Three mixed prairie sites in North Dakota were grazed heavily, moderately, or left ungrazed (exclosure) since 1916. These sites provided treatments to study the effects of long-term grazing on soil organic carbon and nitrogen content and to relate changes in soil carbon and nitrogen to grazing induced changes in species composition. Relative cover of blue grama (Bouteloua gracilis) increased from 25% to 86%. In contrast, under moderate grazing, relative cover fluctuated from 15% to 64%, and returned to 16% at last measurement. Total soil nitrogen was greater in the exclosure than under either grazing regime, while soil carbon was lower only under moderate grazing. The latter finding may be attributed to the increase in blue grama under heavy grazing. Blue grama is a species with a large shallow root mass and possibly greater root turnover or rate of root exudate. These results suggest that changes in species composition from a mixed prairie to predominantly blue grama compensated for soil carbon losses that may result from grazing native grasslands.
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