Field Heterogeneity of Soil Organic Carbon and Relationships to Soil Properties and Terrain Attributes

Our objectives were to: (1) characterize SOC (0 to 1.53-m) within a 37-ha field typical of the Palouse region in eastern Washington; and (2) assess correlations and how much SOC variance could be accounted for by terrain and soil variables. A systematic, non-aligned grid of 177 georeferenced sample locations was established at the Washington State University Cook Agronomy Farm (CAF) near Pullman, WA. Intact soil cores (0-153 cm) were collected, soils were described, classified, the surface divided into 0-30-cm increments and then by soil horizon to a depth of 153 cm and analyzed for soil bulk density and SOC. A digital elevation model (DEM) was developed and terrain attributes (slope, TRASP, curvature, global irradiation, flow accumulation and direction, specific catchment area, and wetness index) derived (10-m grid). Profile (0-153 cm) SOC ranged from 54 to 272 Mg C/ha over the 37-ha field and landscape SOC redistribution via soil erosion was evident. Terrain attributes and soil variables accounted for up to 86% of profile SOC variation; however, results were unique for each soil series. Redistribution of SOC via erosion increases field SOC heterogeneity and must be quantified if SOC sequestration and management impacts are to be adequately assessed.