Landscape Patterns in Soil Microbial Processes in the Serengeti National Park, Tanzania

Abstract

The Serengeti ecosystem contains some of Africa's most geobotanically diverse landscapes and supports some of the highest primary and secondary production on Earth. In an attempt to characterize landscape patterns in soil microbial processes across the Serengeti, 17 study sites from nine landscape regions were sampled for soil physical/chemical characteristics and laboratory determination of soil microbial biomass, 20—d net turnover rates of carbon and nitrogen, and respiratory and nitrogen mineralization responses to carbon and nitrogen amendments. A large variation in soil physical/chemical characteristics across landscapes and a high degree of intercorrelation among these soil properties were found. Soil microbial biomass carbon ranged from 587 to 8971 μg/g soil dry mass, constituting between 3.4 and 9.4% of the total soil carbon at the Central Hills and Southern Plains landscape sites, respectively. Soil respiration rates (as carbon loss per unit soil dry mass) ranged from 9 μ—1.d—1 in the Northwest to 57 mg.g—1.d—1 on the Southern Plains and were positively correlated with soil microbial biomass. Regression models incorporating percent water—holding capacity and total organic carbon were highly predictive of levels of microbial biomass and soil respiration across all landscapes. Net nitrogen mineralization rates per unit soil dry mass, averaging between ‐0.48 and 1.09 mg°g—1°d—1, were positively correlated with soil respiration rates, but unrelated to soil mineral nitrogen pools or soil microbial biomass. (NH4)2SO4 additions significantly reduced both soil respiration rates and net nitrogen mineralization rates, but significantly increased net nitrate production, suggesting that nitrification is limited, in part, by ammonium availability. Low phosphorus availability may not only restrict nitrate production, but also limit ammonium production, thus having fundamental impacts on the nitrogen economy in this ecosystem. The interaction between N and P cycling is likely most significant in the tallgrass Northwest and Northeast landscapes, where granitic, P—deficient parent materials predominate, and nitrogen competition between plants and soil microbes limits aboveground nitrogen flow. On the Southern, Southeast, and Northwest Plains, where grazing intensity is greatest, soils have the highest levels of soil microbial biomass and lower C:P ratios, and microbial growth and nitrogen turnover rates appeared to be more C than N limited. The Eastern and Western Corridor, which support lower levels of sustained, intensive grazing, showed intermediate levels of soil microbial biomass and carbon and nitrogen turnover rates. The observation that mean annual rainfall is positively correlated with net primary production but negatively correlated with soil fertility across African savannas is supported by our data, which generally show that regional patterns of soil fertility and soil microbial processes are negatively correlated with mean annual rainfall across the Serengeti. We found soil microbial processes to be closely correlated with landscape patterns of herbivore use and intensity of herbivory. Herbivores track plant growth, which is highly variable both temporally and spatially, and in doing so, profoundly impact nutrient cycling processes where they feed. Thus, the bursts in soil microbial processes and plant growth that follow rainfall events are accompanied by the direct impact of herbivores on these processes.