Disentangling gross NO production and consumption in soil.

The difficulty of measuring gross NO production and consumption in soil impedes our ability to predict NO dynamics across the soil-atmosphere interface. Our study aimed to disentangle these processes by comparing measurements from gas-flow soil core (GFSC) and NO pool dilution (NOPD) methods. GFSC directly measures soil NO and N fluxes, with their sum as the gross NO production, whereas NOPD involves addition of NO 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 NO production and consumption measured by NOPD were only 10% and 6%, respectively, of those measured by GFSC. However, NOPD remains the only method that can be used under field conditions to measure atmospheric NO uptake in soil. We propose to use different terminologies for the gross NO fluxes that these two methods quantified. For NOPD, we suggest using 'gross NO emission and uptake', which encompass gas exchange within the NO-labelled, soil air-filled pores. For GFSC, 'gross NO production and consumption' can be used, which includes both NO emitted into the soil air-filled pores and NO directly consumed, forming N, in soil anaerobic microsites.