Laboratory experiments have shown oxygen (O2) availability influences the production of nitrous oxide (N2O – a powerful greenhouse gas) and dinitrogen (N2 – an inert gas) during denitrification, however less is known about how field soil O2 availability shapes the production of these gases. We used an intact soil core incubation method to measure N2 and N2O emissions from soils collected at different topographic positions, which experience different O2 availability, in a tropical forest in Puerto Rico. We hypothesized that soils collected at low topographic positions, with lower O2 availability, would produce relatively more N2 and less N2O than cores from more O2-rich upland positions. We sampled soil cores (0-10 cm) across two soil O2 gradients in the Luquillo Experimental Forest: 1) a “macrotopography” gradient, where 4 replicate transects were selected across a ridge-slope-valley continuum, and 2) a “microtopography” gradient, where 16 plots (1m2) that varied in their measured soil moisture were selected across a slope. Soils were collected in plastic sleeves and shipped the same day to the Cary Institute of Ecosystem Studies, where they were analyzed using an N2-free, gas flow incubation system designed to directly measure soil emissions, under variable helium-oxygen headspace concentrations (0%, 5%, 10%, and 20% O2).
At macrotopography sites, we found higher N2 and N2O fluxes from low soil O2 valleys (p<0.0001 and p=0.0002, respectively), than from ridges and slopes. Soils from the microtopography sites, where we have a years worth of continuous O2 measurements, had higher N2 fluxes (p=0.04) from locations with <12% O2. Manipulating O2 in the headspace of the incubation jars had no effect on N2 emissions from soils at either site, but soils from the microtopography sites had higher N2O emissions (p<0.0001) at intermediate (5 and 10%) O2, potentially reflective of a range where nitrification and denitrification can co-occur. At both sites N2 emissions were positively correlated with field soil moisture (p=0.004). The difference between field and laboratory O2 influences on N gas production has implications for how we estimate emission rates across the landscape. Whether this discrepancy is one that results from microbial priming to field O2 conditions, poor macro-aggregate formation in wet soils, or differences in O2 sensitivity between nitrification in macro-aggregates vs. denitrification in micro-aggregates, will require further research.