Seasonal timing of extreme drought regulates N2O fluxes in a semiarid grassland

Abstract

Terrestrial ecosystems are important sources of nitrous oxide (N2O), a powerful greenhouse gas which can be strongly impacted by increasing droughts in association with climate change. However, detailed information on whether and how drought timing regulates N2O fluxes is still lacking. Here, we conducted a 3-year field experiment on a semiarid grassland in which extreme drought was imposed in either early-, mid-, or late-growing seasons repeatedly from 2014 to 2016. We found that early drought affected N2O emission with high interannual variability (increased, decreased and unchanged N2O emission in 2014, 2015, and 2016, respectively), coincident with changes in inorganic nitrogen (SIN), dissolve organic carbon (DOC), microbial biomass carbon (MBC), and soil functional genes (bacterial amoA, nirK, nirS, and nosZ). However, middle drought consistently suppressed N2O emissions due to simultaneous decreases in MBC, DOC and the abundances of archaeal amoA, nirK, and narG genes, causing the largest reduction in N2O emissions across the three years. In contrast, late drought had little effect on N2O fluxes, even though DOC and SIN decreased and the abundance of nirK, nirS, and nosZ increased. As a result, soil organic C and mineral N availability and functional gene abundances were not always robust factors for predicting N2O emissions under droughts across all treatments, except for abundance of AOA and nosZ. Our results highlight the vital role of seasonal timing in regulating the response of N2O emissions to extreme droughts.