COMPARING GIBBS ENERGY RELATIONSHIPS FOR AMMONIA VOLATILIZATIONS FROM AGRICULTURAL SOILS FOR POTATO PRODUCTION

Soil drought, that can be enhanced by global warmin g increases ammonia (NH 3) volatilization. This laboratory study was conducted with two soils: Krom e Gravelly Loam (KGL) from Florida and Warden Silt Loam (WSL) from Washington State and two fertilizer s: Ammonium sulfate [(NH4)2SO4] or ammonium nitrate (NH4NO3). Two water regimes including 20 and 80% Field Cap acity (FC) were used at 20°C which is the average temperature in the potato growing se ason in Washington State. The data demonstrated tha t variation in NH3 volatilization subject to different soil water reg imes can be explained by changes in Gibbs free energy of Nitrogen (N) fertilization in soils with varying water contents. The absolute values of Gibbs free energy of (NH4)2SO4 or NH4NO3 applied to soil at 20% FC soil water regime were 5 -fold greater than at 80% FC. Accordingly, the equilibrium constant (K ) of deprotonation of ammonium ions in soil solutio ns at 20% FC was 3,000or 50-fold greater than that a t 80% FC for (NH4)2SO4 or NH4NO3, respectively. Nitrogen loss via NH3 volatilization was 4-to 7-fold greater at 20% FC t han that at 80% FC. This study suggests potential acceleration of NH 3 volatilization from soils under drought. Therefore , optimal water management is critical to mitigate NH 3 volatilization from agricultural soils.


INTRODUCTION
The global ammonia (NH 3 ) volatilization from Nitrogen (N) fertilizers routinely used for crop production is estimated to be 11 million t N per year (Tg N/yr), which is about one seventh of the nitrogen (N) fertilizers used (FAO/IFA, 2001). While NH 3 is not regarded as a greenhouse gas, it can affect global warming considerably by being oxidized to nitrous oxide (N 2 O) or reacting with nitric and sulfuric acids in the atmosphere to form secondary aerosols (Barthelmie and Pryor, 1998;Turner et al., 2010). Dentener and Crutzen (1994) estimated 3 Mg N/yr of the globally emitted NH 3 can be oxidized by OH radicals and NO 2 (Finlayson-Pitts and Pitts, 2000). A fraction of the oxidized NH 3 is transformed to Nitrous Oxide (N 2 O), a very powerful greenhouse gas and this process accounts for up to 5% of the global N 2 O emission (Ferm, 1998). After deposition on land surface and water bodies, NH 3 acts as a secondary source of N 2 O which contributes to ozone depletion (Martikainen, 1985;Mosier et al., 1998). As a potent greenhouse gas, N 2 O is approximately 310-fold powerful than CO 2 in trapping heat in the atmosphere (Finlayson-Pitts and Pitts, 2000;IPCC, 1996). Thus, NH 3 emitted from agricultural soils contributes to global warming.

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Global warming causes not only climate change but also soil drought. Temperature increase of 1% (0.32°C) can increase evapotranspiration by 15 mm (Goyal, 2004). Evapotranspiration, in turn, can worsen soil drought if the irrigation rate is not adjusted to compensate for the increased loss of moisture. There is evidence that the progression of anthropogenic global warming is accompanied by increased risk of drought, since rising temperature increases the rate of drying (Dai et al., 2004). An increase in either temperature or decrease in soil moisture content significantly accelerates NH 3 volatilization from fertilized soils. Soil drought from 80 to 20% Field Capacity (FC) of soil moisture enhances NH 3 volatilization rate by 113 fold (Liu et al., 2007a); while high temperature from 20 to 29°C increases the NH 3 volatilization rate by 10 fold (Liu et al., 2007b). These increases suggest that NH 3 volatilization induced by soil drought impacts global warming and vice versa.
Ammonia volatilization entails a series of steps including a chemical reaction. The spontaneity of each of these steps is determined by the Gibbs free energy according to the second law of thermodynamics. However, no published literature is available for describing the effects of changes in Gibbs free energy on either NH 3 volatilization or on the acceleration of NH 3 volatilization by soil drought. Therefore, understanding of the relationship between Gibbs free energy and NH 3 volatilization is too limited to enable effective control of NH 3 volatilization from crop production. Our experimental evidence presented on a mechanism whereby soil drought enhances NH 3 volatilization controlled by Gibbs free energy. This mechanism provides clues to alleviate NH 3 volatilization from fertilized by improved water management, which minimizes the negative Gibbs free energy of the deprotonation of 4 NH + ions and in turn minimize NH 3 volatilization.
The objectives of this study were to: (1) quantify Gibbs free energy for NH 3 volatilization in two different soils and (2) assess Gibbs free energy in each step of NH 3 volatilization from the fertilized soils.

