Drivers of hot spots and hot moments of denitrification in agricultural systems

Authors: WEITZMAN, JULIE - Cary Institute Of Economic Studies; GROFFMAN, PETER - New York City University; Adler, Paul; Dell, Curtis; JOHNSON, FRANK - University Of Missouri; Lerch, Robert; Strickland, Timothy

Submitted to: Journal of Geophysical Research-Biogeosciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/21/2021
Publication Date: 6/28/2021
Citation: Weitzman, J.N., Groffman, P.M., Adler, P.R., Dell, C.J., Johnson, F.E., Lerch, R.N., Strickland, T.C. 2021. Drivers of hot spots and hot moments of denitrification in agricultural systems. Journal of Geophysical Research-Biogeosciences. 126(7). Article e2020JG006234. https://doi.org/10.1029/2020JG006234.
DOI: https://doi.org/10.1029/2020JG006234

Interpretive Summary: The form of nitrogen lost from agricultural fields determines the type of effect on the environment, whether nitrous oxide as a greenhouse gas or nitrate on water quality, with dinitrogen as a neutral component. We measured the form of nitrogen across a diversity of agricultural landscapes in the USDA Long-Term Agroecosystem Research Network. We found that most nitrogen gas fluxes were as dinitrogen gas rather than nitrous oxide and that this was a significant sink for excess agricultural nitrogen lost from fields. These results suggest that denitrification as dinitrogen is a significant sink for excess agricultural nitrogen at multiple scales, which may inform agricultural management decisions in the future.

Technical Abstract: Denitrification, the most poorly understood process in the nitrogen (N) cycle, is of great interest as it can significantly reduce pools of reactive nitrogen (N), as well as 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 denitirification frequently account for a high percentage of N gas flux activity. Our study utilized sites within the USDA Long Term Agro-Ecosystem Research (LTAR) Network to understand the importance of potential drivers of such hotspots and hot moments of denitrification in agricultural systems. We quantified in situ denitrification rates in intact soil cores from three LTAR sites – Central Mississippi River Basin, MO, Upper Chesapeake Bay, PA, and Gulf Atlantic Coastal Plain, GA – by directly measuring dinitrogen (N2) 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: soil confining layers (MO), landscape-scale topography (PA), and ecosystem-scale climate (e.g. drying-rewetting events; GA). Results show evidence for denitrification hotspots at the MO site where hard clays act as confining features at depth. At the PA site, wetter, low topographic positions released a higher proportion of N2:N2O compared to more well-drained 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.