Multivariate regulation of soil CO2 and N2O pulse emissions from agricultural soils

Climate and land-use models project increasing occurrence of high temperature and water deficit in both agricultural production systems and terrestrial ecosystems. Episodic soil wetting and subsequent drying may increase the occurrence and magnitude of pulsed biogeochemical activity, affecting carbon (C) and nitrogen (N) cycles and influencing greenhouse gas (GHG) emissions. In this study, we provide the first data to explore the responses of carbon dioxide (CO2) and nitrous oxide (N2O) fluxes to (i) temperature, (ii) soil water content as percent water holding capacity (%WHC), (iii) substrate availability throughout, and (iv) multiple soil drying and rewetting (DW) events. Each of these factors and their interactions exerted effects on GHG emissions over a range of four (CO2) and six (N2O) orders of magnitude. Maximal CO2 and N2O fluxes were observed in environments combining intermediate %WHC, elevated temperature, and sufficient substrate availability. Amendments of C and N and their interactions significantly affected CO2 and N2O fluxes and altered their temperature sensitivities (Q(10)) over successive DW cycles. C amendments significantly enhanced CO2 flux, reduced N2O flux, and decreased the Q(10) of both. N amendments had no effect on CO2 flux and increased N2O flux, while significantly depressing the Q(10) for CO2, and having no effect on the Q(10) for N2O. The dynamics across DW cycles could be attributed to changes in soil microbial communities as the different responses to wetting events in specific group of microorganisms, to the altered substrate availabilities, or to both. The complex interactions among parameters influencing trace gas fluxes should be incorporated into next generation earth system models to improve estimation of GHG emissions.