N and P Cycling in Tanzanian Humid Savanna: Influence of Herbivores, Fire, and N₂-Fixation

• Published in: Ecosystems (New York) 2010-11, Vol.13 (7), p.1079-1096
• Main Authors: Cech, Patrick G, Olde Venterink, Harry, Edwards, Peter J
• Format: Article
• Language: English
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Bioavailability; Biological and medical sciences; Biomedical and Life Sciences; Cattle; Dung; Ecology; Environmental Management; fires; Flowers & plants; Forest and land fires; Fundamental and applied biological sciences. Psychology; General aspects; Geoecology/Natural Processes; Grasses; Grassland fires; Herbivores; Humidity; Hydrology/Water Resources; Legumes; Life Sciences; Mineralization; N-fixation; N:P stoichiometry; National parks; Nitrogen; Nitrogen enrichment; Nitrogen fixation; nutrient balances; Nutrient loss; Phosphorus; Phytopathology. Animal pests. Plant and forest protection; Plant growth; Plant Sciences; Savanna soils; Savanna woodlands; Savannahs; Savannas; Species composition; Synecology; Trees; Vegetation; Weather damages. Fires; Woodlands; Zoology
• ISSN: 1432-9840; 1435-0629
• DOI: 10.1007/s10021-010-9375-9

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.