Fate and Transport of Nitrogen and Carbon Dioxide in Soils Based on a Coupled Nitrogen-Carbon Cycling Model Using the HP1 Code.

In order to predict fate and transport of nitrogen and carbon dioxide diffusion in the air phase as a result of decomposition of organic matters in soils, we described a coupled nitrogen and carbon cycling model based on the LEACHM code (Hutson, 2005) using the PHREEQC program. Two types of principal organic matter were defined: a first cycling pool for litter and a slow cycling pool for stabilized humus. Separate litter pools were used for organic matters having a different decay rate and carbon-to-nitrogen (C/N) ratio such as plant residue and manure. Decay processes from the organic-C to biomass-C, humus-C, and carbon dioxide were described with a first-order kinetic. The biomass-C was recycled into the decomposable litter pool, forming the litter-decomposer complex. Decompositions from the organic-N to biomass-N and humus-N were related to the carbon cycle using the C/N ratios of the organic matter, biomass and humus, respectively. Mineralization of ammonium from the organic matter and immobilization of ammonium to the organic matter were determined based on the available N in the litter pool and the N demand for the formation of biomass and humus. Nitrogen transformations of ammonification, nitrification, and denitrification, were also described with first-order chain reactions. Passive and active root uptakes of ammonium and nitrate were given as a sink term. The nitrogen and carbon cycling model were linked with water flow, root water uptake, solute transport, and gas diffusion in HYDRUS-1D using the HP1 code. Various nitrogen and carbon dioxide transport scenarios as a result of application of organic matters to a variable-saturated soil under a nonisothermal condition were demonstrated.