Ecosystem-scale spatial heterogeneity of stable isotopes of soil nitrogen in African savannas

Soil N-15 is a natural tracer of nitrogen (N) cycling. Its spatial distribution is a good indicator of processes that are critical to N cycling and of their controlling factors integrated both in time and space. The spatial distribution of soil delta N-15 and its underlying drivers at sub-kilometer scales are rarely investigated. This study utilizes two sites (dry vs. wet) from a megatransect in southern Africa encompassing locations with similar soil substrate but different rainfall and vegetation, to explore the effects of soil moisture and vegetation distribution on ecosystem-scale patterns of soil delta N-15. A 300-m long transect was set up at each site and surface soil samples were randomly collected for analyses of delta N-15, %N and nitrate content. At each soil sampling location the presence of grasses, woody plants, Acacia species (potential N fixer) as well as soil moisture levels were recorded. A spatial pattern of soil delta N-15 existed at the dry site, but not at the wet site. Woody cover distribution determined the soil delta N-15 spatial pattern at ecosystem-scale; however, the two Acacia species did not contribute to the spatial pattern of soil delta N-15. Grass cover was negatively correlated with soil delta N-15 at both sites owing to the lower foliar delta N-15 values of grasses. Soil moisture did not play a role in the spatial pattern of soil delta N-15 at either site. These results suggest that vegetation distribution, directly, and water availability, indirectly, affect the spatial patterns of soil delta N-15 through their effects on woody plant and grass distributions.