Using isotope pool dilution to understand how organic carbon additions affect N2O consumption in diverse soils

Nitrous oxide (N2O) is a formidable greenhouse gas with a warming potential similar to 300x greater than CO2. However, its emissions to the atmosphere have gone largely unchecked because the microbial and environmental controls governing N2O emissions have proven difficult to manage. The microbial process N2O consumption is the only know biotic pathway to remove N2O from soil pores and therefore reduce N2O emissions. Consequently, manipulating soils to increase N2O consumption by organic carbon (OC) additions has steadily gained interest. However, the response of N2O emissions to different OC additions are inconsistent, and it is unclear if lower N2O emissions are due to increased consumption, decreased production, or both. Simplified and systematic studies are needed to evaluate the efficacy of different OC additions on N2O consumption. We aimed to manipulate N2O consumption by amending soils with OC compounds (succinate, acetate, propionate) more directly available to denitrifiers. We hypothesized that N2O consumption is OC-limited and predicted these denitrifier-targeted additions would lead to enhanced N2O consumption and increased nosZ gene abundance. We incubated diverse soils in the laboratory and performed a (15) N2O isotope pool dilution assay to disentangle microbial N2O emissions from consumption using laser-based spectroscopy. We found that amending soils with OC increased gross N2O consumption in six of eight soils tested. Furthermore, three of eight soils showed Increased N2O Consumption and Decreased N2O Emissions (ICDE), a phenomenon we introduce in this study as an N2O management ideal. All three ICDE soils had low soil OC content, suggesting ICDE is a response to relaxed C-limitation wherein C additions promote soil anoxia, consequently stimulating the reduction of N2O via denitrification. We suggest, generally, OC additions to low OC soils will reduce N2O emissions via ICDE. Future studies should prioritize methodical assessment of different, specific, OC-additions to determine which additions show ICDE in different soils.