Disentangling gross N₂O production and consumption in soil

The difficulty of measuring gross N₂O production and consumption in soil impedes our ability to predict N₂O dynamics across the soil-atmosphere interface. Our study aimed to disentangle these processes by comparing measurements from gas-flow soil core (GFSC) and $^{15}$N₂O pool dilution ($^{15}$N₂OPD) methods. GFSC directly measures soil N₂O and N₂ fluxes, with their sum as the gross N₂O production, whereas $^{15}$N₂OPD involves addition of $^{15}$N₂O into a chamber headspace and measuring its isotopic dilution over time. Measurements were conducted on intact soil cores from grassland, cropland, beech and pine forests. Across sites, gross N₂O production and consumption measured by $^{15}$N₂OPD were only 10% and 6%, respectively, of those measured by GFSC. However, $^{15}$N₂OPD remains the only method that can be used under field conditions to measure atmospheric N₂O uptake in soil. We propose to use different terminologies for the gross N₂O fluxes that these two methods quantified. For $^{15}$N₂OPD, we suggest using ‘gross N₂O emission and uptake’, which encompass gas exchange within the $^{15}$N₂O-labelled, soil air-filled pores. For GFSC, ‘gross N₂O production and consumption’ can be used, which includes both N₂O emitted into the soil air-filled pores and N₂O directly consumed, forming N₂, in soil anaerobic microsites.