Multiple nutrients control threatened grassland vegetation in eastern South Africa

Grassland types associated with highly weathered upland soils derived from igneous dolerite and sedimentary sandstone in eastern South Africa are rich in restricted range plant species, and have been extensively destroyed for agriculture and other land uses. Anthropogenic increases in nutrient supply, including atmospheric N deposition, and changes in herbivore abundance may alter vegetation and threaten species in remaining fragments. We investigated the influence of multiple nutrient limitation and herbivory on the productivity, diversity, and composition of these distinctive grasslands, as part of the world-wide Nutrient Network. We examined nutrient limitation using three years of factorial additions of N, P + Ca, and K + micronutrients at two sites, one dolerite-derived and one sandstone-derived. Adding N and P + Ca increased aboveground productivity at both sites by 33–55%, indicating that these nutrients co-limited productivity. Adding N reduced species richness (by 11%) and effective species richness (by 24%) at the sandstone site; all nutrient additions also tended to reduce diversity at the dolerite site. At both sites, adding N increased the abundance of grasses at the expense of (N-fixing) legumes, which declined in relative abundance by 44–60%, whereas adding P + Ca tended to increase the abundance of legumes. At the dolerite site, a factorial fencing and nutrient addition experiment showed that large herbivores did not significantly influence vegetation during three years. Widely increasing atmospheric N deposition, and perhaps warming-induced increases in N mineralization at high elevation, will likely increase productivity, reduce diversity, and reduce legume abundance in these dolerite and sandstone grasslands, posing an additional challenge to conserving them.