Drought and small-bodied herbivores modify nutrient cycling in the semi-arid shortgrass steppe

Abstract

Climate change will increase the frequency of droughts over the next century, with severe consequences for ecosystem function in semi-arid grasslands. The shortgrass steppe (SGS) experiences some of the largest interannual variation in precipitation among terrestrial biomes and exhibits extremely high sensitivity to drought. Yet despite decades of research describing the consequences of drought for ecosystem function in the SGS, we currently have little information regarding the impact of drought on bioavailability of important nutrients other than nitrogen, the contribution of herbivores to bioavailable concentrations of these nutrients, and whether drought alters herbivore-derived nutrient cycling. To quantify the impacts of long-term drought and small-bodied herbivores on nutrient cycling and aboveground net primary production (ANPP), we factorially manipulated rainfall and herbivore presence in the SGS of northern Colorado. Specifically, we measured the impacts of drought and herbivores on bioavailability of ten important nutrients: aluminum, calcium, iron, potassium, magnesium, manganese, nitrate, phosphorus, sulfur, and zinc. We then correlated these nutrients with grass production to determine whether reduced plant growth under drought conditions causes a belowground buildup of nutrients. Drought reduced ANPP as expected, and also altered concentrations of many nutrients apart from N, which clustered in their drought response. In contrast, small-bodied herbivores did not affect ANPP or soil N. However, they did contribute to the bioavailable soil concentrations of two important nutrients: PO4-P and S. Importantly, drought generally did not modify the contribution of herbivores to nutrient cycling, suggesting that herbivores might be a critical component of biogeochemical cycling regardless of precipitation in semi-arid grasslands.