Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 4/10/2013
Publication Date: 5/15/2014
Citation: Stott, D.E., Cambardella, C.A., Karlen, D.L. 2014. Assessment of near-surface soil carbon content across several U.S. cropland watersheds. In: Hardemink, A.E., McSweeney, K. (eds.) Soil Carbon. Cham, Zug:Springer International Publishing. p. 249-256.
Technical Abstract: The cropland Conservation Effects Assessment Project (CEAP) was initiated to provide a scientific basis for assessing effectiveness of conservation practices on water and soil quality. In 2006, sampling was initiated within a number of USDA-ARS experimental watersheds to measure and assess management impacts on near-surface (0 to 5 cm) soil quality indicators. In this paper we focus on soil organic carbon (SOC) content from over 3,400 samples, because of its influence on several other soil quality indicators. The sampling schemes for each of the 17 locations were designed to address individual objectives. For instance, in the St. Joseph River Watershed in northeastern Indiana, the question was how pothole topography and management were impacting soil quality. We used the Soil Management Assessment Framework (SMAF) to score the measured data so that climate and inherent soil factors would be taken into account. The SOC-SMAF scoring algorithms, which are modified based on soil classification, texture and climate, use a more-is-better model reflecting the amount of SOC associated with good productivity and minimal environmental impact. Cropping systems within the St. Joseph Watershed included corn (Zea mays)-soybean [Glycine max. (L.) merr.], corn-soybean plus wheat (Triticum aestivum) and/or oat (Avena sativa) in the rotation, alfalfa (Medicago sativa), and crop reserve program (CRP) land. Where possible both tilled and non-tilled systems were evaluated for each cropping system, with samples taken along slope transects that reflected watershed topography. Interactions among many factors including soil type, climate, and management practices such as tillage and crop rotation influenced SOC content at each sampling site. Measured SOC contents ranged from 3.0 to 21.7 g kg-1 and SMAF-SOC scores ranged from 0.09 to 1.00, where 1.00 represents optimum levels of SOC based on soil taxonomic classification and climate. The mean SMAF-SOC scores were 0.43, 0.51, and 0.43 for summit, mid-slope, and toe-slope positions. Analysis by current crop showed SMAF-SOC scores of 0.57, 0.52, 0.44, 0.41, and 0.72 for corn, soybean, wheat, alfalfa, and CRP, respectively. This assessment and those planned for the other watersheds are expected to show that SOC evaluations need to be soil- and site-specific because many factors, including environmental influences and inherent soil characteristics, influence this important soil quality indicator.