A SIMULATION-MODEL OF THE WHEAT CROP IN RESPONSE TO WATER AND NITROGEN SUPPLY .1. MODEL CONSTRUCTION

A simulation model of a fallow-wheat system is described. A total of 2 7 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 previousl y published model that now includes crop response to nitrogen and the alternative fallow management techniques of stubble mulching and reduc ed tillage. The integration interval remains at one day.The soil nitro gen submodel is primarily driven by the decomposition of organic matte r between four state variables: surface crop residues, fresh organic m atter, microbial biomass and stable humus. Mineralization and/or immob ilization is controlled by the corresponding C:N ratios. Loss of NO3-N through denitrification is driven by daily CO2 evolution and total NO 3-. Crop N uptake is achieved by passive and active mechanisms utilizi ng both NO3- and NH4-. Under the active mechanism, NO3-N or NH4-N in t he soil solution may be extracted above the lower limit of water avail ability. The remaining mineral nitrogen is redistributed within the to tal soil water. Within the crop, N is distributed between roots, above -ground and grain components according to demand, but not below predef ined lower limits. Crop growth is determined as a function of transpir ation efficiency (TE) adjusted for temperature extremes and nitrogen d eficiency. Radiation-use efficiency (RUE) varies throughout the growin g period as a function of TE, potential transpiration and intercepted photosynthetically active radiation. Leaf area index (LAI) is determin ed as the product of above-ground bio-mass and leaf area ratio, modifi ed to account for N deficiency. The phenology submodel provides the fr amework for the partitioning of growth to roots, above-ground biomass, dead biomass and grain. Copyright (C) 1996 Elsevier Science Ltd