Nitrogen cycling in southern African soils and plants along rainfall and land-use gradients: A stable isotope study

Understanding nitrogen (N) cycling under different climate and land-use regimes is crucial to predict ecosystem responses to global change because N may limit ecosystem carbon (C) uptake in many systems. This study analyzes N cycling in the soil-plant system along precipitation and land-use gradients in southern Africa. Stable isotopes and elemental composition of plants and soils, acetylene reduction assays, isotope dilution methods and a compartment model were the main tools utilized. There was no evidence of N₂ fixation by woody legumes in arid areas, and soil crusts fixed small amounts of N₂ in the Kalahari during a wet year. Soils and plants were more 15N enriched with decreasing mean annual precipitation, but the correlations were stronger during a wet year, indicating that interannual variability of precipitation drives N cycling in arid areas. Increased mineralization during wet years in the model explained the observed soil 15N enrichment. In addition, Kalahari plants with the C₃ and C ₄ pathways responded differently to precipitation. Nitrogen content and δ15N (natural abundance of 15N) of C₃ plants (trees and shrubs) were more correlated to mean annual precipitation than those of C₄ plants (grasses), indicating different N utilization by trees and grasses. The model suggests that different soil N sources or timing of N uptake by trees and grasses could cause distinct isotopic signatures for these plant types. Observed foliar C, N, and phosphorus varied for different plant types, during wet and dry years, and along a precipitation gradient, suggesting that there are multiple limiting factors for different plants, spatial and temporal scales in these ecosystems. Overgrazing and cultivation modified soil and plant δ15N, but the magnitude and direction of the changes were affected by spatial and probably temporal variability of precipitation. Fires did not clearly affect soil δ15N, probably because of the opposite effects of gaseous N losses and N₂ fixation on δ15N. This study points to the importance of precipitation variability at different temporal and spatial scales on driving N cycling processes and perhaps promoting the coexistence of more than one plant type, trees and grasses, in savannas.