Grazing effects on fungal root symbionts and carbon and nitrogen storage in a shortgrass steppe in Central Mexico

Livestock grazing is the principal driver of land use change in semiarid grasslands of Mexico. Despite the vast expansion of grasslands and the alarming extent of land degradation associated with overgrazing, our understanding of the effects of heavy grazing on root fungal symbiosis and the sizes of soil carbon (C) and nitrogen (N) stocks is still limited. This research was conducted in the shortgrass steppe in Los Llanos de Ojuelos, Jalisco, Mexico, where Bouteloua gracilis is a keystone species. We examined soil C and N storage and root colonization of B. gracilis by fungal symbionts (arbuscular mycorrhizal fungi (AMF) and dark septate endophytes (DSE)) along a grazing gradient. Roots of B. gracilis were examined for root colonization of AMF and DSE. Soil samples were analyzed for total C and N content. Under moderate grazing, soil C and N were 20% higher than under no-grazing and extremely heavy grazing. Grazing intensity did not differently affect root fungal colonization. However, while the two fungal symbionts coexisted on Bouteloua plant roots, a four times higher root colonization by DSE than by AMF suggests that competition for resources may control symbiont abundance. Considering total root fungal colonization by both symbionts (for DSE 37% and for AMF 10%), and a slightly higher colonization in the less fertile sites, we suggest this fungal symbiosis may play an important role linking the aboveground and belowground C cycle in these semiarid grasslands in Central Mexico.