Measurement of NH 3 Volatilization
Three hundred g dry soil of either Krome Gravelly Loam (KGL) collected from Florida or Warden Silt Loam (WSL) from Washington State was placed in an incubation bottle (500 mL in size). The surface area of the soil in the bottle was ~ 60 cm 2 . The soil water content was adjusted to either 20 or 80% of Field Capacity (FC). The percentage of water in the KGL and WSL soils (v/m based on oven dry soils) was 8.4 and 6.7% for 20% FC and 33.4 and 26.0% for 80% FC, respectively. One ml of N solution consisting of an aqueous solution of either (NH 4 ) 2 SO 4 or NH 4 NO 3 was uniformly applied on the soil surface with a micropipette. The amount of N applied was 45 mg N per bottle, equivalent to 75 kg ha −1 based on the surface area. A treatment with only deionized water (no N) was included as a control to measure the NH 3 volatilization from the soils. Thus, this experiment consisted of two soils × three N sources (two N fertilizers plus blank control) × two water regimes × three replications which required 36 incubation bottles. Each incubation bottle was placed in a sealed plastic storage bag (23×30 cm) and maintained at 20°C in an incubator (Precision Incubator, 6DM, THELCO ® High Performance Incubators, Precision, Ottawa, Canada). Ammonia volatilization was measured on 1, 3, 7, 14 and 28 days after incubation. A sponge with trapping solution was sampled at each sampling period and a new sponge (with trapping solution) was placed to trap emitted NH 3 for the subsequent sampling period (Liu et al., 2007a;2007b).

Calculation of Gibbs Free Energy
Because NH 3 volatilization involves three steps: Step 1, dissolution of the applied ammonium fertilizer compound; step 2, deprotonation of 4 NH + ions; and step 3, transformation of aqueous NH 3 into gaseous NH 3 . In the soil solution, the relative stability of the chemical system, i.e., the tendency of the system to react or change is determined by the Gibbs free energy or simply free energy, quantity derived from the relationships between heat and work in thermodynamics of NH 3 volatilization. Systems change toward minimum free energy. Equation (1) is one form of Gibbs fundamental equation: where, G = the Gibbs free energy (kJ); H = the enthalpy (kJ); T = the absolute temperature (°K); and S = the entropy (kJ °K −1 ). For each of the above three chemical steps in soil solution, Equation (1) can be rewritten in Equation (2): where, ∆G = the difference in the Gibbs free energy between products (G products ) and reactants (G reactants ) in the chemical system. ∆H and ∆S = the differences in enthalpy and entropy between the products and reactants, Science Publications AJES respectively. Equation (2) predicts the spontaneity of the steps involved in NH 3 volatilization: • ∆G<0 natural tendency for process to occur (spontaneous) • ∆G = 0 neither the forward nor the reverse process prevails (equilibrium) • ∆G>0 no natural tendency for process to occur (nonspontaneous) The changes in Gibbs free energy of formation for Equation (3a or 3b, 4 and 5) were calculated based on Equation (1). The parameters of enthalpy (H) and entropy (S) for each of the products and reactants in Equation (3a or 3b, 4 and 5) can be found from Haynes (2013):

Calculation of Equilibrium Constant of Deprotonation of Ammonium Ions IN Soil Solutions
The spontaneous equilibrium constant of the deprotonation after fertilization was calculated according to Equation (6) (Majer et al., 2004): where, ∆G = the reaction's Gibbs free energy which is the difference between the sum of the standard Gibbs energies of the products (i.e., NH 3 and H 2 O) and reactants (i.e., 4 NH + and OH − ) in the reaction of deprotonation Equation (4). R is the gas constant (8.314 J K −1 ) and T the absolute temperature (°K).

