Abstract
In the marginally productive rangelands of the semiarid, southwestern USA, the maintenance of organic C (OC) is essential to the stability of the ecosystem. This study was conducted to identify landscape factors responsible for the distribution of OC in watershed soils, its loss from upland areas and subsequent transport within the stream system of a large semiarid watershed (Walnut Gulch Experimental Watershed [WGEW], Tombstone, AZ). Samples were collected along transects from the surface 5 cm of each major soil mapping unit in six subwatersheds (SW). Data were recorded for slope class, landscape position, and aspect at each of the 435 sampling points. Soil analyses consisted of: total C and OC, particle‐size distribution, water dispersible clay, pH, quantitative color, and aggregation index (AI). Sediment samples were collected from flumes at each SW outlet. These 169 bedload and 59 suspended sediment samples were analyzed identically to the soils. Soil data indicated that OC distributions in the SWs were related to parent material with significantly (p ≤ 0.05) greater contents recorded on the steeper slopes (>9%), and backslope and toeslope positions. Fewer significant correlations were identified for aspect. Soil OC was significantly (p ≤ 0.01) correlated with silt and clay contents. Organic C contents of the soils and suspended sediments averaged 11.4 and 24.0 g kg−1, respectively, giving an enrichment ratio (ER) for OC in the suspended sediments of 2.13. Bedload sediment was depleted in OC by an average ratio of 0.65 relative to the soils. The results suggest that OC is transported through this watershed predominantly as silt‐ and clay‐size materials in concentrations controlled by the soil AI.
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