Using isotope pool dilution to understand how organic carbon additions affect N₂ O consumption in diverse soils

Nitrous oxide (N₂ O) is a formidable greenhouse gas with a warming potential ~300× greater than CO₂ . However, its emissions to the atmosphere have gone largely unchecked because the microbial and environmental controls governing N₂ O emissions have proven difficult to manage. The microbial process N₂ O consumption is the only know biotic pathway to remove N₂ O from soil pores and therefore reduce N₂ O emissions. Consequently, manipulating soils to increase N₂ O consumption by organic carbon (OC) additions has steadily gained interest. However, the response of N₂ O emissions to different OC additions are inconsistent, and it is unclear if lower N₂ 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 N₂ O consumption. We aimed to manipulate N₂ O consumption by amending soils with OC compounds (succinate, acetate, propionate) more directly available to denitrifiers. We hypothesized that N₂ O consumption is OC-limited and predicted these denitrifier-targeted additions would lead to enhanced N₂ O consumption and increased nosZ gene abundance. We incubated diverse soils in the laboratory and performed a ¹⁵ N₂ O isotope pool dilution assay to disentangle microbial N₂ O emissions from consumption using laser-based spectroscopy. We found that amending soils with OC increased gross N₂ O consumption in six of eight soils tested. Furthermore, three of eight soils showed Increased N₂ O Consumption and Decreased N₂ O Emissions (ICDE), a phenomenon we introduce in this study as an N₂ 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 N₂ O via denitrification. We suggest, generally, OC additions to low OC soils will reduce N₂ O emissions via ICDE. Future studies should prioritize methodical assessment of different, specific, OC-additions to determine which additions show ICDE in different soils.