Ecologists still do not know why soil bacteria and plants that fix atmospheric nitrogen (N) and convert it into forms useable by plants are so often rare in land ecosystems whose productivity is limited by N. Preferential consumption of legumes by large mammal herbivores potentially limits N fixation, but this has not been adequately studied in the past. We will use new methods based on measuring concentrations of the stable isotope 15-N to explore whether and how large mammal herbivores stimulate or limit N fixation by legumes and soil bacteria in the Serengeti, one of the worlds last great grazing ecosystems and a representative tropical grassland and savanna. Our results should suggest ways that herbivores limit or sustain production of food and fiber in semi-arid grasslands and savannas characteristic of the developing world. Our research will inform different management actions that might inexpensively increase natural N fixation. And should help better estimate the contribution of natural N inputs in ecosystems relative to human-induced elevated N deposition around the world, including the U.S. Our project will employ indigenous Tanzanians to help build the capacity of Tanzania to manage their natural resources and rely less on foreign aid.
