Can Isotopic Maps Reveal Soil N2O Hotspots?

Soil N₂O emissions are laborious and difficult to quantify and upscale to larger areas as they vary greatly in space and time. Nitrogen isotopes are gaining increasing attention in research as a potential tool for improving landscape-scale assessments of N₂O emissions. This is because biochemical processes in the nitrogen (N) cycle discriminate against the heavier N isotope, ¹⁵N, resulting in distinct isotopic signatures in the product and residual substrate. Maps of the spatial distribution of isotopes, isoscapes, offer a promising approach for identifying spatial variability in N processes. However, integrating soil N isotopes into ecosystem models requires a better understanding of the drivers behind their spatial variation.

Moist depressions in crop fields are known to be N₂O hotspots and thus represent key sites for exploring soil ¹⁵N patterns associated with N₂O emissions. This study focused on the spatial patterns of soil N isotopes across two rolling fields in Zealand, Denmark. A total of 148 topsoil samples (0-10 cm) were collected along multiple topographic transects and analyzed for soil N and carbon content and isotopic composition, together with soil texture (at selected locations). Correlations between parameters were assessed using Spearman’s Rank Correlation Coefficient.

Emitted N₂O is expected to be depleted in ¹⁵N relative to its source substrate. Based on this, we hypothesized that soil N in N₂O hotspots (moist depressions) would show higher ¹⁵N enrichment compared to adjacent soils, due to greater losses of ¹⁵N depleted N. Contrary to this hypothesis, the results showed a significant positive correlation between soil δ¹⁵N and elevation, with the lowest δ¹⁵N values observed in the depressions. This suggests that processes other than N₂O emissions play an important role in shaping the isotopic patterns. The study revealed substantial spatial variability in soil δ¹⁵N (3.8-9.8‰) underscoring the importance of sample location in determining isotope fractionation patterns and highlighting the need for further investigation to refine the application of isoscapes.