An approach for calibrating laser-based N2 O isotopic analyzers for soil biogeochemistry research

Bacterial denitrification drives elevated N2O emissions in arid southern California drylands

Soils are the largest source of atmospheric nitrous oxide (N2O), a powerful greenhouse gas. Dry soils rarely harbor anoxic conditions to favor denitrification, the predominant N2O-producing process, yet, among the largest N2O emissions have been measured after wetting summer-dry desert soils, raising the question: Can denitrifiers endure extreme drought and produce N2O immediately after rainfall? Using isotopic and molecular approaches in a California desert, we found that denitrifiers produced N2O within 15 minutes of wetting dry soils (site preference = 12.8 ± 3.92 per mil, δ15Nbulk = 18.6 ± 11.1 per mil). Consistent with this finding, we detected nitrate-reducing transcripts in dry soils and found that inhibiting microbial activity decreased N2O emissions by 59%. Our results suggest that despite extreme environmental conditions-months without precipitation, soil temperatures of ≥40°C, and gravimetric soil water content of <1%-bacterial denitrifiers can account for most of the N2O emitted when dry soils are wetted.

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.