Estimating Surface Soil Organic Carbon Content at a Regional Scale Using the National Resource Inventory

The National Resource Inventory (NRI) may be used to estimate soil organic C (SOC) levels at a regional scale if accurate and precise estimates can be made across a wide range of soils, land uses, and topographic positions. The objectives of this study were (i) to identify important sources of variation in SOC content at a regional scale and (ii) to determine the precision with which SOC content can be estimated. Surface soil samples (0–10 cm) were collected in four Major Land Resource Areas (MLRAs) and analyzed for SOC, sand, silt, and clay content. Land use, hillslope position, and slope aspect effects on SOC levels were evaluated by analysis of covariance with sand or clay content as a covariate to adjust for textural differences at each sample point. Land use was a significant source of variation in all four regions. Hillslope position and slope aspect were significant sources of variation in only one of the four regions and there were no significant interactions between land use and hillslope position or aspect. The SOC content ranged from 26 to 55 Mg ha 10 cm−1 in the Northern Mississippi Valley Loess Hills, 22 to 50 Mg ha 10 cm−1 in the Palouse and Nez Perce Prairies, 9 to 26 Mg ha 10 cm−1 in the Central High Plains, and 5 to 8 Mg ha 10 cm−1 in the Southern High Plains. Standard errors ranged from 0.5 to 5.0 Mg ha−1 and were lowest in regions where SOC levels were also lowest. Results indicate the NRI can be an effective tool for estimating SOC levels under different land use and conservation practices on a regional scale.