SOIL CARBON AND NITROGEN OF NORTHERN GREAT-PLAINS GRASSLANDS AS INFLUENCED BY LONG-TERM GRAZING

Three mixed prairie sites at Mandan, N.D. were grazed heavily (0.9 ha steer(-1)), moderately (2.6 ha steer(-1)), 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 r elate changes in soil carbon and nitrogen to grazing induced changes i n species composition. Blue grams [Bouteloua gracilis (H.B.K.) Lag, ex Griffiths] accounted for the greatest change in species composition f or both grazing treatments, Relative foliar cover of blue grama was 25 % in 1916 and 86% in 1994 in the heavily grazed pasture and 15% in 191 6 to 16% in 1994 in the moderately grazed pasture, Total soil nitrogen content was higher in the exclosure (1.44 kg N ha(-1)) than in either grazing treatment (0.92 and 1.07 kg N ha(-1) for moderately and heavi ly grazed, respectively) to 107-cm depth. Soil organic carbon content avg 7.2, 6.4, and 7.4 kg m(-2) to 30.4 cm soil depth and 14.1, 11.7, a nd 14.0 kg m(-2) to 106.7 cm soil depth for the exclosure, moderately grazed, and heavily grazed treatments, respectively, Compared to the e xclosure the moderately grazed pasture contained 17% less soil carbon to the 106.7 cm depth, Heavy grazing did not reduce soil carbon when c ompared to the exclosure, Based on C-13 analysis and soil organic carb on data to 15.2 cm depth, blue grams or other C4 species contributed 2 4% or 1.2 kg m(-2) of the total carbon in the heavily grazed and 20% o r 0.8 kg m(-2) of the total carbon in the moderately grazed pastures d uring the 1916 to 1991 time period, The increase in blue grama, a spec ies with dense shallow root systems, in the heavily grazed pasture pro bably accounted for maintenance of soil carbon at levels equal to the exclosure, These results suggest that changes in species composition f rom a mixed prairie to predominantly blue grama compensated for soil c arbon losses that may result from grazing native grasslands.