Soil carbon and nitrogen of Northern Great Plains grasslands as influenced by long-term grazing.

Abstract

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 relate changes in soil carbon and nitrogen to grazing induced changes in species composition. Blue grama [Bouteloua gracilis (H.B.K) Lag. ex Griffiths] accounted for the greatest change in species composition for both grazing treatment. Relative foliar cover of blue grama was 25% in 1916 and 86% in 1994 in the heavily grazed pasture and 15% in 1916 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 heavily grazed, respectively) to 107-cm depth. Soil organic carbon content avg 72, 6.4, and 7A kg m-2 to 30.4 cm soil depth and 14.1,11.7, and 14.0 kg m-2 to 106.7 cm soil depth for the exclosure, moderately grazed, and heavily grazed treatments, respectively. Compared to the exclosure the moderately grazed pasture contained 17% less soil carbon to the 106.7 cm depth. Heavy grazing did not reduce soil carbon when compared to the exclosure. Based on 13C analysis and soil organic carbon data to 15.2 cm depth, blue grama or other C4 species contributed 24% or 12 kg m-2 of the total carbon in the heavily grazed and 20% or 0.8 kg m-2 of the total carbon in the moderately grazed pastures during the 1916 to l99l time period. The increase in blue grama, a species with dense shallow root systems, in the heavily grazed pasture probably accounted for maintenance of soil carbon at levels equal to the exclosure. 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.