Impact of grazing management on the carbon and nitrogen balance of a mixed-grass rangeland

Rangeland grazing management strategies have been developed in an effort to sustain efficient use of forage resources by livestock. However, the effec ts of grazing on the redistribution and cycling of carbon (C) and nitrogen (N) within the plant-soil system are not well understood. We examined the p lant-soil C and N balances of a mixed-grass rangeland under three livestock stocking rates using an area that had not been grazed by domestic livestoc k for more than 40 years. We established nongrazed exclosures and pastures subjected to continuous season-long grazing at either a light stocking rate (20 steer-days/ha) or a heavy stocking rate (59 steer-days/ha, similar to 50% utilization of annual production). Twelve years of grazing under these stocking rates did not change the total masses of C and N in the plant-soil (0-60 cm) system but did change the distribution of C and N among the syst em component, primarily via a significant increase in the masses of C and N in the root zone (0-30 cm) of the soil profile. The mass of soil C (0-60 c m) under heavy grazing was comparable to that of the light grazing treatmen t. Grazing at the heavy stocking rate resulted in a decrease in peak standi ng crop (PSC) of aboveground live phytomass, an increase in blue grama (Bou teloua gracilis [H.B.K.] Lag. Ex Steud.), and a decrease in western wheatgr ass (Pascopyrum smithii [Rydb.] A. Love) compared to the light grazing trea tment. The dominant species under light grazing was western wheatgrass, whe reas in the nongrazed exclosures, forbs were dominant and appeared to have increased at the expense of western wheatgrass. The observed increase of so il C and N in the surface soil where roots dominate indicates a greater opp ortunity for nutrient availability and cycling, and hence enhanced grazing quality.