Nitrous oxide (N2O) is a potent greenhouse gas with 298x more warming potential than CO2, but a poorly constrained global budget. More than half of N2O emissions come from soil microbial metabolism, but soil microbes can also remove N2O from the atmosphere by consuming it. N2O consumption, the microbial reduction of N2O to N2, is intriguing because it can significantly reduce net N2O emissions, but it remains understudied. In this study, we sought to better understand what stimulates N2O consumption by measuring it in eight diverse soils collected from Colorado, New Mexico, and Minnesota. We hypothesized that the primary control on N2O consumption is electron donor supply. To address this hypothesis, we incubated soils in the laboratory amended with either aqueous non-fermentable organic carbon (OC; experimental) or deionized water (control) to discern how excess reductants impact gross N2O uptake. All soils were enriched with 99 atom percent excess 15N2O, held at 60% soil saturation, and incubated for 48 hours. We then used 15N2O isotope pool dilution to disentangle gross N2O production and consumption in response to the amendments. We also measured a suite of soil properties before and after incubations to ascertain how the soil environment interacted with the amendments.
Results/Conclusions
Gross N2O consumption increased in six out of eight OC-amended soils, illustrating that the treatment stimulates N2O consumption. However, in three soils the amendment increased consumption and decreased net N2O emissions (hereafter Increased Consumption Decreased Emissions; ICDE), which is ideal from a greenhouse gas management perspective. These ICDE soils had low soil OC content compared to the other five soils (p < 0.0001), and following amendment, ICDE soils showed significant increases in microbial biomass (p = 0.0048), depletion of soil NO3- (p = 0.0123), and increases in transcription of nosZ, the N2O-reducing gene (p = 0.0163) compared to the other five OC-amended soils. We hypothesize that these three OC-amended soils overcame a C-limitation threshold necessary for ICDE. We posit that OC can stimulate microbial growth and deplete the soil NO3- pool, necessitating increased transcription of nosZ and a shift from NO3- to N2O as an alternative reductant. These findings support our hypothesis, electron donor supply is a primary control of N2O consumption, and we predict that all soils could overcome their C-limitation threshold and attain ICDE if given enough OC. This has implications for managing ecosystems, in particular agroecosystems, to reduce N2O emissions.