Drivers of Hot Spots and Hot Moments of Denitrification in Agricultural Systems

Denitrification, the anaerobic microbial conversion of nitrate (NO₃^-) and nitrite (NO₂^-) to the gases nitric oxide (NO), nitrous oxide (N₂O), and dinitrogen (N₂), is the most poorly understood process in the nitrogen (N) cycle. Yet, it is of great interest, as it can significantly reduce pools of reactive N or lead to diverse pollution problems. However, it is notoriously difficult to quantify. A particular challenge is the fact that small areas (hot spots) and brief periods (hot moments) frequently account for a high percentage of N gas flux activity. While N₂O fluxes from agricultural systems have long been measured, few studies have directly quantified denitrification. Globally, agriculture is the largest anthropogenic source of N₂O. Thus, directly quantifying denitrification rates in agricultural systems, where the opportunity for large N fluxes exists due to the high rate of industrial N application, is especially critical. Our study utilized sites within the Long Term Agro-Ecosystem Research (LTAR) Network to understand the importance of potential drivers of hotspots and hot moments of denitrification in agricultural systems. We took intact soil cores from three LTARs - Central Mississippi River Basin in Columbia, MO, Upper Chesapeake Bay in Leck Kill, PA, and Gulf Atlantic Coastal Plain in Tifton, GA - in order to quantify in situ denitrification rates by directly measuring N₂ and N₂O production in the cores using the Nitrogen-Free Air Recirculation Method (N-FARM). Each site provided us the opportunity to study a different potential driver of denitrification: soil confining layers (MO), landscape-scale topography (PA), and ecosystem-scale climate (e.g. drying-rewetting events; GA). Our results show evidence for denitrification hotspots at the MO site where hard clays act as confining features at depth. At the PA site we also saw a clear pattern of denitrification, with the wetter, low topographic positions releasing a higher proportion of N₂:N₂O compared to the more well-drained mid topographic positions. Similarly, denitrification hotspots (at depth) and hot moments (drying-rewetting) were apparent at the GA site. These results suggest that denitrification is a significant sink for excess agricultural N at multiple scales, which may inform agricultural management decisions in the future.