How well can APSIM simulate nitrogen uptake and nitrogen fixation of legume crops?

Abstract

It is important to quantify the nitrogen (N) uptake and dinitrogen (N2) fixation of legumes and estimate the N contribution that these crops make to subsequent crops for sustainable agricultural production. The growth and development of legumes and their impact on soil N fertility can be simulated by the Agricultural Production Systems Simulator (APSIM). However, the model performance has not been evaluated in simulating the dynamic processes of N accumulation and N2 fixation. The parameterised model was tested for the simulation of N uptake and N2 fixation in above-ground biomass of four crop legumes (lupin, chickpea, field pea and peanut). The simulations varied in location, cultivar, sowing date, climate, soil type, water regime (irrigated or dryland) and starting soil N and applied fertiliser N in tropical, subtropical, semiarid and Mediterranean environments across Australia. In general, the absolute amount of N uptake and N2 fixation in above-ground biomass (unit: kgha−1) were reasonably well simulated, with 92% of the variation in observed N accumulation in above-ground biomass and 84% in N2 fixation being explained by APSIM. The model was also able to simulate the responses of N2 fixation by chickpea and peanut to differences in soil mineral N status. However, the simulations of N2 fixation efficiency (NFE, calculated as fixed N2 per unit above-ground dry matter (DM; unit: gNkg−1 DM) were much less accurate, especially for lupin. Sensitivity analysis showed that improving the definition of the model parameter of crop N2 fixing capacity (the potential to fix atmospheric N2 per unit above-ground DM; unit: gNg−1 DM) would improve the simulations of NFE. We therefore propose that to successfully simulate the absolute amount of N accumulation and N2 fixation, the above-ground biomass as the major driving factor must first be simulated well, and future work should focus on accurately determining the parameter of crop N2 fixing capacity through optimisation of N2 fixation data obtained from field or controlled experiments to fine-tune the simulations of the relative efficiency of N2 fixation.