Using isotope pool dilution to understand how organic carbon additions affect N O consumption in diverse soils.

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