................................................................37 10. Denitrifying enzyme activity.................................................................................38 11. Short term nitrifier activity....................................................................................39
Analysis of variance
Biological nitrogen fixation
Cation exchange capacity
Denitrifying enzyme activity
Geographic information system
Global positioning system
International Fertilizer Industry
Nitrogen use efficiency
Urea, ammonium, and nitrate
UAN and Agrotain
UAN and Agrotain Plus
Water filled pore space
Water holding capacity
CHAPTER 1 INTRODUCTION Increased use of N fertilizer in agriculture The use of nitrogen fertilizers for food production has continually increased globally since the mid 20th century. After World War II, the Haber-Bosch process was used to convert large amounts of atmospheric dinitrogen gas into reactive nitrogen (Nr). The International Fertilizer Industry Association (IFA; 2006) estimates that in 2004 there were approximately 90 Mt (million tonnes) of N fertilizer used worldwide. That was roughly 7.5 times more N fertilizer than was used in the 1960’s. The use of N fertilizer has also increased in the United States by approximately three and half times over the last 40 years. The estimate of total N fertilizer use in the United States in 2004 was about 11 Mt (http://www.fertilizer.org). The ability to easily convert atmospheric N2 into Nr increased human inputs of Nr from zero in the late 19th century to about 82 Mt in the year 2000 (Fixen and West 2002). Three-quarters of the N2 fixed by humans is used for agricultural fertilizer production (Galloway et al. 2004). Currently, Asia, Europe, and N. America are the top three producers of Nr (Galloway et al. 2004). The commercialization of the Haber-Bosch process coupled with the industrial revolution and human population explosion has led to a worldwide increase in the application of synthetic N fertilizers to agroecosystems. Nitrogen fertilizer is a broad term that includes a variety of different types of nitrogen- based soil additives. The IFA combines about seven different types of N fertilizers
in order to estimate total fertilizer-N application. Ammonium sulfate, urea, ammonium nitrate, ammonia, calcium ammonium nitrate, and also mixtures of various N forms like urea, ammonium, and nitrate (UAN) are common types of N fertilizers that are available to farm managers today (http://www.fertilizer.org). In addition, there are synthetic fertilizers that contain potassium and/or phosphorus in addition to nitrogen. Some of these fertilizer categories include monoammonium phosphate (NK-N) and diammonium phosphate (NPKN) (Bouwman 1997). The N fertilizer best suited for a particular agricultural application is dependent on a variety of factors which include but are not limited to crop, soil type, and climate. Despite the variety of synthetic N fertilizers available, urea-based fertilizers are the fastest growing and most commonly used source of N worldwide for agriculture (Bremner 1995). Urea can be applied as a synthetic fertilizer and is a major component in animal manure fertilizers. Currently, it is estimated that more than half of all fertilizer used globally is in the form of urea (Gilbert et al. 2006). The benefits of using urea as a fertilizer are due to its high nitrogen content (approximately 46% nitrogen), nonpolarity, high solubility, and low expense to manufacture, store, and transport (Prasad 1998). However, there are some problems associated with the use of urea fertilizers. Once urea is in soil, it rapidly decomposes (Eq. 1) to form ammonia (NH3) and carbon dioxide through hydrolysis (Bremner 1995): NH2CONH2 + H2O J 2NH3 + CO2
In addition to increasing NH3 volatilization, urea fertilizer can cause an increase in soil pH, and nitrite (Krogmeier et al. 1989; Bremner 1995). The increase in soil NH3 (or NH4+,
depending on soil pH) concentration from urea has been linked to a multitude of environmental and human health concerns (Bremner 1995; Townsend et al. 2003). A major pathway by which N is lost from agricultural settings is through NH3 volatilization. Mosier (2001) estimated that 65% of the global NH3 flux to the atmosphere is emitted from agricultural systems, including animal production and the use of synthetic fertilizers. Bouwman et al. (2002) estimated that during the 1990s 11.2 of the 78.5 Mt of synthetic fertilizer-N used in agriculture was lost to NH3 volatilization. Further, 7.3 of the 34.4 Mt of field-applied urea was lost to volatilization. The two categories of fertilizer that have the largest contribution of losses of N through volatilization are synthetic urea fertilizer and animal manure (Bouwman et al. 2002).
Plant uptake of nitrogen Plants can accrue N through the uptake of NH4+ and NO3- in soil (Glass 2003; Miller and Cramer 2004). However, because NO3- is typically more available, root uptake of N occurs mostly as NO3- (Miller and Cramer 2004). Nitrate can be directly applied to soil or can become available for plant uptake as a result of the microbial process, nitrification. As such, a major factor in determining the amount of NO3- that is available for plant uptake is partly determined by the activity of different microbial communities in soil.
Nitrification Nitrification is the term that describes the microbial conversion of NH4+ to NO3-, which follows a two step reaction (Schlesinger 1997): 2NH4+ + 3O2 J 2NO2- + 4H+ + 2H2O
2NO2- + O2 J 2NO3-
The total oxidation reaction is NH4+ + 2O2 J NO3- + 2H+ + H2O. The transformation of ammonium to nitrite (Eq. 2) is carried out by ammonium-oxidizing bacteria, and nitrateoxidizing bacteria are responsible for the second reaction (Eq. 3) where nitrite is further oxidized to nitrate (Brady and Weil 2002). Nitrification is influenced by soil physical and chemical properties. Soil structure and texture can influence the growth rate of nitrifying bacteria by providing substrate for growth and determining how tightly cations like NH4+ are bound to soil particles through the cationexchange capacity (CEC) of the soil. Soils with a high CEC can decrease the bioavailability of NH4+ to nitrifying bacteria and thereby limit nitrification rates (Subbarao et al. 2006). The moisture, temperature and amount of O2 and CO2 in the soil can also influence nitrification rates (Gilmour 1984). Soil moisture and oxygen levels are inherently connected, as decreased soil moisture increases the diffusion of O2 in soils. Nitrification decreases in soil with high moisture content due to oxygen limitation. Nitrification is an aerobic process with optimal rates occurring when soil oxygen concentrations are similar to that found in the atmosphere (20%) and approximately 60% of the soil pore space is filled with water (Brady and Weil 2002; Subbarao et al. 2006). Nitrification is all but inhibited in saturated soils due to the absence or low diffusion of oxygen. The ideal temperature range for nitrification is between 25 and 35°C (Subbarao et al. 2006). Although nitrification can take place in highly acidic soils (pH L 4.0), pH ranges from 6.7-8.5 are ideal (Subbarao et al. 2006). In general, low rates of nitrification are observed in soils with little oxygen, extremely high or low soil moisture, and low pH (Schlesinger 1997).
Denitrification Nitrate in soil can undergo three fates; denitrification, assimilation into plant and microbial biomass, or loss through leaching or runoff. Denitrification is the term used to describe the conversion of NO3- into nitrogen gas (NO, N2O, N2) and has the following general reaction (Schlesinger 1997): 5CH2O + 4H+ + 4NO3- J 2N2 + 5CO2 + 7 H2O
Nitrogen gas formed through denitrification can be emitted to the atmosphere. Denitrifying bacteria are facultative anaerobes, meaning they can metabolize nutrients and remain active under oxic or anoxic soil conditions (Schlesinger 1997). Nevertheless, optimum denitrification rates are observed in anoxic and water saturated soil conditions (Brady and Weil 2002). However, depending on soil structure, denitrification can occur in anoxic microsites in soil that is for the most part well oxygenated (Schlesinger 1997). Temperatures that optimize denitrification range from 25-35°C. In addition, denitrifying bacteria are sensitive to pH. In acidic soil, denitrification occurs more slowly and N2O is more likely to be the end product (Brady and Weil 2002).
Mineralization and immobilization Nitrogen mineralization describes the process by which organic forms of N are converted to inorganic-N (NH4+, NO3-). During N mineralization soil microbes decompose plant material or other organic matter as an energy source to break down amino compounds and release N (Brady and Weil 2002). Nitrogen immobilization is the opposite of mineralization, involving the incorporation of inorganic-N into organic material, either as microbial biomass or a component in soil humic matrices. As the term suggests, immobilized
N is biologically unavailable for plant uptake. Preference toward net nitrogen mineralization or immobilization in soil is dependent on the carbon to nitrogen ratio in soil (Brady and Weil 2002).
Volatilization Volatilization is another fate of soil nitrogen. Nitrogen as NO, N2O, and N2 can be volatilized from the soil through microbial mediated processes such as nitrification and denitrification. In addition, NH3 produced through the breakdown of organic residues can be volatilized from soil (Brady and Weil 2002). Much like nitrification and denitrification, rates of NH3 volatilization are impacted by soil characteristics. Soils that are calcareous, high in pH, and/or low in CEC are all prone to increased NH3 volatilization (Follett and Delgado 2002; Meisinger and Randall 1991). Also, soils that have manure or urea fertilizers surfaceapplied often show high rates of NH3 volatilization (Follett and Delgado 2002). Brady and Weil (2002) report that by incorporating fertilizers a few centimeters below the soil surface ammonia volatilization can be reduced by up to 75%.
Nitrogen Fixation Previously described nitrogen transformations involve the cycling of the extant biologically available pool. New nitrogen can be made biologically available through biological nitrogen fixation (BNF), which involves the transformation of atmospheric N2 into NH3 in the pedosphere and hydrosphere. However due to the triple bond that forms between the two N atoms in dinitrogen gas, there is an extremely high energetic cost to this conversion (Schlesinger 1997). Some bacteria exist in soils that are capable of independent
BNF and other bacteria form symbiotic relationships with plants, specifically plant roots, in order to fix nitrogen. Irrespective of the organism, the enzyme responsible for nitrogen fixation is nitrogenase (Brady and Weil 2002).
