A simulation model of the wheat crop in response to water and nitrogen supply. I. Model construction

A simulation model of a fallow-wheat system is described. A total of 27 state variables are grouped in six submodels, viz. soil water, soil carbon, soil nitrogen, crop biomass, crop nitrogen and phenology. Two further submodels, environment and management, provide input data and controls for model operation. The model is an extension of a previously published model that now includes crop response to nitrogen and the alternative fallow management techniques of stubble mulching and reduced tillage. The integration interval remains at one day. The soil nitrogen submodel is primarily driven by the decomposition of organic matter between four state variables: surface crop residues, fresh organic matter, microbial biomass and stable humus. Mineralization and/or immobilization is controlled by the corresponding C:N ratios. Loss of NO3-N through denitrification is driven by daily CO2 evolution and total NO3-. Crop N uptake is achieved by passive and active mechanisms utilizing both NO3- and NH4-. Under the active mechanism, NO3-N or NH4-N in the soil solution may be extracted above the lower limit of water availability. The remaining mineral nitrogen is redistributed within the total soil water. Within the crop, N is distributed between roots, above-ground and grain components according to demand, but not below predefined lower limits. Crop growth is determined as a function of transpiration efficiency (TE) adjusted for temperature extremes and nitrogen deficiency. Radiation-use efficiency (RUE) varies throughout the growing period as a function of TE, potential transpiration and intercepted photosynthetically active radiation. Leaf area index (LAI) is determined as the product of above-ground biomass and leaf area ratio, modified to account for N deficiency. The phenology submodel provides the framework for the partitioning of growth to roots, above-ground biomass, dead biomass and grain.