Tan, Zhengxi (South Dakota State Univ., Brookings, SDUSGS, EROS Data Center, 47914, 252nd St., Sioux Falls, SD, 57198; Phone: 605-594-6903; Email: ztan@usgs.gov)

Soil Organic Carbon Budget as Related to Land Use History in the Northwestern Great Plains

Z.X. Tan *, S. Liu, C.A. Johnston, J. Liu, T.R. Loveland

The strategies for mitigating global greenhouse effect need to understand soil organic carbon (SOC) dynamics and driving factors at both spatial and temporal scales, which is usually challenging due to limitation in data and approach. This study was conducted to characterize the SOC dynamics associated with land use change history in the Northwestern Great Plains Ecoregion. A sampling framework (40 sample blocks of 10km x 10km randomly located in the Ecoregion) and the General Ensemble Biogeochemical Modeling System (GEMS) were used to quantify the spatial and temporal variability in the SOC storage from 1972 through 2001. The results indicate that the C source and sink coexist in the study area and overall SOC storage in the top 20 cm depth over the 30 years increased by 3.93 Mg/ha. About 17.5% of the area was evaluated as a C source at a rate of 0.122 Mg C/ha/yr. The spatial variability of total SOC storage was determined by the dynamics of both slow and passive fractions but the temporal variation depended on slow fraction only. The magnitude of SOC budget at the block scale was significantly related to either grassland areal proportion positively or cropland fraction negatively. It was concluded that C sink was generally associated with grassland use and C source with cropland but their strength depended on antecedent SOC contents and land use change history.