Drivers Of Hot Spots And Hot Moments Of Denitrification In Agricultural Systems

Denitrification, the most poorly understood process in the nitrogen (N) cycle, is of great interest as it can significantly reduce pools of reactive N and act as a source of the potent greenhouse gas nitrous oxide (N2O). A particular study challenge is that small areas (hot spots) and brief periods (hot moments) of denitrification frequently account for a high percentage of N gas flux activity. Our study utilized sites within the U.S. Department of Agriculture Long-Term Agroecosystem Research (LTAR) Network to understand the importance of potential drivers of such hot spots and hot moments of denitrification in agricultural systems. We quantified in situ denitrification rates in intact soil cores from three LTAR sites - Upper Chesapeake Bay (UCB), PA, Central Mississippi River Basin (CMRB), MO, and Gulf Atlantic Coastal Plain (GACP), GA - by directly measuring dinitrogen (N-2) and N2O production via the Nitrogen-Free Air Recirculation Method (N-FARM). Each site provided an opportunity to study a different potential driver of denitrification: landscape-scale topography (UCB), soil confining layers (CMRB), and ecosystem-scale climate (e.g., drying-rewetting events; GACP). At the UCB site, wetter, low topographic positions released a higher proportion of N-2:N2O, suggesting that N removal does not result in increased greenhouse emissions. Results at the CMRB site show evidence that confining features, like hard clays, could drive denitrification hot spots. Similarly, such features at the GACP site could drive denitrification hot spots (at depth) and hot moments (drying-rewetting). These results suggest that denitrification is a significant sink for excess agricultural N at multiple scales, which may inform management decisions in the future.