Grazing and the Dynamics of Nutrient and Energy Regulated Microbial Processes in the Serengeti grasslands

Microbial biomasses and N and P mineralization potentials were determined on soils collected from short-, mid- and tallgrass communities along a gradient of increasing rainfall and decreasing grazing intensity in the Serengeti National Park, Tanzania. Total N, organic C, pH and clay content were highest on shortgrass sites which experienced the highest grazing intensities, and declined successively on soils from mid- and tallgrass communities. Microbial biomass ranged across sites from an average of 47.1 ± 6.3 to 315.8 ± 8.4 mg C 100 g dry soil-1, and was directly related to grazing intensity and organic carbon, and inversely related to soil C/N ratio. Net N mineralized over the 20 d incubation period ranged from 1.0 +± 1.3 to 63.4 ± 4.6 ig N g dry soil-' 20 d-1, was a function of both total N and soil microbial biomass, and varied inversely with soil C/N ratio. Periods of net N immobilization lowered the mean mineralization rates in soils from midgrass sites. Net N mineralized ranged from 0.09 ± 0.10% of total soil N at Seronera, the site with the lowest grazing intensity, to 2.24 + 0.17% of total soil N at Southeast Kopjes, a site with high grazing intensity. In order to study microbial growth and the concomitant N and P mineralization responses to varying substrate quality, soil samples were subjected to a laboratory experiment involving untreated soils, and three C amendment treatments which included either glucose, wildebeest dung or plant litter. Dung-amended soils showed higher net N mineralization levels in comparison to litter-amended soils, despite insignificant differences in total respiration. The more efficient utilization of dung compared with plant litter, appeared to be related to more balanced C/nutrient ratios of dung and the energy or nutrient limited site-specific characteristics of microbial growth. On sites of low nutrient availability, the increased flow of P through dung may be important in maintaining accelerated nutrient cycling rates in the grazing food web.