N2O emissions from emission hotspots in agricultural soils – linking crop root growth and distribution with soil moisture and nitrate dynamics

Agricultural soils are a major source of the potent greenhouse gas nitrous oxide (N₂O). However, these emissions have high spatial variability and are highly heterogeneous in their distribution in agricultural landscapes. In Eastern Denmark, N₂O “hotspots” or disproportionately high emission zones, have been observed in the vicinity of glacial depressions. These depressions form a topographical slope, where the base of the slope is typically inundated for 1-3 months over the winter and spring seasons. Preliminary work showed that the highest N₂O emissions have been observed at the base of the slope, along the perimeter of the ponding zone of these depressions. In the present study we now aim to identify the mechanistic drivers of these N₂O hotspots.

N₂O emissions were measured weekly at four positions along a gradient from upslope to the centre of a glacial depression over a winter to spring wheat growth season, at a farm in Eastern Denmark. The potential driving factors were  measured weekly along the slope: soil moisture at depths of 10 cm, 20 cm and 100 cm; soil solution nitrates and DOC at these same depths; and root growth using photographs at different depths using minirhizotrons to a depth of 100 cm. Additionally the groundwater level was monitored using a well and pressure logger. To underpin the results of this core site, further N₂O measurements and soil samples were taken along the slope of three additional glacial depressions nearby with comparable topography and soil properties .

The N₂O emissions were found to be highest at the base of the slope, around the perimeter of the depression. We were able to demonstrate that root growth is directly linked to the moisture regime and thus the overall fate of nitrate and N₂O emissions.

Given the locally high N₂O emissions of these hot spots, this study provides a lens into an important source of N₂O emission heterogeneity in the Danish agricultural landscape.