Management zone-based estimation of positive and negative nitrous oxide flux in organic corn fields

Accurate estimation of field-scale nitrous oxide (N₂O) fluxes is hindered by their considerable variability and the fact that soils can be both sources and sinks for N₂O. This is particularly challenging for organic systems that have complex rotations and inputs. This study used digital soil mapping and survey datasets to explore spatial controls of N₂O “hot moments” induced by precipitation with strategic sampling designed to identify covariates that influence N₂O emission patterns. Soil N₂O fluxes after rain events were measured within three management zones (MZs, “High,” “Medium,” “Low”) delineated by crop productivity, soil fertility, and hydrological features in eight organic fields during the 2018 and 2019 corn (Zea mays L.) growing seasons. Hot moments typically occurring 1 d after rain events included both positive and negative N₂O fluxes. The MZ-based design identified regions with different patterns in positive and negative flux, with hotspots for both being co-located with areas of poorer drainage and higher soil fertility. Covariates that best explained hot moments included corn growth stage, soil moisture, slope, texture, and soil organic matter. Negative fluxes were large enough to offset positive fluxes so that averaged net N₂O fluxes were only significantly different between the “High” and “Low” MZs. Had negative fluxes been omitted, averaged N₂O fluxes would have increased estimates by 37%. Processes that lead to N₂O consumptions must be better quantified to improve the estimation of management-associated net N₂O flux. Use of strategic sampling can efficiently capture needed information, but spatial and temporal weighting is needed to scale up results.