DOES GRAZING MEDIATE SOIL CARBON AND NITROGEN ACCUMULATION BENEATH C-4, PERENNIAL GRASSES ALONG AN ENVIRONMENTAL GRADIENT

An experiment was conducted to evaluate the influence of long-term (>2 5 yrs) grazing on soil organic carbon (SOC) and total soil nitrogen (N ) accumulation beneath individual plants of three perennial grasses al ong an environmental gradient in the North American Great Plains. The zone of maximum SOC and N accumulation was restricted vertically to th e upper soil depth (0-5 cm) and horizontally within the basal area occ upied by individual caespitose grasses, which contributed to fine-scal e patterning of soil heterogeneity. Long-term grazing mediated SOC and N accumulation in the tall-, mid-and shortgrass communities, but the responses were community specific. SOC and N were lower beneath Schiza chyrium scoparium plants in long-term grazed sites nf the tall- and mi dgrass communities, but higher beneath Bouteloua gracilis plants in th e long-term grazed site of the shortgrass community. SOC, but not N, w as greater in soils beneath compared to between S. scoparium plants in an abandoned field seeded in 1941, indicating that this caespitose gr ass accumulated SOC more rapidly than N. SOC and N were greater in the 0-5 cm soil depth beneath a caespitose grass (S. scoparium) compared to a rhizomatous grass (Panicum virgatum) in the tallgrass community, with no significant accumulation of either SOC or N beneath P. virgatu m plants. Grazing appears to indirectly mediate nutrient accumulation beneath caespitose grasses along the environmental gradient by modifyi ng the size class distribution of plants. Populations with a greater p roportion of large plants have a greater potential for biomass incorpo ration into soils and may more effectively capture redistributed organ ic matter from between plant locations. Contrasting plant responses to grazing at various locations along the environmental gradient conform to the predictions of the generalized grazing model, as the selection pressures of grazing and aridity may have also influenced the ability of caespitose grasses to accumulate nutrients in soils beneath them b y mediating grazing resistance, competitive ability and population str ucture.