Interpreting Apsim and DSSAT Maize Model Responses to Carbon Dioxide, Temperature, Water, and Nitrogen Under Degraded Soil Conditions.

It is important, in climate impact assessment, to understand how different crop models respond to CO₂, temperature, water (rainfall), and nitrogen (hereafter, CTWN). Regional teams in West Africa, East Africa, South Africa, Southeast Africa, and South India obtained farm survey yield data for maize from households at multiple sites in their regions, along with farmer management, historical weather, soils, and local cultivar calibrations. Representative fields were selected for evaluating DSSAT and APSIM maize models for their sensitivity to CTWN. Simulated response to N fertilization from 0 to 180 kg N ha^-1 revealed that stable or inert soil C pools for the models had to be set correctly to mimic the farmer-observed yields under low to zero N fertilization, and that the yields at high N fertilization represent the genetic potential of the cultivar. The need for correct N response is very important because the teams used N fertilization as a favored management intervention. The two maize models had very modest similar responses to CO₂, giving less response at low N. Increasing temperature reduced yield for both models at most sites, with DSSAT CERES-Maize being more sensitive to high temperature. Response to temperature was influenced by model differences in temperature parameterization for life cycle, assimilation, grain growth rate, as well as soil N mineralization. Response to rainfall was less than expected for West and East Africa, because N was so limiting that the low LAI created low transpiration demand. In Southern Africa, where rainfall was lower, the sensitivity to rainfall was stronger, with greater yield reductions for DSSAT than for APSIM. The CTWN exercise was valuable for understanding differential model sensitivity to climatic factors, and guiding model calibration for response to N fertilization for degraded soil conditions.