Data Analysis
The Statistical Analysis System (SAS) version 9.1 was used for the statistical analysis. The data were analyzed by one-way ANOVA. Results were considered significant at p<0.05. After running the SAS program, the critical ranges (LSD 2,0.05 ) of Duncan's Multiple Range Test were used to identify significantly different means (Hubbard, 2012).

Results
Ammonia volatilization entails the above three steps, which occur after the application of N fertilizer and culminate in gaseous N loss via NH 3 emission from agricultural soils. These three steps are as follows: (1) dissolution of the applied fertilizer in soil solution (Equation 3a or 3b); (2) deprotonation of ammonium ( 4 NH + ) ions from the dissolved fertilizer (Equation 4) and (3) transformation of aqueous NH 3 into gaseous NH 3 (Equation 5). Each of the three steps influences the rate and extent of NH 3 volatilization from agricultural soils. Figure 1 presents the differences in Gibbs Free Energy for each of the three steps of NH 3 emission from the two tested soils fertilized with ammonium sulfate and ammonium nitrate at two soil moisture regimes. Table 1 compared the equilibrium constants of depratonation of ammnium sulfate and ammonium nitrate applied to the two tested soils with 20% or 80% FC. Figure 2 shows the cumulative loss of N via NH 3 volatilization.

Change in Gibbs Free Energy of the Three Steps
The change in Gibbs free energy (∆G) of step 1 (dissolution of the applied 4 NH N + − fertilizer) was negative and small for both ammonium sources, step 2 (deprotonation of 4 NH + ions) was negative and large, but step 3 (transformation of aqueous NH 3 into gaseous NH 3 ) was positive and small. Therefore, steps 1 and 2 are exergonic (proceed spontaneously); but step 3 is endergonic (does not proceed spontaneously). Accordingly, steps 1 and 2 favor NH 3 volatilization, but reduce N use efficiency and worsen air quality.
Step 2 is always the dominant driver of NH 3 volatilization. Therefore, any agricultural practice intended to reduce NH 3 volatilization should focus on step 2. Thus, either the avoidance of deprotonation of 4 NH + ions or the improvement of protonation of NH 3 in the soil solution may greatly decrease the rate of step 3 and reduce net NH 3 volatilization. For example, based on Equation 2 and 5, the value of -∆G becomes 90% less if the pH value of the soil solution is changed from 7 to 6. Therefore, a combinational application of N fertilizers and physiologically acidic fertilizers, such as potassium sulfate, may greatly reduce NH 3 volatilization and enhance N use efficiency (Dalling and Green, 1943;Hershey, 1991).  At 20% FC (soil drought), -∆G was always 4-to 5fold greater than that at 80% FC. This suggests that soil drought accelerated the deprotonation of 4 NH + ions and enhanced NH 3 emission from both N sources. Deprotonation of 4 NH + ions is closely associated with the soil water content. In potato production systems in Florida, intermittent soil droughts occur during the growing season. Global warming may increase the frequency of occurrence of soil droughts (Sheffield and Wood, 2008). Thus, optimal water management is critical for mitigation of NH 3 volatilization (Liu et al., 2007a).

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Both (NH 4 ) 2 SO 4 and NH 4 NO 3 were applied at the same N rate (75 kg ha −1 ). However, (NH 4 ) 2 SO 4 dissociates into two fold greater 4 NH + ions than that from NH 4 NO 3 (Fig. 1). Accordingly, the absolute value of -∆G for (NH 4 ) 2 SO 4 was twice that for NH 4 NO 3 . The magnitude of -∆G was dependent on 4 NH + concentration.

