Abstract: Although stable nitrogen isotopes (δ¹⁵N) can provide a powerful integrative tracer of localized nutrient cycling across heterogeneous savannas, the relative influence of top-down versus bottom-up drivers in shaping these spatial patterns remains largely unresolved. Here, we leveraged long-term monitoring and extensive spatial sampling of 253 sites across a tropical savanna in Kruger National Park, South Africa. By applying a Generalized Additive Model (GAM) and variance partitioning, we evaluated the distinct contributions of top-down disturbances (fire and herbivory) and bottom-up drivers (vegetation structure, soil properties, topography, and climatic variables). Our model explained 60.9% of total deviance and revealed high spatial heterogeneity in soil δ¹⁵N (2.99–9.07‰). Variance partitioning demonstrated that bottom-up drivers overwhelmingly governed spatial variation, driven predominantly by two factors: first, the non-linear effects of high N-fixing tree cover, which dilute soil δ¹⁵N with depleted atmospheric N; and second, increasing soil C:N ratios, which potentially slow mineralization and consequently limit fractionating N losses. Conversely, top-down drivers, while traditionally central to savanna ecology, played a comparatively minor role, primarily limited to the local enriching effects of extended fire-free intervals. Furthermore, the patterns predicted by the GAM model highlight localized hotspots of ¹⁵N enrichment alongside broader spatial trends in soil δ¹⁵N that contrast sharply with total soil N. Ultimately, our study demonstrates that spatial variation in savanna soil δ¹⁵N is primarily dictated by localized bottom-up drivers rather than top-down disturbances, suggesting these local factors act as strong proxies for the underlying biogeochemical processes shaping savanna N cycling.

Link to the paper: Zhou, Y., Coetsee, C. 2026. Top-Down and Bottom-Up Drivers of Soil δ¹⁵N Spatial Patterns in a Tropical Savanna. Journal of Geophysical Research: Biogeosciences, 131(9), e2026JG10008. https://doi.org/10.1029/2026JG010008