Loss of nitrogen to surface waters and leaching Besides volatilization, nitrogen can also be lost from soil through leaching and surface runoff. Nitrogen in the form of NO3- is the most susceptible to leaching through the soil profile or lateral transport in surface water runoff because it is negatively charged and does not participate in cation exchange reactions (Follett and Delgado 2002). The amount of nitrate that is lost to surface water runoff is influenced by the infiltration potential of the soil, the severity of rain-fall events and the slope or topography of the land (Follett and Delgado 2002). Globally it is estimated that approximately 20 Tg of N is lost annually due to soil erosion and runoff (Smil 1999). Factors such as soil and crop type, tillage practice and climate can also influence N leaching (Goulding 2004). Plant growth and uptake of N during the spring and summer months help to prevent the net downward movement of NO3- . However in the absence of growing plants, such as in the fall and winter months, there is a net downward movement of N through the soil profile (Kirchmann et al. 2002). In no till or conservation tillage agriculture, microbial communities are less disturbed and able to immobilize larger amounts of N than if conventional tillage agriculture is practiced (Kirchmann et al. 2002). Agricultural Best Management Practices (BMPs) involving nitrogen aim to reduce the amount of NO3found in the soil rooting zone (0-15cm) beyond what is needed for plant growth and thereby
reduce the likelihood that N will be leached or incorporated into surface runoff (Keeney and Follett 1991).
Nitrogen use efficiency Despite advances in the understanding and forecasting when and how plants utilize nitrogen, there remain large inefficiencies in plant uptake of N per amount of fertilizer applied. Recent estimates indicate that only 30 to 40% of N applied as fertilizer is used by crops and converted to grain (Lea and Azevedo 2006). Nitrogen use efficiency (NUE) is the term used to describe the ratio of N utilized by the plant to the amount of N applied to the soil (Glass 2003; Lea and Azevedo 2006). NUE includes two main components; the ability of plants to remove N from the soil, and the efficiency in the transfer of N within the plant to the production of grain (Lea and Azevedo 2006). In a more encompassing definition, Cassman et al. (2002) describes NUE as the proportion of N inputs to the soil that are removed by a crop in the form of biomass harvested, remaining in the soil as a part of crop residue, or immobilized in inorganic pools.
Pollution of surface and ground waters with nitrogen As described previously, applied nitrogen that is not utilized by plants or soil microbes is subject to offsite loss through leaching to ground waters, lateral transport in surface runoff, or volatilization. Leaching of NO3- into groundwater can cause contamination of drinking water wells and other groundwater resources (Fletcher 1991; Townsend et al. 2003; Follett and Delgado 2002). In areas where agriculture is extensive, such as the Midwest region of the United States, agriculture is a significant contributor to NO3--N
contamination of ground and surface waters (Udawatta et al. 2006; Fletcher 1991). High nitrate levels in drinking water can cause health problems especially for babies and young children. Methemoglobinemia, also known as ‘Blue-baby-syndrome’ can affect infants when high nitrate concentrations in drinking water reduce the ability of red blood cells to carry oxygen (Fletcher 1991; Kirchmann et al. 2002). There are many environmental problems associated with the transport of NO3--N in surface waters. For instance, high N levels can cause eutrophication of fresh and marine waters by promoting primary production (Follett and Delgado 2002; Townsend et al. 2003; Grant et al. 2006). Subsequent decomposition of phytoplankton blooms consume oxygen and can kill other aquatic life and otherwise disrupt the entire food web (Galloway 2003). Furthermore, Vitousek et al. (1997) noted that anoxic or hypoxic conditions resulting from increased N loading can lead to a loss of biodiversity in aquatic ecosystems.
Nitrogen loss due to volatilization In addition to losses of excess N fertilizer through leaching and surface water runoff, N can be lost through volatilization. Microbial activity in soil can lead to NO, N2O, and N2 emissions through nitrification and denitrification. Typically, nitrification is largely responsible for NO emissions, and denitrification for N2O efflux (Liu et al. 2006). Ammonia can also be volatilized directly from the soil surface. Once in the atmosphere, the various forms of nitrogen can have different fates. Nitric oxide in the atmosphere can become oxidized to form HNO3 in addition to interacting with hydrocarbons, light, and ozone to form nitric acid and atmospheric particulate nitrate (Mosier 2001). Small amounts of NH3 in the atmosphere can be oxidized to form N2O (Ferm 1998).
The major sink for N2O in the atmosphere is stratospheric destruction (Schlesinger 1997). Tropospheric N2O is a greenhouse gas, while stratospheric N2O is involved in reactions that reduce ozone, allowing more ultraviolet to reach the Earth’s surface. In contrast, NO and NH3 can be returned to the earth’s surface through wet and dry deposition. The deposition of these gases can cause eutrophication in aquatic ecosystems and soil acidification (Bouwman et al. 2002).
Future fertilizer use During the last 35 years of the 20th century global food production has doubled (Tilman et al. 2001). This doubling has resulted in 6.87-fold and 1.1-fold increases in N fertilization and area of land under cultivation, respectively. If this trend continues, it is estimated that N fertilizer use will increase 1.6 times by year 2020 (Tilman et al. 2001). In addition to increases in N fertilizer use, a doubling in food production will result in an 18% increase in land devoted to agriculture (Tilman 1999). An 18% increase represents the conversion of 268 million hectares of previously uncultivated land to agriculture. More recently, Zhang et al. (2006) estimated that by the year 2030 land devoted to agriculture will increase 6.6% from 2006 values. Strategies such as improved timing of fertilizer application, fertilizer placement, and use of chemical additives can be used to improve NUE in agroecosystems.
Timing of fertilization In addition to intrinsic plant properties such as the genetic makeup and C3 or C4 photosynthetic pathways, extrinsic factors such as timing and frequency of fertilization can
affect the NUE of plants (Dobermann and Cassman 2002). Fertilizer that is applied in one large application or during the fall has a greater chance of being lost from the system compared to fertilizer that is applied in correspondence with plant growth demands. In addition, timing fertilization application with plant demand for N can increase the amount of N utilized by plants (Dobermann and Cassman 2002). Inherent plant physiological limitations on NUE are unavoidable. However, matching N availability with plant N demand reduces the loss of N to the natural environment, minimizing the negative offsite environmental impacts that are commonly associated with agricultural activity.
Urease and nitrification inhibitors Chemical additives can also be used to increase NUE in crops and reduce N lost from soil to the surrounding environment. One such example of a chemical additive is a urease inhibitor. A urease inhibitor can be applied with urea fertilizers to reduce the rate that the urease enzyme catalyzes the hydrolysis of urea. By slowing the urease enzyme, N remains as urea-N longer, which increases the likelihood that urea-N will move deeper into the soil, thus reduce ammonia volatilization. A urease inhibitor that has been identified as being particularly effective in upland soils is N-(n-butyl) thiophosphoric triamide (NBPT) (Bremner 1995). Researchers have found that it can be used to slow the transformation of urea-N to NH3. In addition, NBPT has been useful in preventing ammonia volatilization from a broad range of agricultural settings such as cattle feedlots and swine lagoons to crops (Bremner 1995; Hendrickson and Douglass 1993; Watson et al. 1994; Varel 2002). The inhibitory action of NBPT is due to the formation
of its oxon analog (Creason et al. 1990) and its interaction with the two active Ni atoms in the urease enzyme (Manunza et al. 1999). In addition to a urease inhibitor, a nitrification inhibitor can also be added to fertilizer alone or in concert with a urease inhibitor. Nitrification inhibitors slow the conversion of NH4+-N to NO3--N and thereby reduce the likelihood that N will be lost from soil. Specifically, nitrification inhibitors can help to reduce N loss from leaching or denitrification (Fixen and West 2002). Research has shown that the incorporation of the nitrification inhibitor dicyandiamide (DCD) to urea fertilizer containing a urease inhibitor can increase NUE in crops and increase grain yield (Schwab and Murdock 2005).
Precision agriculture Precision agriculture (PA) is another method used to restrict N loss and improve fertilizer use efficiency and farm profitability. Precision agriculture is defined as a “management strategy that uses information technologies to bring data from multiple sources to bear on decisions associated with crop production” (National Research Council 1997). There are three main components to PA which include; collection of data at an appropriate scale, interpretation and analysis of gathered data, and implementation of management response at an appropriate spatial and temporal scale (National Research Council 1997). There are an array of technologies used in PA that include but are not limited to; microcomputers, geographic information systems (GIS), global positioning systems (GPS), yield mapping systems, ground based sensors and remote sensing (National Research Council 1997; Robert 2002). Through assembling a variety of data, farm managers can adjust
management practices so that timing and placement of field operations is site specific (Cassman 1999). Although PA is more economically and environmentally beneficial than uniform management practices, there are obstacles preventing it from widespread adoption. Specifically, the cost associated with acquiring new technologies, the education or skill level of the farm manager, agronomic barriers, and development of technologies are all hurdles that must be overcome in order for the adoption of PA (Robert 2002). Daberkow and McBride (2003) conducted a study to identify farm and farm operator characteristics that are indicative of the likelihood that PA would be adopted. Factors such as farm size and locality, computer literacy, full time farming, and farm type were all identified as playing a role in the decision to adopt PA as a management strategy. As the knowledge gap narrows and technologies become more accessible, it is inevitable that PA practices will become more widespread.
CHAPTER 2 OBJECTIVES The overall objective of this research was to evaluate rates of N transformation and transport from the rooted zone in an agricultural soil treated with UAN–type (ureaammonium-nitrate) fertilizer. This study was conducted with bulk soils or intact soil cores from 0 to 20 cm in depth under controlled laboratory conditions. Plants were excluded to focus on the effects of physical and microbiological variables that influenced the transformation and transport of N applied at doses simulating field application practices, with an eye toward gaining a better understanding of the fate of N fertilizer applied to this soil. Specific goals were to compare for three fertilizer types (UAN, UAN amended with a urease inhibitor, UAN amended with a urease and nitrification inhibitor): 1) The post-application rate of urea decomposition and rate of NH3 volatilization; 2) The influence of soil moisture on rates of urea decomposition and subsequent transformations of fertilizer N; 3) The potential for fertilizer loss below the rooted zone during rainfall; and 4) The post-fertilization development and activity of the overall soil microbial population and microbes specifically involved in N cycling dynamics (nitrifying and denitrifying bacteria) The study site, Open Grounds Farm, is located on a drained pocosin and is therefore characterized by highly organic soils. Relatively little information is available regarding the
impact of physical and microbiological variables on N cycling dynamics in fertilizeramended, peaty agricultural soils. Information from this study is therefore of general interest to farm managers, but is also critical for developing informed fertilizer management strategies at the study site which is located adjacent to a sensitive estuarine complex that could serve as receiving waters for nutrients transported offsite.
