Soil Biogeochemistry, Aridity and Plant Adaptation Responses in Southern Africa Savannas

Savannas cover about 20 0f the Earth's land area and 50 0f Africa, crossing a wide range of climatic conditions. It remains unclear how the relative importance of water and nutrient limitations varies with the mean climatic conditions in the savanna ecosystems. This dissertation used the Kalahari Transect (KT) in southern Africa as a representative savanna ecosystem and combined multiple tools to understand the variations in soil biogeochemistry and subsequent responses of plants on this immense rainfall gradient. The large spatial scale field manipulation experiment shows that nitrogen (N) may not be a limiting factor in tropical savanna ecosystems. The fertilization experiment demonstrates that even at the wet end of the transect, water remains the principal factor limiting grass productivity. Natural abundance of foliar  15 N and  13 C reveals different water and N use for C 3 and C 4 plants. The consistently higher foliar  15 N for C 3 plants suggests that C 4 plants may be superior competitors for N. The foliar  13 C data may indicate that the C 3 plants have an advantage over C 4 plants when competing for water resources. The differences in water and nitrogen use between C 3 and C 4 plants likely collectively contribute to the tree-grass coexistence in savannas. The soils along the KT rainfall gradient show clear differences in belowground nutrient content and  15 N distributions between wet and dry seasons. The results also show how the formation of "fertility islands" is not necessarily coupled with belowground processes in that the distribution of soil nutrients at the surface does not match belowground patterns. The modeling framework largely agrees with the field observations and shows that, at daily time scales, there are distinct dynamics for soil moisture, decomposition and nitrogen 3 mineralization between soil plots located under tree canopies and in open canopy areas. The modeling study shows that in savanna ecosystems, water availability determines the patterns and rates of nutrient cycling at large scales, while at the local scales, vegetation patchiness plays an important role in nutrient cycling. Lastly, this dissertation discusses the potential combination of isotopic and remote sensing techniques, which holds promise to allow spatially continuous estimates of isotopic compositions.

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