SIMULATED EFFECTS OF GRAZING ON SOIL-NITROGEN AND MINERALIZATION IN CONTRASTING SERENGETI GRASSLANDS

The roles of edaphic characteristics and grazing in regulating nitrogen cycling and belowground processes were simulated for the shortgrass and tallgrass regions of the Serengeti National Park, Tanzania, by coupling a model of grassland productivity with a model of belowground decomposition processes. Grasslands of each region were simulated under ungrazed and observed grazing intensities to establish reference points for sensitivity analyses using a wide range of grazing intensities and different microbial biomasses. The consequences of patchy carbon and nitrogen deposition were also simulated. Results indicate that nitrogen cycling processes are affected differentially by grazing in the shortgrass and tallgrass regions. Net nitrogen mineralized by the microbial biomass in the shortgrass simulations declined from the ungrazed condition at low grazing intensities, and proceeded to increase from moderate to high grazing intensities. Plant nitrogen uptake was significantly correlated with net mineralization, and soil mineral nitrogen levels were greatest at moderate grazing intensities. In tallgrass simulations, soil mineral nitrogen increased with grazing intensity; net nitrogen immobilization occurred over much of the grazing intensity range because of low decomposition substrate quality. Tallgrass plant nitrogen uptake responded to urine nitrogen input to the soil and was uncorrelated with net mineralization. The potential for nitrogen competition between grasses and microbes was greater in the tallgrass than in the shortgrass because of lower total system nitrogen and lower grazing intensity in the tallgrass. The ecosystem-wide soil nitrogen gradient of the Serengeti governs most properties of nitrogen cycling, while within-region edaphic conditions and grazing create significant variation in local ecosystem processes.