Modeling Root Water and Nitrogen Uptake Based on a Coupled Nitrogen-Carbon Cycling Model Using the HP1 Code.

Passive and active root inorganic-nitrogen uptake is an important process for carbon and nitrogen cycle in a soil. We described a coupled nitrogen and carbon cycling model based on the LEACHM code (Hutson, 2005) in the PHREEQC program. We implemented a root passive and active inorganic-nitrogen uptake model in the nitrogen-carbon model. The nitrogen-carbon model were linked with water flow, root water uptake, solute transport, and gas diffusion in HYDRUS-1D using the HP1 code. Passive inorganic-nitrogen uptake is described in PHREEQC using the root water uptake rate from HYDRUS. Active inorganic-nitrogen uptake is described with the Michaelis-Menten kinetics in PHREEQC. 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. We investigated the effect of root passive and active inorganic-nitrogen uptake on ammonium and nitrogen transport.