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.