Equilibrium Constant (K) of Deprotonation of Ammonium Ions in Soil Solutions
The equilibrium constant (K) of deprotonation of 4 NH + ions were much greater than 1 across both N sources, soils, as well as soil water regimes ( Table 1). This indicates that the deprotonation of 4 NH + ions has a high potential across all variables in this study. The K values of (NH 4 ) 2 SO 4 at 20% FC were 3,060-and 3637-fold greater than those at 80% FC, in the KGL and WSL soils, respectively. The corresponding values for NH 4 NO 3 were 52-and 61-fold greater.
These data indicate that the concentration of 4 NH + ions in the soil solution is a major factor in determining the extent of NH 3 volatilization. Thus, the magnitude of -∆G increased with increasing concentrations of 4 NH + ions. Both N fertilizers were applied to attain a similar N rate (75 kg ha −1 ) but the concentration of 4 NH + ions in the soil solution of the soil which received (NH 4 ) 2 SO 4 was twofold greater than that for NH 4 NO 3 . Thus at 20% FC, the K values of (NH 4 ) 2 SO 4 was 425-and 498-fold greater than those of NH 4 NO 3 in the KGL and WSL soils, respectively. The corresponding values for 80% FC were 7-and 8-fold. These results imply that dilution of 4 NH + ions in the soil solution decreases the absolute value of-∆G and can, therefore, reduce NH 3 volatilization.

Nitrogen Loss Via Ammonia Volatilization
Cumulative NH 3 volatilization (28 d) at 20% FC was 4-to 5-fold greater than that at 80% FC in both soils across both N sources (Fig. 2). Soil droughts, which occur frequently during the growing season, in some production regions, increase the concentrations of 4 NH + ions in soil solutions.
At the USDA Experiment Site in Paterson, WA in the Colombia River Basin, the alfalfa reference evapotranspiration reached 14 mm d −1 particularly in July and August, 2010 (Fig. 3). Under such evapotranspiration rates, soil drought develops rapidly if water management is inadequate or inappropriate. Global warming is expected to increase evapotranspiration, reduce soil moisture (Zavaleta et al., 2003) and exacerbate the incidence of elevated soil droughts. Soil properties can also significantly affect the NH 3 emissions. For example, soil acidity (pH<6.3) can reverse the deprotonation of 4 NH + ions, i.e., protonate NH 3 molecules to form 4 NH + ions. This mechanism can be exploited by applying physiological acidic fertilizers with 4 NH + -N fertilizer together to reduce NH 3 volatilization from fertilized soils. Furthermore, soil particles can bind considerable amounts of 4 NH + ions by chemical, physicochemical, or physical adsorption. In particular, a high cation exchange capacity that favors increasing 4 NH + adsorption can prevent NH 3 volatilization (Freney et al., 1981). For example, even though the -∆G for ammonium fertilizers were greater in the WSL soil than in the KGL soil, net NH 3 volatilization from the former soil was less than that from the latter soil (Fig. 2). The pH and electrical conductivity were both lower for the WSL soil than those for the KGL soil (Liu et al., 2007b).

CONCLUSION
Ammonia volatilization from agricultural soils following the application of N fertilizers involves three steps: Fertilizer dissolution; deprotonation of 4 NH + ions; and phase transformation of NH 3 . Among the three steps, the first two are spontaneous, i.e., -∆G, while the third step is not spontaneous, i.e., +∆G.
Step 2, deprotonation of 4 NH + ions dominates the overall process of fertilizer dissolution leading to NH 3 emission.
Step 2 contributes over 95% of -∆G of the overall process which culminates in NH 3 emission. The management strategies aimed to reduce NH 3 volatilization should focus on delay or restriction of deprotonation of 4 NH + ions. Soil drought greatly accelerates deprotonation and, hence, increased N loss via NH 3 emission. The value of -∆G in the soil at 20% FC was 5-fold greater than that at 80% FC. Soil drought greatly accelerates NH 3 volatilization from fertilized soils. Potential increases in temperature, a possible outcome of continued global warming, increases evapotranspiration and in turn enhances soil drought that can lead to increased NH 3 volatilization from fertilized soils. Therefore, optimal soil water management is an important strategy to reduce N loss via NH 3 volatilization from cropping systems.

ACKNOWLEDGMENT
This study was funded by USDA-ARS, specific cooperation agreement with the University of Florida. Drs. Paul W. Chun, Waldemar Klassen and Kati W. Migliaccio at the University of Florida have given constructive suggestions and comments to improve the manuscript.