Nitrous oxide emission hotspots in temporarily flooded cropland depressions: year-round measurements and regional estimation

Temporarily flooded depressions within cropland have been identified as substantial hotspots of nitrous oxide (N₂O) emission, releasing up to 80 times more N₂O than surrounding field areas during the flooded period. Despite their significant contribution, the temporal dynamics of N₂O emissions from these depressions and their impact on regional annual N₂O budgets remain inadequately quantified. The primary drivers of these high emissions are poorly understood, limiting the accuracy of regional estimates and the development of effective mitigation strategies.

To address this knowledge gap, we established two elevation transects in two Danish croplands, each comprising five positions (0, 1, 2, 3, 4; with three replicate plots per position) along a slope gradient from depressions to the uphill areas. Biweekly in-situ N₂O flux measurements were conducted at each plot over a year (March 2020 to March 2021) using static chambers. Concurrently, soil samples were collected for laboratory analysis of physicochemical properties along with each field measurement, and soil water content and temperature were monitored at 30-minute intervals in the depression areas. Additionally, daily photographs of each transect were captured using installed cameras, and daily remote sensing images at 3-m resolution (PlanetScope) were utilized to evaluate relative wetness for each plot. Based on the field data, daily photos, and relative wetness, the study year was divided into three distinct periods:  flooded period (with water above the soil surface), flood recover period (characterized by high soil water content typically after flooding), and drained period (with comparable soil moisture between depression and uphill areas).

Our results reveal significant spatial and temporal variability in N₂O fluxes along the transects. Positions within the depressions exhibited significantly higher annual mean N₂O fluxes, ranging from 93.4 to 204.6 µg N₂O m⁻² h⁻¹, compared to 20.6 to 58.2 µg N₂O m⁻² h⁻¹ in the transition areas and 12.1 to 26.4 µg N₂O m⁻² h⁻¹ in the uphill areas. Temporally, flood recover period in depressions showed the highest N₂O fluxes compared to any other periods, whereas the uphill areas maintained consistent emissions throughout the year. Annual cumulative N₂O emissions from positions within the depressions were estimated to be 0.64 to 1.5 g N₂O m⁻², significantly higher than the emissions of 0.16 to 0.39 N₂O m⁻² from transition areas and 0.09 to 0.27 g N₂O m⁻² from uphill areas. Regionally, although depressions cover less than 1% of the total cultivated area, they contribute approximately 10% to the total annual N₂O emissions. Our analysis identified soil moisture and temperature as key drivers for the spatial and temporal variabilities in N₂O emissions along the transects. These findings highlight the importance of incorporating high-emitting depressions into local and regional N₂O inventories to improve the accuracy of agricultural greenhouse gas estimates and inform the development of effective mitigation strategies.