CHAPTER 3 MATERIALS AND METHODS 2.1 Site Description The study site, Open Grounds Farm, is located at 34°54’N, 76°50’W in Carteret County, NC, USA (Fig 1). Open Grounds Farm was established in 1974 on undeveloped pine forest, swamp forest and pocosin and consists of 18,220 ha of row crops in a corn-wheatsoybean rotation. The farm is divided into a network of 260 ha blocks that are surrounded by field ditches that drain into canals. The canals have water control structures that can regulate water flow and can increase the water table during the growing season. During heavy rain events a portion of runoff drains into headwater creeks of the South River, a coastal plain estuary and tributary to the Neuse River Estuary which is part of the Albemarle-Pamlico Sound complex. Management practices include no-till agriculture and precision application of liquid nitrogenous fertilizer by point or line injection with GPS-equipped planters that fertilize according to detailed soil fertility maps developed annually. The typical fertilizer is a 32% N solution by mass, consisting of a 16:8:8 mixture of urea, ammonium and nitrate (UAN). In some blocks, the standard UAN fertilizer is amended with Agrotain™ or Agrotain Plus™. The former is a urease inhibitor and the latter includes both a urease and a nitrification inhibitor. The N content by total mass and stoichiometry for these alternate fertilizers is consistent with the standard UAN fertilizer.
Soils are of the hydric series and are mapped as Deloss fine sandy loam (fine-loamy, mixed, semiactive, thermic Typic Umbraquult) overlying relic sand and shell. Soils are circumneutral (pH = 7.4) with a water holding capacity (WHC) of 67% and an organic content of 310 g kg-1. Bulk and particle densities are 860 and 1370 kg m-3. The average annual precipitation at the study site is 146 cm and the average daily winter and summer temperatures are 7.9 and 25.9°C, respectively.
Figure 1. Location of the study site in eastern North Carolina. 2.2 Sample Collection and Preparation Soils were obtained from field block 4 on five occasions beginning in the post-harvest period in October 2005 and ending in January 2007. Block 4 was planted with soybeans during 2006. Depending on the intended use, bulk soils from the surface 20 cm layer or intact soil cores to a similar depth were collected. Bulk soils were obtained with a hand trowel, while intact soil cores were collected with a hammer corer fitted with removable 17
stainless steel sleeves (4.8 cm i.d. x 30 cm length). Samples were stored at 4oC until experimentation. Bulk soils were homogenized and sieved (4-mm mesh) prior to distribution into experimental vessels, while soil cores were maintained in stainless steel collection sleeves during experimentation. 2.3 Experimentation Analysis of the fertilizer showed it was composed of 84 mg mL-1 NH4+-N and NO3--N and 168 mg mL-1 of urea. Laboratory experimentation was conducted in an attempt to simulate field realism in fertilizer application. Based on farm data, fertilizer is line-applied at 14 mL 100 cm-1. Therefore, 0.71 mL of fertilizer was applied to a 5.1 cm diameter soil core. In order to ensure even distribution on the surface of soil cores, 0.71 mL of fertilizer was in all cases diluted in 5mL of deionized water prior to application. A similarly diluted volume of fertilizer was added per ~200g dry mass of homogenized soil in experiments involving bulk soils, as this approximates the dry mass of an intact soil core of the dimensions used here. Experiments were about 2 to 3 weeks in length (except for the simulated rainfall experiment) as this is the typical time frame between fertilizer applications in the field. Abbreviations used throughout for fertilizer treatments are as follows: UAN fertilizer (UAN), UAN with Agrotain™ (UI), and UAN with Agrotain Plus™ (UNI). All experiments were conducted at 20°C. 2.3a Urea hydrolysis. Two experiments were conducted to assess the fate of urea (transformation to NH4+ and volatilization) as a function of fertilizer type. Both experiments consisted of two treatment groups. In the first experiment, 48 cores were collected; 24 were fertilized with UAN and the remaining with UI. All cores were sealed at the bottom with a polyethylene cap
and were fitted on the top with two horizontally stacked, acid-impregnated sorption pads (described below), allowing for a ~6 cm headspace between the soil surface and the lower sorption pad. The breathable sorption pads allowed air exchange between the atmosphere and soil surface. However, the lower pad absorbed NH3 volatilized from the soil surface, while the upper pad prevented NH3 contamination of the lower pad by ambient air. Triplicate soil cores for each treatment were destructively sampled on days 0, 1, 2, 3, 5, 8, 14, and 21. The second experiment consisted of a total of 42 soil cores; half of the cores were fertilized with UAN and the other half with UNI. Triplicate cores for each treatment were destructively sampled in the second experiment on days 0, 1, 3, 5, 8, 14, and 21. The handling of soil cores for both experiments was identical. On days when cores were sacrificed, soil was divided into 10 cm sections (0-10cm, 10-20cm). Each section was then homogenized, subsampled (10g) and analyzed for urea-N, NH4+-N, and NO3--N. Nitrogen lost to volatilization was determined in association with each soil nutrient analysis by analyzing the NH4+-N content of each lower sorption pad. 2.3b Moisture dependence The moisture-dependence of N cycling as a function of fertilizer type was assessed in homogenized soil samples. Percent water holding capacity (WHC) was adjusted to 22% higher or lower than the value for field moist soil (64% WHC) by misting or air drying soils with continuous mixing to bracket soil conditions that are typically observed in fields. The equivalent of 210g dry mass homogenized soil was added to 27 teflon-lined polycarbonate cylinders (9 cm i.d. x 35 cm long). Fertilizer treatments (UAN, UI, UNI) were triplicated at each of the three levels of %WHC (42%, 64%, and 86%) and cylinders were capped with duplicate horizontally stacked, acid-impregnated sorption pads as described above. On days
0, 1, 3, 6, 14, and 20 soil was subsampled (10g) from each cylinder and nutrient concentrations (urea-N, NH4+-N, NO3- -N) were determined. Nitrogen lost to volatilization was measured in association with nutrient analysis by changing the sorption pads on days 1, 3, 6, 14, and 20 and analyzing the lower pad for NH4+-N content. 2.3c Simulated rainfall Intact soil cores were used to determine the influence of fertilizer type on the rate and form of N leached. Fertilizer treatments included UAN, UI, UNI, and a control that had the equivalent volume of deionized water added (S). Treatments were triplicated and subjected to simulated rainfall based on a historic record of volume and timing of rainfall at the time of planting near the site (http://www.nc-climate.ncsu.edu/). Rainfall of 2.5 cm over a 1 h period was added at weekly intervals for four weeks followed by a 5 cm rainfall over 2 h on the fifth week. A peristaltic pump was used to deliver rainfall as deionized water with the dripper positioned in the middle of the core to prevent channeling down the side of the core and to ensure constant delivery over an hourly period. Artificial rainwater was allowed to infiltrate the core and the leachate was collected. The volume of leachate was measured after each rainfall event and assessed for nutrient content. Cores were sealed at the bottom, but were left uncapped at the top between rainfalls to allow for natural gas exchange across the soil surface. At the termination of the experiment, cores were dismantled into 5 cm sections that were analyzed for residual N. 2.3d Microbial activity. Overall microbial community metabolism and the activity of the microbial groups specifically active in N cycling dynamics (nitrifiers and denitrifiers) were measured as a function of time and fertilizer treatment. Fifteen samples (1000g) of homogenized, field
moist soil were randomly assigned to five treatment groups which included field moist soil only (S), and soil amended with 35mL of deionized water (C) , or a similar volume of UAN, UI or UNI solutions. Treated soils were distributed into 2L Erlenmeyer flasks that were covered with perforated foil to allow gas exchange between the atmosphere and flask headspace. Flasks were subsampled (~100 g wet weight) on days 0, 1, 3, 5, 7, 10 and 14 to measure any or all of the following: gravimetric moisture content; community respiration (CO2 production); short term nitrifier activity (NA); or denitrifying enzyme activity (DEA). On day 14, soil, subsamples were rendered anaerobic and assayed for DEA again after 4 days. 2.4 Analytical Soil nutrients were extracted (10:1 volume/soil wet weight) with 2M KCl and filtered (Whatman no. 42 paper). Nutrient extracts were analyzed colorimetrically (Shimadzu Model UV-1201V spectrophotometer) for NO2--N + NO3--N (hereafter referred to as NO3--N) and NH4+-N by the Cu-Cd reduction and indophenol blue methods, respectively (Bundy and Meisinger 1994). Urea-N was determined by the modified diacetyl monoxime method (Bremner 1982). Concentrations of N are expressed per gram of dry soil mass (g dw-1). Concentrations of NO3--N, NH4+-N and urea-N in leachate were measured using similar colorometric methodologies adapted for aqueous samples (Parsons et al. 1984; Grasshoff 1999), while NO2--N (NA determination; below) was determined by diazotization (Parsons et al. 1984). Ammonia volatilization was determined by the closed chamber, sorption pad technique (Marshall and Debell 1980). Duplicate polyfoam sorption pads impregnated with a 2:1 (v/v) solution of 2.2 N H3PO4 and glycerine were emplaced in the headspace above
