Opportunities for mitigating nitrous oxide emissions in subtropical cereal and fiber cropping systems: A simulation study

Abstract

23% of global agricultural land are situated in the subtropics. Nitrous oxide (N2O) emissions were estimated to be higher under subtropical than under temperate climates. So mitigation of N2O emissions from subtropical farming systems can make an important contribution to reducing global warming. Accordingly, in this study we explored long-term N2O emissions and possible mitigation options for representative subtropical cropping systems (e.g., summer versus winter crops, inclusion of a legume in the rotation) and management practices (nitrogen fertilizer, irrigation) by calculating scenarios with the agricultural systems model APSIM. The model was tested against high temporal frequency data from experiments conducted on an oxisol and a vertisol in subtropical Australia, which comprised a number of fertilization and irrigation treatments. The threshold of water filled pore space above which denitrification starts was calibrated on a subset of the data while the rest of the large number of parameters controlling the carbon and nitrogen cycles were kept to default values. The validity of the model was confirmed with 11 validation data sets for yields of four different crops (R2=0.92) and 16 validation data sets for seasonal N2O emissions during crop and fallow periods (R2=0.77). While these results show that the model performs well in sub-tropical environments, this modeling skill might not translate to other environments and the model would benefit from wider testing. In the scenario analyses, long-term average N2O emissions from wheat, cotton, maize and sorghum were predicted to vary between 0.2 and 6.1kgNha−1yr−1 and showed large interannual variability of N2O emissions. This highlights the risk that results from short-term experiments may not be representative for the long-term behavior of these agro-ecosystems, and thus the value simulation studies add to experiments. The scenario analysis revealed that long-term average yields and N2O emissions increased in response to the same management practices (e.g., increase in nitrogen rate), leading to a trade-off between maximizing yield and minimizing N2O emissions. When crop yields were limited due to water stress either by low seasonal rainfall or by lack of irrigation, average N2O emissions increased. Given the annual variability in climate and soil nitrogen stocks, mitigating N2O emissions without compromizing in yield is not a simple task but requires an optimal nitrogen management considering other limiting factors such as water supply.