Submitted to: Soil Science Society of America Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/19/2012
Publication Date: 11/19/2012
Citation: Veum, K.S., Goyne, K.W., Kremer, R.J., Motavalli, P.P. 2012. Relationships among water-stable aggregates and organic matter fractions under conservation management. Soil Science Society of America Journal. 76(6):2143-2153. Interpretive Summary: Soil quality entails the ability of soil to function within a landscape to provide favorable plant productivity while maintaining environmental quality. Assessment of soil quality provides a measure of the effectiveness of crop production systems and soil conservation management practices. Some soil conservation practices are use of vegetative filter strips (VFS) composed of forage or native grasses and legumes, sometimes with trees, planted in narrow strips in fields following the landscape contour (i.e., perpendicular to slope). Established vegetation in these strips retard water runoff and filter suspended soil contained in runoff after rainfall. Numerous soil quality indicators can be measured to show beneficial effects of VFS in conserving and restoring soil. Soil organic carbon (SOC) is considered a key soil quality indicator because it influences several soil properties; thus determination of SOC may provide insight to performance of other soil functions. SOC is a broad measure of the total C present in soil; recent research, however, suggests that components making up SOC may relate better with other soil quality indicators and may increase the sensitivity in detecting improved soil quality resulting from use of soil conservation practices. Our objective was to measure different SOC fractions and soil aggregate stability to assess soil quality on landscapes managed with VFS and no-tillage systems. Soils were collected in November 2007 from management systems established in 1997 within a small watershed that included VFS with grass, VFS with grass plus oak trees (agroforestry VFS), and no-till corn and soybean crops. The different SOC fractions included water-soluble C that is available to microorganisms for metabolism; and partially decomposed C that is important for nutrient retention and soil structure. Stable soil aggregates were measured as indicators of good soil aeration and water infiltration. These measures detected differences between the VFS and no-till cropping systems but were unable to detect differences among positions on the landscape. The gradual slope (0-3%) of the overall landscape within the watershed likely reduced chances of detecting effects due to landscape position. Strong, positive correlations of water-soluble and partially decomposed C with water stable aggregates suggested that these are components of the total SOC playing major roles in building up stable soil aggregates. Thus, indicators based on C fractions were effective in detecting improvements in soil quality 10 years after establishment of conservation practices. Results are important to farmers, conservationists, extension personnel, and other scientists because they illustrate the value of conservation management in improving soil quality within watershed sites; that soil quality assessment can be improved using soil C fractions rather that total soil C; and that the measurements reported here can be easily applied to other areas for assessing soil conservation effects.
Technical Abstract: Conservation management practices may improve soil quality in agroecosystems. The objective of this study was to investigate differences in water-extractable organic carbon (WEOC) and aggregate stability in soils using air-dried (WSAAD) and field moist (WSAFM) samples collected from three conservation management practices (i.e. no-till, grass vegetative filter strips [VFS], and agroforestry VFS) and four landscape positions (i.e. summit, shoulder, backslope and footslope) ten years after the VFS were planted. Treatment effects were evaluated on a gravimetric, soil volume, soil mass, and carbon normalized basis. Regression and correlation relationships were explored among WSAFM, WSAAD, WEOC, total soil organic carbon (SOC), particulate, occluded and adsorbed organic carbon (PAOC), bulk density, water content and pH. In the surface layer (0-5 cm), VFS soils had significantly greater gravimetric, soil volume and soil mass based WEOC and WSAAD than no-till soil and significantly greater PAOC and SOC normalized WSAAD than no-till. No significant landscape effects on any parameter were detected, likely due to the gentle (0 – 3%) slope of the study site, the small catchment area, and the similarity of the soil series found at each landscape position. Simple regression between bulk density and WEOC and between bulk density and PAOC explained 80% and 78% of the variation in WSAAD. In this study, WEOC and WSAAD were effective early indicators of improved soil quality under VFS after 10 years. Additionally, these results suggest that WEOC and PAOC fractions may play a greater role in aggregate stability than total SOC.