intact soil cores or homogenized soils in teflon-lined polycarbonate cylinders. The top pad prevented atmospheric contamination of the lower pad that intercepted NH3 diffusing upward from the soil surface. At prescribed intervals (above), the top pad was discarded and the bottom pad was rinsed with deionized water. Collected rinse water was analyzed for ammonium as described above. Other soil physicochemical properties were determined following standard techniques (Klute et al.1986). Soil pH was determined potentiometrically on 1:1 soil-deionized water slurries. Gravimetric soil moisture was determined by oven drying (105°C) for 24 hours. Particle density was measured pycnometrically. Organic matter content was determined by loss on ignition of oven-dried soil at 550°C for four hours. Water holding capacity (WHC) was determined by the soak and drain technique. Bulk density was determined as the ratio of oven-dried mass to volume of soil in core. Carbon dioxide production was measured as an indicator of community microbial activity. Subsamples (20g) of field-moist soil equilibrated with laboratory air were sealed in air-tight 133-cc glass jars equipped with O-seal fittings and septa for syringe sampling of headspace gases. Using a syringe, 3mL of headspace gas was extracted at 0, 90, and 180 minutes from the time of sealing and CO2 was measured by thermal conductivity gas chromatography (Shimadzu GC-14A). Short-term nitrifier activity (NA) was measured using the chlorate block technique (Schmidt & Belser 1994). Chlorate inhibits the oxidation of NO2--N to NO3--N. Therefore, in the presence of excess NH4+-N, the accumulation of NO2--N provides an index of the population size of ammonium oxidizing bacteria. Subsamples (20g) of field moist soil in 250mL Erlenmeyer flasks were amended with 90mL of a phosphate buffer containing 0.2mL
of 0.25mM (NH4)2SO4 and 1mL of 1 M KClO3 solutions. Flasks were swirled and 2mL of slurry was immediately extracted and syringe-filtered (0.8µm polycarbonate filter). Flasks were placed on a rotary shaker at 150 rpm. At 24 and 48 h, additional samples were withdrawn and similarly filtered. All filtrate was analyzed for NO2--N. Denitrifer enzyme activity (DEA) was measured as an indication of the potential for denitrification in soil samples following Tiedje (1994). The assumption is that the rate of N2O production is commensurate with the denitrifying enzyme concentrations when no other factors are limiting. Subsamples (20g) of field-moist soil placed in 133-cc glass jars (described previously) were amended with a deoxygenated medium consisting of 25mL of 1mM glucose, 1mM KNO3-, and 1g L-1 chloramphenicol. The latter inhibits de novo synthesis of denitrifying enzymes. Jars were then sealed with air tight screw top lids and the headspace was rendered anaerobic by repeated evacuation and filling with ultrahigh purity N2. Acetylene was added to 10kPa to inhibit reduction of N2O to N2. Jars were then hand shaken vigorously to ensure equilibration of C2H2 between the gas and aqueous phases and placed on a rotary shaker (150 rpm). A 3mL sample of jar head space gas was withdrawn and analyzed for N2O by 63Ni electron capture gas chromatography (Shimadzu GC 14A) at 30, 120, and 210 minutes after being placed on the shaker. 2.5 Calculations and Statistical Analysis Rates of NA and DEA and total microbial activity were calculated by the time-linear rate of NO2--N, N2O-N and CO2-C accumulation, respectively. Total N2O-N at each time point in DEA experiments was calculated as the sum of the gas and liquid phase (determined from Bunsen solubility coefficients [Moraghan and Buresch 1977]) concentrations. Total NH3 volatilization over the 2-3 week experiments was determined by adding the masses of
NH3-N captured on sorption pads while total CO2 production was calculated by timeintegrating data collected at discrete points throughout the observational period. Microbial activities are normalized per unit dry soil mass. Data were analyzed by a one, two or three-way Analysis of Variance (ANOVA) using SAS software version 9.1.3 (SAS Institute, 2003). In the case of a three-way ANOVA, data were analyzed using a repeated measure approach. ANOVAs were performed using either the General Linear Model procedure (PROC GLM) or the Mixed procedure (PROC MIXED), and the MEANS procedure was used to calculate the mean and standard error of the various treatment replicates. Tukey’s or Scheffe’s methods were used for post hoc comparison of significant differences among means. All statistical analyses were performed at Y = 0.05.
CHAPTER 4 RESULTS 3.1 Urea Hydrolysis Urea-N declined rapidly from the time of fertilization for both UAN and UI treatments (Figure2a). However, urea-N was retained in the soil significantly longer with UI relative to UAN. By day 8, less than 1% of urea-N remained in the soil in the UAN treatment, while 20% was still present for the UI treatment. By day 14, all of the initial ureaN was depleted from the UAN treatment and less than 1% remained in UI treated soil. The disappearance of urea corresponded with a rapid increase in soil NH4+-N for several days for both fertilizers, indicating a high level of urease activity. In agreement with the rate of urea-N losses from soil, accumulation of NH4+-N in soil was more rapid for UAN than for UI (Figures 2a and 2b). Maximum soil NH4+-N concentrations occurred on day 3 for the UAN treatment and were 81% higher than the day 0 value. In contrast, maximum NH4+N concentrations occurred on day 8 in UI treatment and were 101% higher than initial concentrations. On day 21, soil in the UAN treatment had an increase in NH4+-N from 0.40 ± 0.01 mg g dw-1 ( x ± 1 SEM) on day 0 to 0.50 ±0.08 mg g dw-1 soil. In contrast, the UI treatment declined from 0.37 ± 0.02 mg g dw-1 on day 0 to 0.35 ± 0.16 mg g dw-1 on day 21. These values represented changes of +27% and -6% with respect to initial concentrations. Nitrate concentrations in soil cores amended with both fertilizer types were relatively constant up to day 8 followed by a slight increase until day 14 (Figure 2c). During the final 7 days of the experiment there was a small decrease in NO3--N concentrations (Figure 2c). The
highest soil NO3--N concentration of 0.60 ± 0.08 and 0.40 ± 0.05 mg g dw-1for the UAN and UI was observed on day 14 for both treatments. These concentrations represented increases of 87% and 52% over day 0 values for UAN and UI treatments, respectively. On day 21, soil NO3--N concentrations increased from initial values of 0.30 ± 0.06 to 0.35 ± 0.03 mg g dw-1 for the UAN treatment and decreased for the UI from 0.29 ± 0.04 to 0.20 ± 0.04 mg g dw-1. Most of the fertilizer (97%) remained localized in 0 to 10cm zone of the 20 cm cores throughout the experiment.
Figure 2. Time course for change in concentrations of nitrogenous nutrients in intact soil cores fertilized with a urea-ammonium-nitrate (UAN) fertilizer or a similar fertilizer amended with Agrotain™ (UI), a urease inhibitor. Error bars represent ± 1 SEM for triplicate soil cores.
Ammonia volatilized from soil cores at least to day 14 for both fertilizer treatments (Figure 3a). With respect to time, the rate of volatilization from soil was often (days 5, 8 and 14) significantly more rapid for UAN than for UI. However, the total mass of N volatilized on day 21 from soils amended with UAN, 20.2 ± 1.7 mg, was not significantly different than the total of 16.7 ± 2.1 mg emitted from soils fertilized with UI. Loss to volatilization after 21 days represented 7% and 6% of the total N applied in the respective fertilizers.
Figure 3. Time course for total NH3 volatilized from soil cores fertilized with a ureaammonium-nitrate (UAN) fertilizer or a similar fertilizer amended with Agrotain™ (UI), a urease inhibitor, or Agrotain Plus™ (UNI), a urease and nitrification inhibitor. Error bars represent ± 1 SEM for triplicate soil cores. The rate of urea-N decline in soil was more rapid for UAN than the UNI treatment (Figure 4a). By day 8, less than 2% urea-N remained in the UAN treatment, while 99% was still present for the UNI treatment. After 21 days, approximately 1% of the initial urea-N remained in soils treated with UAN. However, 50% of the urea-N that was present in the soil on day 0 remained on day 21 in the UNI treatment.
Differences in the soil concentrations of NH4+-N were also observed between the UAN and UNI treatments (Figure 4b). There was a rapid increase in soil NH4+-N concentrations for the UAN treatment and a slower rate of increase for UNI. Maximum soil NH4+-N concentrations occurred on day 8 for the UAN treatment and were 1.5 times higher than on day 0, at 0.94 ± 0.13 mg g dw-1. Unlike the UAN treatment, maximum soil NH4+-N concentrations for UNI treatment occurred on day 21 and were 1.3 times higher than initial concentrations. On day 21, soil in the UAN treatment had an overall increase in NH4+-N from 0.38 ± 0.08 mg g dw-1on day 0 to 0.60 ± 0.07 mg g dw-1. A more pronounced increase of 0.29 ± 0.03 mg g dw-1 on day 0 to 0.68 ± 0.07 mg g dw-1on day 21 was observed in soil concentration of NH4+-N for the UNI treatment. These values represented changes of +28% and +133% with respect to initial concentrations. Nitrate concentrations in soil cores amended with both UAN and UNI fertilizer solutions were relatively constant until day 8 and then increased slowly until day 21 (Figure 4c). The highest soil NO3--N concentration of 0.56 ± 0.03mg and 0.43 ± 0.03mg for UAN and UNI was observed on day 21 for both treatments. These concentrations represented increases of 57% and 55% over day 0 values for UAN and UNI treatments, respectively. A majority of the fertilizer (97%) remained in the top 10cm of the 20cm cores throughout the experiment. Ammonia volatilized from soil cores to day 21 for both fertilizer treatments UAN and UNI (Figure 3b). However, the rate of N volatilization from soil was significantly more rapid for UAN than for UNI on days 5, 8, 14, and 21. Accordingly, the total mass of N volatilized from soils amended with UAN (± 1 SEM) was significantly higher than for UNI, 14.7 ± 1.1
mg versus 5.2 ± 0.9 mg on day 21. Loss of N to volatilization represented approximately 5% and 2% of the total applied in the respective fertilizers.
