Assessing the effects of soybean–maize cropping on soil N biogeochemistry and N2O emissions in the Brazilian Amazon using the DNDC model
The rapid expansion and intensification of crop agriculture from single cropping of soybeans to double cropping of soybeans with maize in the Brazilian Amazon is now dramatically increasing the use of mineral nitrogen (N) fertilizer. This increased N application has potentially important environmental consequences in the form of emissions of nitrous oxide (N2O), a potent greenhouse gas, and increased leaching of N to streams and rivers. Evaluations of the environmental effects, magnitude and controls on these important processes, both in the Amazon and more generally on the highly weathered soils on which most global agricultural expansion and intensification now take place, are very limited. Mathematical models, such as the DNDC (DeNitrification DeComposition) model, are powerful tools to examine the potential impacts of agricultural management and climate change for current and future agriculture. DNDC can simulate the processes responsible for production, consumption and transport of carbon and N in the soil. During the last 20 years DNDC has been modified and adapted for specific regions and questions, but it has not yet been used to simulate responses to the intensification of tropical and Amazon cropland agriculture.
We show results of our calibration and validation of DNDC for simulating N2O emissions from soils under the soy-maize cropping system that now covers more than 30 thousand square kilometers in the southern Brazilian Amazon state of Mato Grosso. We tested DNDC against field N2O fluxes from representative soy-only cropped fields (with no applied N fertilizer) and soy-maize cropped (that received 70 kg N ha-1 during the maize cropping phase) at Tanguro Ranch, a farm in eastern Mato Grosso. After modifying the model's soil hydrological parameters, DNDC successfully simulated soil moisture and soil temperature as well as annual N2O fluxes and crop productivity. Sensitivity analysis showed that maize crop productivity was highly responsive to N fertilization and reached a plateau at application of 90 kg N ha-1, which was in line with literature and practice in the Amazon. N2O emissions increased along with maize productivity, but beyond this yield threshold, the model suggested a sharp increase in N2O emissions per crop grain produced. Our next steps will be to validate the model against field-measured N2O fluxes over a range of a N application levels and to use information on double-cropping area, climate and N fertilizer use to predict N2O fluxes at the large regional scale.