Abstract
Availabilities of nitrogen (N) and phosphorus (P) have a strong influence on plant growth and the species composition of savannas, but it is not clear how these availabilities depend on factors such as fire, N2-fixation, and activities of wild herbivores and cattle. We quantified soil N and P availabilities in various ways (extractable pools, mineralization, resin adsorption) along vegetation gradients within a recently abandoned cattle ranch and a former game reserve in Tanzania (both areas now part of the Saadani National Park). We also assessed annual N and P balances to evaluate how long-term availabilities of N and P are affected by large herbivores, symbiotic N2-fixation, and fire. The results show that cattle ranching led to a spatial re-distribution of nutrients, with the local accumulation of P being stronger and more persistent than that of N. In the former game reserve, intensively grazed patches of short grass tended to have elevated soil N and P availabilities; however, because quantities of nutrients removed through grazing exceeded returns in dung and urine, the nutrient balances of these patches were negative. In dense Acacia stands, N2-fixation increased N availability and caused a net annual N input. Fire was the major cause for nutrient losses from tallgrass savanna, and estimated N inputs from the atmosphere and symbiotic N2-fixation were insufficient to compensate for these losses. Our results call into question the common assumption that N budgets in annually burned savanna are balanced; rather, these ecosystems are a mosaic of patches with both N enrichment and impoverishment, which vary according to the vegetation type.
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Acknowledgments
We greatly acknowledge Thomas Kuster for his help with the combustion experiments, as well as Stephanie Halsdorf and Christoph Rohrer for collecting fresh dung of almost all herbivore species in Saadani, and for measuring the N and P concentrations in it. We thank Sabine Güsewell for statistical advice, and Michael Scherer-Lorenzen for helpful comments on the manuscript. We thank Prof. S. L. S. Maganga, Markus Schneider-Mmary, and the authorities of Saadani National Park for their logistical support in Tanzania, and Benjamin Donald, John Williams and Hamis Williams for their assistance in the field. This study was financed by the Swiss National Science Foundation grant No. 2-77502-04.
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P.C., H.O.V., and P.E. designed the study and wrote the paper; P.C. performed the research and analyzed the data.
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Cech, P.G., Olde Venterink, H. & Edwards, P.J. N and P Cycling in Tanzanian Humid Savanna: Influence of Herbivores, Fire, and N2-Fixation. Ecosystems 13, 1079–1096 (2010). https://doi.org/10.1007/s10021-010-9375-9
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DOI: https://doi.org/10.1007/s10021-010-9375-9