Figure 4. Time course for change in concentrations of nitrogenous nutrients in intact soil cores fertilized with a urea-ammonium-nitrate (UAN) fertilizer or a similar fertilizer amended with Agrotain Plus™ (UNI), a urease and nitrification inhibitor. Error bars represent ± 1 SEM for triplicate soil cores. 3.2 Moisture dependence The rate of urea-N decline was faster in homogenized soil that was treated with UAN fertilizer compared to UI or UNI treatments at all levels of %WHC levels (42%, 64%, and 86%). On day 6, urea-N was not detectable in soil treated with UAN at all moisture levels. In contrast, there was a decrease in mean concentrations of urea-N by only 15 ± 14% and 37± 13% in soil treated with UI and UNI, respectively. At each of the three moisture levels there was no significant difference in the rate of urea-N transformation in soil treatments UI and 29
UNI. In addition, comparison of urea-N transformation rates among moisture levels within a fertilizer treatment showed that there were no differences in treatments UAN or UI. However, UNI-treated soil at 86% WHC had a significantly faster rate of urea-N decline than at 64% WHC. Ammonium concentrations in soil increased at the end of the experiment from initial day 0 concentrations for all fertilizer treatments at 86% WHC (Figure 5a). On day 20, soil NH4+-N increased from initial concentrations by 65 ± 19%, 123 ± 31%, and 156 ± 24% for the UAN, UI, and UNI treatments, respectively. There were significant differences in soil NH4+-N concentrations when the UAN treatment was compared to UI or UNI. However there was no significant difference in soil NH4+-N concentrations between the UI and UNI treatments. The UAN and UI treatments showed a net decrease in soil NH4+-N concentrations between days 0 and 20 at both 42% and 62% WHC. In contrast, NH4+-N concentrations in soil treated with UNI increased at 42% and 62% WHC by 78 ± 43% and 73 ± 21%, respectively. All fertilizer treatments showed significantly higher soil NH4+-N concentration at 86% WHC than 42%WHC. However, there was no significant difference between soil NH4+-N concentration between 42% and 64% WHC within a fertilizer treatment. In contrast to increasing NH4+-N soil concentrations seen at 86% WHC between days 0 and 20 for all fertilizer treatments (Figure 5a), soil NO3--N concentrations decreased by 40 ± 14%, 32 ± 9%, and 63 ± 1% at the highest moisture level for the UAN, UI, and UNI treatments, respectively (Figure 5b). There were no significant differences in soil NO3--N concentrations at 86% WHC among fertilizer treatments (UAN, UI, UNI). Soil treated with UNI showed an overall decrease in NO3--N concentrations at 42% and 64% WHC, whereas NO3--N concentrations in soil treated with UAN and UI increased at the respective % WHCs.
At 64% and 42% WHC, soil NO3--N concentrations in the UNI treatment were significantly different as concentrations declined at both moisture levels, but increased for other fertilizer treatments. Within the UAN treatment there were significant differences in soil NO3--N concentrations among all moisture levels, with the observed value at 64% WHC representing a 277 ± 12% increase on day 20 relative to the initial NO3--N concentration. In contrast, for the UI and UNI treatments there were only significant differences within fertilizer treatment when 86% WHC was compared to 42% or 64% WHC.
Figure 5. Percent change in concentrations of nitrogenous nutrients at 20 d relative to day 0 values in homogenized soil samples fertilized with urea-ammonium-nitrate (UAN) fertilizer; a similar fertilizer amended with Agrotain™ (UI), a urease inhibitor; or Agrotain Plus™ (UNI), a urease and nitrification inhibitor. Triplicate soil samples for each fertilizer were held at each of three values for percent of water holding capacity (WHC). Error bars represent ±1 SEM.
The mass of ammonia volatilized from homogenized soil was significantly higher at 86% WHC for all fertilizer treatments than at 42% or 64% WHC (Figure 6). The respective amounts of NH3-N volatilized after 20 days for the fertilizer treatments at 86%WHC were 13.0 ± 0.8, 17.1 ± 0.3 and 18.0 ± 1.1 mg for the UI, UAN and UNI treatments. Among the fertilizer treatments at the three moisture levels, soil treated with UI had significantly less N volatilized than fertilizer treatments UAN or UNI. In addition, there was no statistical difference in mass of NH3-N volatilized between the UAN and UNI treatment at any moisture level.
Figure 6. Total mass of NH3 volatilized after 20 days from homogenized soil at three values for percent of water holding capacity (WHC) for soils fertilized with urea-ammonium-nitrate (UAN) fertilizer; a similar fertilizer amended with Agrotain™ (UI), a urease inhibitor; or Agrotain Plus™ (UNI), a urease and nitrification inhibitor. Error bars represent ±1 SEM (n=3). 3.3 Simulated Rainfall Leaching of nitrogenous nutrients occurred from all soils in response to simulated rainfall. Unsurprisingly, the mass of N leached from soil was related to the addition of fertilizer. Unfertilized soil (S treatment) had consistently lower masses of N leached (urea-N, 32
NH4+-N, and NO3--N) than fertilized (UAN, UI, and UNI) soil. Only trace amounts of urea-N mass leached from soil cores irrespective of treatment (data not shown). The highest mass of urea-N leached from soil occurred in the UI soil treatment during the first rain event (0.05 ± 0.00 mg), followed by 0.003 ± 0.000 mg urea-N during the fourth rain event in the S treatment. Ammonium was leached from fertilized soil cores during every simulated rainfall (Figure 7). Considering all fertilizer treatments, week 1 NH4+-N leachate values varied from 0.02 ± 0.00 (UNI) to 1.6 ± 1.6 mg (UI). The mass of NH4+-N leached from fertilized cores showed a generalized increase with each successive rainfall, at least to the fourth event. At that time, the mass of NH4+-N leached varied from 0.9 ± 0.4 to 8.1 ± 7.7 mg in the UNI and UI treatment, respectively. However, due to high variability and low sample size, the only significant differences among weeks in the mass of NH4+-N leached were observed for the UI treatment. Week 4 had significantly more NH4+-N leached (8.1 ± 7.7 mg) than week 1, 2, or 3, where the mass of N leached varied from 1.6 ± 1.5 to 1.9 ± 1.5 mg. In contrast to the fertilized cores, the unfertilized cores (S treatment), showed little NH4+-N loss to leaching. The weekly mass of NH4+-N lost varied from 0.005 ± 0.000 to 0.016 ± 0.004 mg. These values represent 27% and less than 1% of the smallest and largest losses recorded for fertilized cores. In accord with the data for NH4+-N, NO3--N was leached from fertilized soil cores during every simulated rainfall (Figure 7). Considering all fertilizer treatments, the mass of NO3--N leached through soil cores on week 1 varied from 0.04 ± 0.01 (UNI) to 0.80 ± 0.70mg (UI). All fertilizer treatments showed increasing amounts of NO3--N leached as the weeks progressed. For the UAN and UI treatments, there was significantly more NO3--N
leached during week 5 than previous weeks. For the respective treatments, 3.0 ± 0.2 and 2.9 ± 0.7 mg of NO3--N was leached. The largest mass of NO3--N was also leached from the UNI treatment on week 5 (1.4 ± 0.1mg). However, the mass of leachate was not significantly different compared to week 4 (1.1 ± 0.2mg). In further agreement with the data for NH4+-N, the unfertilized cores showed comparatively little NO3--N loss to leaching. The weekly mass of NO3--N lost varied from 0.003 ± 0.001 mg to 0.040 ± 0.014 mg. These values represent 8% and 1% of the smallest and largest mass losses recorded for fertilized cores.
Figure 7. Time course for NO3--N and NH4+-N leached from intact soil cores following simulated rainfall events of 2.5 cm (weeks 1 through 4) or 5 cm (week 5). Soil cores (n=3; each treatment) were amended with one of three fertilizer treatments one week prior to the first simulated rainfall. Fertilizer treatments were a urea-ammonium-nitrate fertilizer (UAN) solution, or similar fertilizer solution amended with Agrotain™ (UI), a urease inhibitor; or Agrotain Plus™ (UNI) a urease and nitrification inhibitor. A fourth treatment included unfertilized, field moist cores (data not shown).
The S treatment showed the lowest total mass of NH4+-N leached, 0.1 ± 0.0mg (Figure 8a). In contrast, the total mass of NH4+-N leached for fertilized cores was 16.9 ± 3.2, 4.9 ± 0.7 and 3.6± 1.1 mg for the UI, UAN and UNI treatments, respectively. Thus, the total mass of NH4+-N leached from soil in the S treatment ranged from 0.2% to 1.1% of that leached from fertilized soil treatments (UAN, UI, and UNI). Among fertilized treatments, there was significantly more NH4+-N leached from soil for the UI treatment than either the UAN or UNI treatments. In agreement with the data for NH4+-N, the S treatment showed the lowest total mass of NO3--N leached, 0.1 ± 0.0 mg (Figure 8a and 8b). The total mass of NO3--N leached through soil differed significantly among fertilizer treatments, ranging over a factor of approximately 2. The UNI treatment had the smallest value (3.8 ± 0.3mg) followed by the UAN (6.1 ± 0.3mg) and UI (7.7 ± 0.3mg) treatments. Nitrate captured in leachate from the S treatment represented from 0.9% to 1.8% of that leached from the treatments amended with fertilizer.
Figure 8. Total mass of NO3--N and NH4+-N leached from intact soil cores following simulated rainfall events of 2.5 cm (weeks 1 through 4) or 5 cm (week 5). Soil cores were amended with one of three fertilizer treatments one week prior to the first simulated rainfall. 35
Fertilizer treatments were a urea-ammonium-nitrate fertilizer (UAN) solution, or similar fertilizer solution amended with Agrotain™ (UI), a urease inhibitor; or Agrotain Plus™ (UNI) a urease and nitrification inhibitor. A fourth treatment included unfertilized, field moist cores (S). Error bars represent ± 1 SEM for triplicate soil cores, except for the UI treatment (n=2). The total mass of N (NO3--N, NH4+-N, and urea-N) that was recovered as leachate from the UAN, UI, and UNI treatments was relatively small. For the respective fertilizer treatments, the percentage of fertilizer N that was collected in leachate was 3.9 ± 0.4%, 8.8 ±1.0 %, and 2.6 ± 0.4%. The differences among means were not significant. The percentage of total recovered N that was in the form of NO3--N was 55.8 ± 2.2%, 31.9 ± 4.9%, and 53.4 ± 7.5% for the UAN, UI and UNI treatments, respectively. The remainder of N leached was largely in the form of NH4+-N, as urea comprised less than 1% of all N leached in the fertilizer treatments. In contrast, the relative distribution of N leachate from the S treatment was 50.0 ± 14.5%, 40.6 ± 11.6%, and 9.4 ± 2.9% NO3--N, NH4+-N, and urea-N, respectively. The total mass of N leached from the S treatment was 0.12 ± 0.03 mg, which represents less than 1% of the total mass of N leached from fertilized cores.
3.4 Microbial Activity The rate of CO2 production was used as an index of community microbial activity. Carbon dioxide production was significantly higher for soil in fertilizer treatments (UAN, UI, and UNI) than soil without fertilizer (S and C) (Figure 9). The rates of soil CO2-C production were similar in the UAN and UI treatments at approximately 0.9 mg g dw-1. However, soil in the UNI treatment showed a significantly lower CO2-C production rate (0.79 ± 0.04 mg g dw-1) than soil treated with UAN or UI. The rate of CO2-C 2 production in soil for the S and C treatment were not significantly different from one another. The
respective rates of CO2-C production in soil for the S and C treatments were 0.29 ± 0.01 and 0.35 ± 0.002 mg g dw-1. The highest rate of soil CO2-C production among all treatments was observed in the UAN and UI treatments and was approximately 1.8 times the CO2-C production observed in the S and C treatments.
