Simulation of Nitrogen Demand and Uptake in the Maize Model MaizSim.

Nitrogen (N) uptake as nitrate is largely regulated by plants. For example, low nitrate concentrations near plant roots will turn on genes that control membrane permeability and metabolic processes in order to increase nitrate uptake in the presence of low or negative gradients. As a result, uptake of nutrients such as nitrogen appears to be largely active, i.e., regulated by the plant. Complex and mechanistic approaches to modeling uptake processes have been proposed over the years. Few of these models have been incorporated into mechanistic models for crop growth where nitrogen uptake and demand are dependent on root growth and carbon assimilation by the plant. The goal of this study was to evaluate the nitrogen demand and uptake components in the maize model MAIZSIM and resultant growth and development of the crop. Nitrogen movement in the soil is calculated using a 2D finite element model of soil processes, 2DSOIL.  In MAIZSIM, nitrogen demand is a function of N content in the biomass. Sufficiency or deficiency of N is quantified using N-dilution curves (nitrogen content of cumulative biomass) determined from experiments. We compared two methods of simulating N uptake. The first is a convective-diffusive model of N uptake from soil. This approach calculates influx of N toward the root based on mass flux of water to the roots, concentration gradients between the bulk soil and root surface and the radius of the root. The second approach utilizes a Michaelis-Menton type kinetics equation where uptake is driven primarily by diffusion. Nitrogen uptake was linked to demand by comparing cumulative actual N uptake to cumulative demand adjusted for current plant growth and nitrogen stress. This method allows adjustment of uptake parameters as the plant develops and N needs change. The simulation results were compared to data from outdoor growth chambers and field experiments. The use of N dilution curves provided accurate representation of nitrogen contents in the plants as compared to field measurements under sufficient and excess N.  The largest error was in simulation of nitrogen uptake when the plant was small. Linking N uptake to growth using N dilution curves provided realistic estimates of N contents in plant tissue as compared to measured values.