A 2.5-Year Eddy Covariance Study of Nitrous Oxide Fluxes in Winter Barley, Sugar Beet and Winter Wheat: Responses to Environmental and Management Factors

Nitrous oxide (N₂O) fluxes exhibit a high temporal and spatial variability, making their accurate quantification challenging. Recently, the eddy covariance (EC) technique has become available for N₂O measurements, providing non-intrusive spatially integrated flux data at high temporal resolution (half-hourly fluxes from 10 Hz raw data), which can give valuable insights in the underlying drivers when combined with ancillary measurements.

Since April 2022, we measure N₂O fluxes by EC using a fast response closed-path N₂O analyzer (Los Gatos Research, San Jose, USA) in an agricultural field in central Germany (51.49° N, 9.93° E; Reinshof, DE-Rns). The crop sequence during the study period was winter barley - white mustard (as cover crop) - sugar beet - winter wheat. Additionally, net ecosystem exchange (NEE) of carbon dioxide, ancillary meteorological and soil variables as well as plant performance were measured at the EC tower and crop management was recorded.

The N₂O flux measurements over the 2.5 years showed a large temporal variability across seasonal conditions and management activities. Ten particularly large N₂O emission peaks with fluxes higher than 120 and up to 950 μg N₂O-N m^-2 h^-1 that last around three days were detected. Those peaks were induced by nitrogen (N) fertilization, rainfall, freeze-thawing or tillage events in the field. From April 2022 to September 2024, we observed cumulative N₂O emissions of 3.57 kg N₂O-N ha^-1 and calculated an emission factor of 0.89% of applied N. We estimated that 23.4% of the cumulative N₂O emissions were induced by three rainfall events and one freeze-thawing event, and were thus not directly associated with fertilizer application, while 32.2% occurred within three weeks after N fertilizations. Beside the N₂O emission peaks, we observed periods with N₂O uptake in 2023 and 2024 when plants showed their highest leaf area index.

This data set, providing several years of N₂O fluxes at half-hourly resolution, can help us to better constraint N₂O budgets of crop cultivation. Improving our understanding about the interactions of management practices and environmental conditions that lead to higher N₂O emissions can help developing strategies for mitigation of N₂O emissions.