N₂O emissions are laborious to quantify and upscale to larger areas, as they vary greatly in space and time. Isotopes can be an important tool to improve landscape scale quantification of emissions, because biochemical processes in the nitrogen (N) cycle result in distinct isotopic signatures in the product and residual substrate due to isotopic fractionation. Maps of the spatial distribution of isotopes, isoscapes, can therefore help identifying spatial variations in the N-cycling. However, to incorporate soil N isotopes in ecosystem models, the drivers of the spatial variations need to be better understood. Moist depressions in crop fields are known to be N₂O hotspots. Hence such sites offer potential for identifying spatial patterns in soil ¹⁵N associated with N₂O emissions. The first objective of this project was to test the following hypothesis:

• Isotopic fractionation during denitrification will cause N₂ or N₂O to be depleted in ¹⁵N relative to the initial nitrate (NO₃^-). Over time, residual NO₃^- could enrich the bulk soil in ¹⁵N in N₂O hotspots relative to ambient drained soils.

This was tested on approx. 150 soil samples from top 10 cm. depth across two undulating crop fields in Zealand, Denmark. Notable variations in soil ¹⁵N (δ¹⁵N 3.8-9.8‰) was observed and there was a significant correlation between elevation in the terrain and soil δ¹⁵N in both study sites (p<0.01* and p<0.001*). Contrary to the hypothesis, the lowest ¹⁵N values were observed in depressions. Soil texture and C% indicate downhill movement of fine soil particles due to erosion, which should enhance denitrification rates downhill. However, this process might at the same time mask the fractionation effects.

As isoscapes are receiving growing attention, the current results are essential because sample location do have an impact on the apparent extent of isotope fractionation. The next step of the project is to map N isotopes in a landscape with varying land use intensities and soil texture. Together with N₂O flux measurements and the isotopic signatures of leached NO₃^- and NH₄⁺, correlations between soil biogeochemical processes and the isotopic signatures of bulk soil N will be studied to reveal the drivers of the spatial variations of soil δ¹⁵N.

*Tested with Spearmann’s Rank Correlation Coefficient.