Figure 9. Total CO2-C production from homogenized soil for five treatments; field moist soil only (S), soil with the equivalent volume of deionzed water added to soil as is added in fertilizer solutions (C), soil fertilized with urea-ammonium-nitrate (UAN) fertilizer solution, or a similar fertilizer amended with Agrotain™ (UI), a urease inhibitor, or Agrotain Plus™, a urease and nitrification inhibitor (UNI). Error bars represent ± 1 SEM for triplicate soil samples. Denitrifying enzyme activity (DEA) was used as a measure of the relative population size of denitrifying microbes in soil. Denitrifying enzyme activity increased for all soil treatments to day 10 followed by a decrease on day 14 (Figure 10). On day 10, soil in the UAN treatment had the largest increase in DEA, from an initial rate of 4.4 ± 1.5 ng N2O-N g dw-1 h-1 on day 0 to 121.1 ± 23.4 ng N2O-N g dw-1 h-1. Thus, the rate increased by a factor of 27. Denitrifying enzyme activity on day 10 in the treatments UI and UNI increased to a lesser extent from initial levels, by factors of 2.3 and 5.5, respectively. Soil without fertilizer, the S 37
and C treatments, had low (< 10 ng N2O-N g dw-1 h-1) DEA throughout the experiment. In addition, there was no significant difference in DEA between the S and C treatments. After soil was kept in anoxic conditions for four days there was an increase in DEA for all soils (data not shown) from the highest observed values (day 10). The soil in the UI treatment showed the greatest relative increase (factor of 73) from day 10 values. Soils in both the C and UAN treatments showed similar increases, by factors of 44 and 42, respectively. The UNI treatment showed an increase in DEA by a factor of 11 which was the lowest observed for all fertilizer treatments. Rates of DEA on day 18 were 348 ± 84, 304 ± 141, 5230 ± 705, 1712 ± 582, and 922 ± 322 ng N2O-N g dw-1 h-1 for the S, C, UAN, UI and UNI treatments, respectively.
Figure 10. Time course for denitrifying enzyme activity (DEA) in homogenized soil for five treatments: field moist soil only (S); soil with the equivalent volume of deionzed water added to soil as is added in fertilizer solutions (C); soil fertilized with urea-ammonium-nitrate (UAN) fertilizer solution; a similar fertilizer amended with Agrotain™ (UI), a urease inhibitor; or Agrotain Plus™, a urease and nitrification inhibitor (UNI). Error bars represent ± 1 SEM for triplicate soil samples. Short-term nitrifier activity provided an index of the response of the ammoniumoxidizing microbial population to each treatment. Similar to DEA, NA increased in the UAN 38
and UI treatments to day 10 followed by a decline (Figure 11). On day 10, NA increased by 103% and 130% from day 0 values for the UAN and UI treatments, respectively. Maximum rates of NA for these treatments exceeded 500 ng NO2--N g dw-1 h-1. In contrast to these two fertilizer types, rates of NA in soil treatments S and C remained low and fairly constant, varying from 161.4 ± 3.8 to 216.0 ± 0.8 ng NO2--N g dw-1 h-1. Mean rates between these two treatments differed significantly only on day 5. Soil treated with a nitrification inhibitor (UNI) had the lowest rates of NA observed on all dates. Rates were only 24 to 57% of values recorded for the C treatment and 9 to 34% of the values observed for the UAN treatment. Short term nitrifier activity in soil was significantly lower in the UNI treatment than the C treatment throughout the experiment. In addition, NA rates declined significantly (57%) between day 0 to 14 for soil in the UNI treatment, from 91.7 ± 10.5 to 39.2 ± 3.5 ng NO2--N g dw-1 h-1.
Figure11. Time course for short-term nitrifier activity (NA) from homogenized soil for five treatments; field moist soil only (S); soil with the equivalent volume of deionzed water added to soil as is added in fertilizer solutions (C); soil fertilized with urea-ammonium-nitrate (UAN) fertilizer solution; a similar fertilizer amended with Agrotain™ (UI), a urease inhibitor; or Agrotain Plus™, a urease and nitrification inhibitor (UNI). Error bars represent ± 1 SEM for triplicate soil samples. 39
CHAPTER 5 DISCUSSION 4.1 Urea Hydrolysis The extracellular urease enzyme is responsible for catalyzing urea hydrolysis, the transformation of urea-N to ammonia and carbon dioxide (Bremner 1995). Soil properties such as pH, texture, moisture, cation exchange capacity, and CaCO3 content can influence urease activity (Mulvaney and Bremner 1981). However, soil organic content is positively correlated with and perhaps exhibits the greatest effect on urease activity in soils (Myers and McGarity 1968; Zantua et al. 1977). Studies assessing the relationship between soil organic content and urease activity are limited to soils of 3.0 to 203.9 g kg-1 organic carbon (Zantua and Bremner 1976; Zantua et al. 1977; Watson et al. 1994; Chakrabarti et al. 2004). Furthermore, it is difficult to compare rates of urea hydrolysis in soils with varying organic matter content because quality of organic material can influence urease activity (Perucci et al. 1982; Gill et al. 1999; Gioacchini et al. 2000). Accordingly, the experiments conducted in this study were to gain a better understanding of rates of urea hydrolysis and N transformations in soil with exceptionally high organic content. Complete urea hydrolysis occurred between day 6 and 8 in all experiments when unamended UAN fertilizer was added to soil in this investigation. Previous laboratory experiments showed that in urea-fertilized soil with considerably lower organic content (2.8 to 24.6 g kg-1) over 90% of applied urea-N was hydrolyzed within 10 d of application (Carmona et al. 1990; Zhengping et al. 1991b; Gill et al. 1999). Despite differences in
organic content and possibly other physiochemical properties, rates of urea hydrolysis from urea or UAN fertilized soil are comparable across studies. In a laboratory study involving a variety of soils with soil organic content ranging from 7.0 to 45.0 g kg-1, Kumar and Wagenet (1984) found no definitive relationship between urease activity and any one soil physiochemical property. Ammonia volatilization ranged from 5.2 to 7.2% of N applied to soil cores treated with unamended UAN fertilizer during the 20 d incubation period in the current study. Similarly, in a laboratory experiment using homogenized Typic Ustochrept soil with a lower organic content (2.8 g kg-1), 7.12% of N applied as urea fertilizer volatilized after 16 days (Gill et al. 1999). However, in another study using 16 different homogenized soils from Northern and Southern Ireland with varying organic content (16.3 – 203.9 g kg-1), NH3 volatilization from urea-fertilized soils ranged from 5.8 to 38.9% during a 9 day observational period (Watson et al. 1994). Analysis of the relationship between soil physicochemical properties and the rate of NH3 loss from unamended urea- fertilized soil indicated that volatilization was most highly correlated with soil acidity. Few studies have used (or cited the use of) Agrotain™ and Agrotain Plus™ products as enzyme or microbial activity inhibitors to study their effects on nitrogen transformations in soil. However, the chemical in Agrotain™ that acts as a urease inhibitor, N-(n-butyl) thiosphophoric triamide (NBPT), and its effectiveness as an amendment to urea- based fertilizers has been extensively studied. The two major chemicals in Agrotain Plus™ that serve as urease and nitrification inhibitors are NBPT and dicyandiamide (DCD), respectively. While there have been studies conducted on the effectiveness of these inhibitors and their
ability to increase fertilizer and nitrogen use efficiency in agroecosystems, there is limited information on their effectiveness in highly organic soils. Results of the first urea hydrolysis experiment showed that less than 1 and 20% of the initial urea-N remained in the soil on day 8 for the UAN and UI treatments, and hydrolysis was near complete by day 14 in both treatments (Figure 2a). These results compare favorably with previous laboratory studies. Hendrickson and Douglass (1993) found that 0 and 30% of urea-N remained in acidic soil fertilized with urea and urea plus NBPT, respectively, and hydrolysis was complete on day 14 for the NBPT-amended soil. Similarly, Vittori Antisari et al. (1996) reported that approximately 20-30% of applied urea-N remained in soil by day 8 when fertilized with urea plus NBPT with an application rate of NBPT equivalent to that used here (0.1% w/wurea). Both studies assessed rates of urea hydrolysis under varying physicochemical conditions (e.g. pH, texture, etc.) but soil organic contents ranged from 0.153 to 47 g kg-1. The time course for urea hydrolysis and its retardation by NBPT in this wide range of soil organic contents were similar possibly due to poor quality organic material or the interactive effects of physicochemical factors affecting hydrolysis and the effectiveness of NBPT. Peak soil concentrations of NH4+-N occurred between day 3 and 8 in the first urea hydrolysis experiment (Figure 2b), with the UAN treatment showing the most rapid NH4+-N accumulation. A previous report (Vittori Antisari et al.1996) also found that concentrations of NH4+-N increased in general more rapidly in soil that was fertilized without a urease inhibitor versus soil that was fertilized with a urease inhibitor. The addition of Agrotain™ to UAN fertilized soil in the first urea hydrolysis experiment resulted in a significant (45%) reduction in NH3 volatilization over 21 days
(Figure 3a). Agrotain™ was more effective here at preventing NH3 volatilization than in a laboratory study involving sandy soil cores (Singh et al. 2004), where a 27% reduction in volatilization was observed in urea-treated soils amended with Agrotain™. Differences in the percentage of N fertilizer lost to NH3 volatilization could be due to differences in buffering capacity and pH of soil (Hargrove 1988). The second urea hydrolysis experiment is the first study to directly assess the effectiveness of Agrotain Plus™ at delaying urea hydrolysis. A temporal disjunction between urea hydrolysis experiments precludes direct comparison of the efficacy of Agrotain™ and Agrotain Plus™ at delaying urea hydrolysis, as critical soil properties may have differed between sampling dates. In agreement with results of the first urea hydrolysis experiment comparing UAN and UI, addition of NBPT and DCD resulted in a significant reduction (64%) in NH3 loss to volatilization relative to UAN fertilizer. Although the percent reduction in NH3 volatilization appears greater than the 45% observed for UI, simultaneous examination of the performance of all three fertilizers is necessary for confirmation. An 80 day field study on sandy loam soil (Gioacchini et al. 2002) showed that the addition of NBPT plus DCD to urea fertilizer reduced NH3 volatilization by 58% relative to soil fertilized with unamended urea. This performance is comparable to results found here.
