A major feature of southern African savannas is the co-dominance of grasses, and trees / shrubs with the C4 and C3 photosynthetic pathways respectively. Several hypotheses try to explain this feature. Fires and grazing are thought to limit tree growth, allowing grasses to co-exist with trees. Niche differentiation regarding water uptake could also explain the shared dominance, with grasses taking water from the surface and trees from deeper soil layers. Nutrients are not commonly used to explain tree-grass interactions. In this study, data is presented to support the hypothesis that nutrient dynamics affects C3 and C4 distribution. Foliar P and \delta15N, root profiles and abundances of C3 and C4 plants, soil \delta13C, and gross N mineralization and nitrification rates in soils under C3 and C4 plants were analyzed along a precipitation gradient in southern Africa. The \delta15N and foliar P of both plant types respond differently to precipitation. Plants with the C3 metabolism are more enriched in 15N with increasing aridity, and this enrichment is associated with a higher abundance of C4 plants. The delta15N of C4 plants is not related to precipitation. Root profiles and soil \delta13C indicate that C4 roots dominate in mass and abundance from the surface to >60 cm depth, under C3 and under C4 plants. Gross N mineralization rates are significantly higher under C3 plants. Our results suggest that competition for N may be a factor in determining the abundance of C3 and C4 plants in southern African savannas, with grasses being stronger competitors. Does Nutrient Dynamics Determine C3-C4 Plant Abundance in Southern African Ecosystems?. Keywords: 1040 Isotopic composition/chemistry, 1615 Biogeochemical processes (4805), 4815 Ecosystems, structure and dynamics, 4845 Nutrients and nutrient cycling, 4870 Stable isotopes.