Simulation of Nitrogen Dynamics in Wheat Cropping Systems

Abstract

Some 8 million tonnes of fertilizer N are applied annually to the world's wheat crop. The efficiency with which this is used is very variable but in general poor. Much of this variability in fertilizer efficiency and hence crop response to N is due to differences in climate. Soil physical and chemical properties, crop residues, crop variety, and management practices also influence crop response to fertilizer and efficiency. A tool which is able to reliably predict crop response to N and determine the causes and magnitude of poor fertilizer recovery could greatly assist crop and fertilizer management. Various approaches to describing the response to N and - their ability to capture the effects noted above are reviewed in the thesis. The shortcomings of traditional response research are highlighted and dynamic computer simulation modelling is advanced as an appropriate methodology. A review of modelling methodologies and models of N dynamics in cropping systems is presented. The review indicated that one comprehensive model with a management or user focus and which is able to accommodate the effects of the factors listed above is lacking. This thesis describes the CERES-WHEAT model and the development of an N component for it. This model utilizes a daily time step and is designed to be able to simulate the growth, yield, and response to N of a wheat crop grown anywhere in the world. The model requires daily climatic data as well as data describing soil water storage characteristics and data pertaining to soil factors which influence the supply of N to the crop. The model utilizes several genotype specific coefficients to characterize a cultivar's response to environment. Management data defining the time of planting, plant population, fertilizer rate, source, and time of application are also required. The CERES-WHEAT model describes the processes of evapotranspiration, soil water balance, crop ontogeny as affected by temperature, photoperiod, and vernalization, and the growth of leaves, stems, roots, ears, and grain. The nitrogen component of the model adds to this the description of mineralization and/or immobilization of N associated with the decay of crop residues, mineralization of "stable" organic matter, movement of nitrate in the profile, nitrification, denitrification, uptake of N by the plant, plant N effects on growth processes, and redistribution of N within the plant associated with grain filling. The derivation of the functions for each of these processes and their relationship to published information is described.