Evidence of Gibbs Free Energy of Drought Accelerating Nitrogen Loss Via Ammonia Volatilization From Fertilized Soils.

Ammonia volatilization (AV) from agricultural soils is one of both increasing environmental and economic concerns worldwide. To better understand the thermodynamic equilibrium of AV and control AVs from fertilized soils, this study was carried out by using four soils in a laboratory incubation system at mean temperature: 20^oC and two water regimes: 20% and 80% field capacity (FC). The soils included Biscayne Marl Soil (BMS) and Krome Gravelly Loam (KGL) from Florida and Quincy Fine Sand (QFS) and Walden Silt Loam (WSL) from Washington State. They are often used for potato production and fertilized with ammonium nitrate. Based on concentration of ammonium ions in the soil solution and the definition of Gibbs free energy (G), G of AVs was calculated for each tested soil under a specific water regime:

G = H – TS

Where H is the enthalpy (SI unit: joule); T is the temperature (SI unit: kelvin); and S is the entropy (SI unit: joule per kelvin). The results show that the values of G of AVs were -612.5, -4005.0, -159.1, and -493.2 J mol^-1 for BMS, KGL, QFS, and WSL at 20% FC, respectively. The corresponding values were -4.2, -8.4, -7.1, and -0.6 for the soils at 80% FC, respectively. The difference of the G values ranged from 22.6- to 774-fold between the two water regimes. These results suggest that dry (20% FC) condition of the tested soils drive nitrogen loss via AVs hundred-fold more spontaneously than the appropriately moisture (80% FC) condition.