4.2 Moisture dependence Moisture content appeared to have a fertilizer-specific impact on rates of urea hydrolysis as rates were not different among % WHCs for UAN and UI treatments, but soil at 86% WHC showed a significantly higher rate of urea hydrolysis than soil at lower
moisture levels for the UNI treatment. There are other references to inconsistencies in the relationship between urease activity and soil moisture in previous research (Mulvaney and Bremner 1981; Agehara and Warncke 2005). However, the results for the UNI treatment agree with most research, in that urea hydrolysis increases as moisture levels approach field capacity (Kumar and Wegnet 1984; Vlek and Carter 1983; Hongprayoon et al. 1991). Allowing for a direct comparison, Agehara and Warncke (2005) reported that urea-fertilized soil at 90% WHC had higher rates of urea-N decline than soil at 50 and 70% WHC. The reason for differences among fertilizers in the moisture sensitivity of urease activity in the current study is unknown. In agreement with the results of the two urea hydrolysis experiments, urea hydrolysis in the moisture dependence experiment was delayed significantly in the UI and UNI treatments relative to the UAN treatment at all moisture levels. In addition, there was no significant difference in the rate of urea hydrolysis between the UI and UNI treatments within a moisture level. These results are consistent with previous research that the addition of DCD to NBPT in urea fertilizers does not further affect urea hydrolysis (Mulvaney and Bremner 1981). This further suggests that temporal differences in physicochemical properties influencing rates of urea hydrolysis account for the difference in performance of the UI and UNI treatments relative to the UAN treatment in the two urea hydrolysis experiments. Results from the moisture experiment showed that soil at 86% WHC had an overall increase in soil NH4+-N concentrations for all fertilizer treatments (Figure 5). Soil conditions that are unfavorable for nitrification, such as high moisture and low oxygen levels, can cause
accumulation of NH4+-N in soil (Tisdale and Nelson 1975). It is likely that these factors contributed to the accumulation of soil NH4+-N at 86% WHC. The simultaneous decline in soil NH4+-N and rise in NO3--N for the UAN and UI treatments at 42 and 64% WHC in contrast to the increase in soil NH4+-N and decrease in NO3--N in the UNI treatment, indicates that the DCD was effective at inhibiting nitrification throughout the 20 day experiment. In previous laboratory experiments using soils with varying physiochemical properties, the effectiveness of DCD ranged from 21 d to over 42 d (Bronson et al. 1989a; McCarty and Bremner 1989). The rate of DCD degradation in soil varies with soil temperature, texture, and moisture regime (Bronson et al. 1989). There was an overall decrease in soil NO3--N concentrations at 86% WHC in all fertilizer treatments. The decline in soil NO3--N concentration suggests that soil moisture conditions were favorable for denitrification. As soil moisture increases, denitrification rates also increase (Tisdale and Nelson 1975) until optimum conditions are met when 100% of soil pore space is filled with water (Brady and Weil 2002). The decrease in soil NO3--N concentrations in the UNI treatment at all moisture levels suggests that denitrification was occurring in all soil fertilizer treatments. Although denitrification is a facultative anaerobic process that has faster rates in anaerobic soils, denitrification can take place in well aerated soil due to anoxic zones in soil aggregates (Schlesinger 1997). Because nitrification is more rapid in aerobic soils, it was not until nitrification was inhibited that denitrification was inferred in soil at 42 and 64% WHC. In aerated soils, denitrification rates can be masked by high nitrification rates which allow a net accumulation of nitrate (Burns et al. 1996). The highest net accumulation of soil NO3--N occurred in the UAN treatment at 64% WHC, suggesting that this moisture level is ideal for nitrification in this soil. Brady and Weil
(2002) reported that the optimum moisture condition for nitrifying bacteria occurs when around 60% of total soil pore space is filled with water. Similarly, in a field study Abbasi and Adams (2000) measured the greatest accumulation of NO3--N in soil at 63% water filled pore space (WFPS). In a laboratory study, soil NO3--N concentrations increased with increasing water content in the range from 40 to 60% WFPS, leading researchers to conclude that nitrification rates increased with increasing moisture for moderately wet soils (Burns et al.1996). This is consistent with previous reports that nitrification rates are lowest in excessively dry and wet soil conditions (Tisdale and Nelson 1975; Brady and Weil 2002). Ammonia volatilization was significantly higher in all fertilizer treatments at 86% WHC than at 42 or 64% WHC. Previous research has shown that NH3 volatilization increases with soil moisture content (Fenn and Hossner 1985; Clay et al. 1990). Additionally, research has shown that maximum NH3 volatilization occurs when fertilized soil is at field capacity (Hargrove 1988). In a laboratory study conducted by Al-Kanani et al. (1991) NH3 volatilization increased with soil water content in UAN-fertilized soil. Moreover, field and laboratory experiments have shown that NH3 volatilization losses increase in urea-N fertilized soil following a rainfall event which leads to higher soil water content (Kissel et al. 2004). Soil in the UI treatment had significantly less NH3 volatilized at all moisture levels compared to the other fertilizer treatments. Results from both laboratory and field studies conducted by Bronson et al. (1989b) showed that NH3 volatilization was significantly less in soil when urea fertilizer was amended with the urease inhibitor NBPT. Zhengping et al. (1991) showed similar results in a laboratory study, concluding that the addition of NBPT prevented the hydrolysis of urea and therefore reduced NH3 volatilization. In contrast to
NBPT, the addition of DCD increased NH3 volatilization from soil as shown in these results and in previous studies. In a laboratory experiment using sandy loam soil the application of DCD increased volatilization when applied with a urea fertilizer compared to unamended urea fertilizer (Praksa Roa and Puttanna 1987). In another experiment, NH3 volatilization losses were higher in both sandy loam and clay loam soil when DCD was added to soil with urea and NBPT in contrast to a treatment of urea-fertilizer amended with NBPT (Gioacchini et al. 2002) Since both types of soil showed a similar response to the addition of a nitrification and urease inhibitor, the authors concluded that DCD reduced the efficiency of NBPT. As an alternative explanation, Zhengping et al. (1991) speculated that the increased NH3 volatilization observed in aerated soils is due to the competition between nitrification and volatilization processes.
4.3 Simulated Rainfall Essentially no urea-N leached from soil cores regardless of fertilizer treatment. However, previous laboratory research using repacked soil columns found measurable concentrations of urea-N in collected leachate (Paramasivam and Alva 1997; Prakash et al. 1999). In a laboratory study using sandy soil, it was not until the second leaching event, approximately two weeks after fertilization, that urea-N was leached through the repacked soil column (Prakash et al. 1999). Additionally, there was no significant difference in the amount of urea-N leached between the urea and Agrotain™ fertilizer treatments. In another laboratory experiment using similar sandy soil Paramasivam and Alva (1997) also reported that the largest percentage of urea-N was leached from urea-fertilized soil approximately two
weeks after addition. In both experiments leaching of urea-N was complete by approximately four weeks. Contrary to expectations, the proportion of N leached as NH4+-N was relatively large in this simulated rainfall experiment. There are conflicting reports of the percentage of N leached as NO3--N and NH4+-N in previous field and laboratory studies. In a laboratory study using repacked soil columns 8 cm in height with homogenized sandy soils and fertilized with NH4NO3, approximately 85% and 37% of the NH4+-N and NO3--N applied was recovered in 1000 mL of leachate (Alva 1992). Further, it was determined that less NO3--N leached because soil cores were nearly water-saturated, favoring denitrification. However in field experiments, researchers have found larger portions of N leached in the form of NO3--N than NH4+-N regardless of N fertilizer type and irrigation practice (Timmons and Dylla 1981; Gioacchini et al. 2002; Portela et al. 2006). Despite six different treatments with varying combinations of granular or liquid nitrogen fertilizer and irrigation method, NH4+-N comprised a small percentage (0.02 to 0.12%) of the total N leached compared to NO3--N in a field experiment conducted in a sandy Minnesota loam (Timmons and Dylla 1981). Similarly, Gioacchini et al. (2002) found that negligible amounts of NH4+-N leached from either sandy loam or clay loam soils fertilized with urea. Finally, a five year study involving silt loam soil fertilized with a variety of N fertilizers including urea and ammonium sulfate showed that NH4+-N concentrations in the leachate were below the detection limit (Portela et al. 2006). Overall, less NO3--N leached in the UNI treatment than either the UAN or UI treatments, although differences among treatments were not significant due to high withintreatment variability in the mass of NO3--N leached. Qualitatively, however, reduced NO3--N
in leachate from the UNI treatment is in agreement with results of a laboratory experiment (Alva 1992) that showed reduced leaching of NO3--N from soil columns amended with controlled release versus traditional fertilizers. The nitrification inhibitor in the UNI treatment is functionally similar to the controlled release fertilizer by slowing the oxidation of NH4+-N to NO3--N. The total percentage of fertilizer lost to leaching ranged from 2.6% to 8.8% in the simulated rainfall experiment. These percentages are considerably lower than the values of 43% and 36% of applied N leached through soil columns containing moderately well drained sandy soil treated with urea and Agrotain™ fertilizer, respectively (Prakash et al. 1999). However, these data compare favorably with field studies assessing leaching loss of nitrogenous fertilizers. Portela et al. (2006) estimated that 0 to 3.5% of fertilizer derived N leached from silt and sandy loam soils in a field experiment, with the percentage loss linked to the crop and fertilizer type. Similarly, a field experiment conducted on a corn-soybeanwheat rotation showed that approximately 5% of urea fertilizer leached below the root zone (Riley et al. 2001). Direct comparison of the percentage of fertilizer-derived N lost to leaching is difficult due to an array of factors that influence N movement, including timing and volume of rainfall and soil texture.
