Submitted to: Soil Science Society of America Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/1/2006
Publication Date: 9/15/2006
Citation: Rhoton, F.E., Emmerich, W.E., Goodrich, D.C., Miller, S.N., Mcchesney, D.S. 2006. Soil Geomorphological Characteristics of a Semiarid Watershed: Influence on Carbon Distribution and Transport. Soil Science Society of America Journal. 70:1532-1540. Interpretive Summary: Quantifying carbon movement in watersheds is essential for the assessment of ecosystem stability on the basis of carbon and nutrient distribution, and from the standpoint of comprehensive carbon sequestration studies related to global warming issues. We measured carbon and nitrogen contents over a wide range of soil types and topography in a large, semiarid watershed setting, and related these data to the contents of these same elements in sediments being transported from the watershed. The results indicated that the distribution of carbon and nitrogen was closely related, and primarily concentrated in the silt-size fraction of the watershed soils. The greatest concentrations were observed on the steeper slopes, and on the lower hillslope positions, indicating a high level of susceptibility to erosion processes. Further, stream sediments were enriched in these nutrients to the extent that they are being removed from the watershed in concentrations twice that of the source soils. These findings show the need for implementation of conservation measures in the form of sediment retention ponds and/or check dams that will reduce the losses of the silt-size component of the watershed soils to stabilize the nutrient status of such watersheds and contribute to carbon sequestration efforts.
Technical Abstract: In the marginally productive rangelands of the semiarid, southwestern United States, the maintenance of organic C and nutrients is essential to the stability of the ecosystem. This study was conducted to identify those factors responsible for the distribution of organic C and N in watershed soils, their loss from upland areas, and their transport in the stream system of the Walnut Gulch Experimental Watershed (WGEW) at Tombstone, AZ. Samples were collected along transects from the surface 5 cm of each major soil mapping unit in six of the subwatersheds (SW). Data were recorded for slope class, position, and aspect at each sampling point. Soil analyses consisted of: total and organic C, total N, particle size distribution, water dispersible clay, pH, quantitative color, aggregation index (AI), and magnetic susceptibility. Suspended and bedload sediment samples collected at the mouth of the SW were characterized for similar properties. Soils data indicated that C and N distributions in the SW were related to parent material and past erosion, with significantly (P < 0.05) greater contents recorded on the steeper slopes (> 13%), and backslope and toeslope positions. Fewer significant correlations were identified for aspect. Soil C was correlated with silt (r = 0.467) and clay contents (r = 0.389) at the 1% level of significance. Total N was most highly correlated with soil C (r = 0.873; P < 0.01). Organic C and N contents of the soils and suspended sediments averaged 11.4 and 0.73 g kg-1, and 24.0 and 1.45 g kg-1, respectively. Enrichment ratios for C and N in the suspended sediments were 2.13 and 2.02, respectively. Bedload sediment was depleted in C by an average ratio of 0.65 relative to the soils, and N contents were below detectable limits. The results suggest that C and N are transported in WGEW predominantly as silt and clay size materials whose concentrations are controlled by the soil AI.