4.4 Microbial Activity The addition of water to soil did not stimulate the microbial community in the microbial activity experiment, as there was no difference CO2 production between soils in the S (no amendment) and C (water addition) treatments (Figure 9).
Soil wetting has frequently
been demonstrated in field and laboratory studies to enhance CO2 production as much as
500% compared with unwetted soils, with the CO2 pulse persisting as long as 6 d following the wetting event (eg. Kieft et al. 1987; Franzluebbers et al. 2000; Borken et al. 2003). Enhanced CO2 production following soil wetting has been attributed to improved C availability to active microbes from either previously killed microbial biomass or nonbiomass soil organic matter (Fierer and Schimel 2003). The lack of a respiratory response on wetting in the current study likely relates to soil moisture. Field moist soil (S treatment) was at 64% WHC, close to the optimum of 60% WHC for microbial respiration across diverse soils (Linn and Doran 1984), suggesting that the additional moisture would not stimulate microbial CO2 production. Soils treated with a volume of liquid UAN type fertilizer consistent with the volume of deionized water added in the C treatment show significantly elevated rates of CO2 production relative to the C or S treatments (Figure 9), indicating that the enhanced CO2 evolution was associated with fertilization rather than wetting. Previous studies assessing changes in CO2 flux following application of urea fertilizers and inhibitors give mixed results. In a laboratory experiment conducted on sandy loam soils in Minnesota, Clay et al. (1990) found that CO2 evolution was lower in soils treated with urea amended with NBPT and NBPT plus DCD relative to unamended controls. Although no explanation was given for the decline in CO2 emission, soil pH increased from 6.5 to 9.0 in all fertilized samples, which could have influenced the distribution of CO2 between the aqueous and gaseous phases. In contrast, a laboratory experiment conducted with 12 German soils showed either an increase and decrease in CO2 production in soils amended with urea plus NBPT, depending on the soil (Guettes et al. 2002). Results likely reflect the sensitivity of urease activity to soil physicochemical properties as noted previously.
The reason for the CO2 flush in the current study is unclear. If the time-integrated mass of CO2-C emission from the S and C treatments (0.3 mg C g dw -1) is considered to be the basal metabolic rate of unfertilized soil, the CO2 efflux of 0.9 mg C g dw -1 can be explained simply by urease activity, as total decomposition of the urea in the fertilizers used here will liberate 0.7 g CO2-C g dw -1 soil, at the rate of fertilizer application in this experiment. An increase in denitrification in response to the increased availability of NO3--N following fertilization (Figure 10) may also be responsible for elevated CO2 emission in fertilizer treatments. Simultaneous increases in denitrification and CO2 production have been reported for natural soils (Robertson and Tiedje 1984; Groffman and Tiedje 1991) and DEA has been positively correlated with CO2 evolution in a Kentucky soil (Parsons et al. 1991). Increases in the population of nitrifiers (Figure 11), should lead to net incorporation of CO2 into microbial biomass. However, Miller and Johnson (1964) reported the simultaneous occurrence of maximum rates of nitrification and CO2 production in Colorado soils. Carbon dioxide is an end product of the degradation of DCD, the nitrification inhibitor in the UNI fertilizer (Gioacchini et al. 2002), but will contribute insignificantly to the CO2 mass balance at the application rate used here. Proximal controls on denitrification (reviewed by Firestone and Davidson 1989; Robertson 2000) include O2 and the availability of reductants (labile organic-C) and electron acceptors (nitrogen oxides). Soil moisture is often used as a proxy for soil O2 status, as O2 concentrations are inversely related to moisture (Davidson and Schimel 1995). Increasing rates of DEA in the fertilizer treatments from day 0 to 10 (Figure 10), and persistently low rates in the S and C treatments indicate that denitrification in these soils is at least in part constrained by NO3--N availability. Denitrification has been frequently documented in the
anoxic interior of soil aggregates in otherwise oxic bulk soils (Sextone et al. 1985). Moreover it is closely coupled with nitrification at low NO3--N concentrations (Groffman 1994) due to diffusional constraints on substrate supply (Strong and Fillery 2002). The endogenous NO3-N concentration of ~10 µg g dw-1 soil (= 0.8 mM) approaches the threshold of 5 µg g dw-1 for denitrification (Ryden 1983) and is lower than Km values (half-saturation constant) which range to 12 mM for denitrification in unstirred soil samples (Bowman and Focht 1974; Strong and Fillery 2002).
Apparently, fertilization relaxed or eliminated diffusion
limitation, as soil NO3--N increased to about 800 µg g dw-1, allowing for denitrification uncoupled to nitrification (e.g. Nielsen and Revsbech 1998). It is unclear whether increased DEA in fertilized soils resulted from population growth of denitrifiers or de novo enzyme synthesis by existing microbes. Whalen and DeBerardinis (2007) demonstrated an increase in total microbial biomass in N-fertilized agricultural soils on a time horizon similar to that of the current experiment, but did not distinguish among functional groups. The marked increase in DEA for all treatments following four days of anoxic incubation indicates that O2 was the dominant control over denitrification in this soil. The relative increase among fertilizer treatments (UAN > UI > UNI) was consistent with the rank order for NO3 availability. The DEA values of about 900 to 5200 ng N2O-N g dw-1 fertilized soils following anaerobic incubation rank toward the high end of previous reports for agroecosystems, which show DEA values ranging to 2000 ng N2O-N g dw-1 soil (Bergstrom and Beauchamp 1993; Sotomayor and Rice 1996; Loro et al. 1997; Mulvaney et al. 1997; Pelletier et al. 1999; D’Haene et al. 2003). Nitrogen loss to denitrification in agroecosystems has been estimated to account for 10 to 40% of fertilizer inputs worldwide (Galloway et al. 2004). High DEA following anaerobic incubation and the high organic content of these soils
point to a high potential for loss of fertilzer-N from denitrification during rainfall and argue for the use of the UNI fertilizer to restrict substrate availability for denitrifiers, and hence, gaseous N loss. Active nitrification in all treatments in NA experiment (Figure 11) indicate that O2 was present and corroborates the inference that denitrification was limited to anoxic microsites in these soils. In accord with data for DEA, at least some fertilizers (UAN and UI) elicited an increase in NA relative to S and C treatments. Initial soil concentrations of NH4+N were around 1 to 2 µg g dw-1 in the S and C treatments, but values were increased to around 800 µg g dw-1 in fertilized soils. Laboratory studies have shown an increase in nitrification with increasing NH4+-N in the 50 to 200 ^g g dw-1 range (Malhi and McGill 1982; Nishio and Fujimoto 1990), while the reported Km for soil nitrification is 154 to 186 ^g NH4+-N g dw-1. Thus, the fertilizer-induced increase in NA is at least partly related to an increase in substrate supply. It is unclear whether population growth of ammonium oxidizers occurred, as Wheatly et al. (1997) demonstrated that NA can show wide temporal fluctuations without change in population size. Further, ammonium oxidizers have shown an uncommonly slow maximum specific growth rate (Hagopian and Riley 1998). The observed range of values for NA in this study when all treatments were considered (4 to 540 ng NO2-N g dw-1 h-1) is similar to the 0 to 500 ng NO2-N g dw-1 h-1 given in previous reports for arable land and pastures (Berg and Roswall1985, 1989; Hopkins et al. 1988; Bramley and White 1989). The significant reduction in NA for soils in the UI treatment relative to the S and C treatments (Figure 11) is consistent with mode of action of DCD, namely suppression of activity of extant ammonium oxidizers (Subbarao et al. 2006). Previous research in mineral
soils (Amberger 1989) has demonstrated that DCD remains effective for 4 to 8 weeks. The current study clearly indicates that DCD inhibited NH4+ oxidation to at least 2 weeks in this highly organic soil. This direct demonstration of the effectiveness of DCD as a nitrification inhibitor in this soil provides further evidence that its use will reduce offsite loss of fertilizer N from leaching and denitrification.
CHAPTER 6 SUMMARY This study simulated field conditions to evaluate rates of N transformation and transport in highly organic agricultural soils treated with UAN type fertilizers. Rates of urea hydrolysis from UAN fertilizer were similar to previously reported rates for urea or UAN fertilizers in mineral soils, suggesting that the quality and/or quantity of organic material at the study site had no influence on urease activity. The urease inhibitor NBPT effectively slowed urea hydrolysis both alone and combined with nitrification inhibitor DCD. The influence of soil moisture on rates of urea hydrolysis appeared to be fertilizer specific, as only the UNI fertilizer showed sensitivity to % WHC. However, soil moisture did influence other nitrogen transformations that affect both the availability of fertilizer for plant assimilation and susceptibility for loss from the point of application. Specifically, soils fertilized with UAN and UI fertilizers at 42 and 64% WHC showed high rates of nitrate accumulation which can enhance offsite transport by subsequent denitrification or by leaching and surface runoff. Simulated rainfall experiments showed that soils subjected to UNI fertilizer experienced less leaching loss of both NO3--N and total measured N (urea-N, NO3--N, NH4+-N) than soils fertilized with UAN or UI. However, differences among treatments in the latter were not significant due to high variability in the data. Although nitrification and denitrification were not specifically assessed in this study, soils fertilized with UAN and UI showed high potential for both microbial activities. In contrast, UNI soils
exhibited a reduction in NA relative to all other treatments over 14 d and severely limited DEA after a 4 d anaerobic incubation. Thus, UNI fertilizer can effectively limit the oxidation of NH4+-N supplied directly as fertilizer or from hydrolysis of fertilizer-urea, reducing the NO3--N available for denitrification and offsite transport in these soils. Fractional N loss to volatilization was similar among fertilizer types, suggesting that this should have little influence in fertilizer selection at this site. Based on the effectiveness of the UNI fertilizer on suppressing nitrification and the pivotal role of NO3--N in fertilizer loss from the site of application, use of this fertilizer is recommended for this agroecosystem, which abuts nutrient-sensitive marine waters.
Appendix I: Urease Activity Experiment Treatments UAN and UI
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MATH 049: Introduction to MESA ... 13) RHC/CSU GE/IGETC Request. Item 1516-128. SPCH 150: Intercultural. Communications. RHC Area: 6. CSU GE Area: D3 & D7. IGETC Area: 4C. 14) Degree Change. Item 1516-129. AA-T â Communication Studies for. Transfer
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