Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/22/2005
Publication Date: 3/21/2006
Citation: Tomer, M.D., Cambardella, C.A., James, D.E., Moorman, T.B. 2006. Surface-soil properties and water contents across two watersheds with contrasting tillage histories. Soil Science Society of America Journal. 70:620-630. Interpretive Summary: The long-term economic and environmental performance of an agricultural management system will be partly determined by its influence on soils and soil moisture. Spatial distributions of soil moisture have been examined in several settings, but effects of management on soil moisture and its spatial pattern in the long term have not been documented. This study compared soil moisture distributions in two small watersheds (30 and 43 ha) in Iowa's loess hills, where from 1972 through 1995, one watershed was under conventional tillage and was severely eroded and the other was under conservation (ridge) tillage and had little erosion. We found that soils in the eroded watershed were denser and had less organic matter than those in the conservation-tilled watershed, even eight years after a no-till system was implemented in the eroded watershed. This reduced the amount of water stored in the surface soils of that watershed; this difference was clearly detected when soils were recently wetted by rain. Also, the spatial patterns of soil moisture, as related to landform and soil organic matter, were clearly evident in the conservation-tilled watershed, but not in the eroded watershed. Therefore long-term erosion reduced soil moisture and obscured its spatial pattern, even though the area's soils are very deep and uniform. Conservation practices, in the long term, helped soils retain carbon and moisture. These differences can clearly provide benefits including increased availability of water to crops, reduced off-site losses of water and nutrients, and enhanced carbon storage. Results are of interest in agricultural, environmental, and policy arenas where benefits of conservation need to be documented.
Technical Abstract: Spatial distributions of soil moisture have been examined in various landscape settings. However, soil management may alter soil water content and its spatial pattern. This study's objective was to compare surface-soil water contents in two small watersheds (30 and 43 ha) in Iowa's loess hills with contrasting management. Both watersheds (W) were in continuous corn from 1972 through 1995, one (W1) under conventional tillage and the other (W3) under conservation (ridge) tillage; increased erosion and runoff from W1 is well documented. Surface-soil (0'0.2 m) moisture samples were collected along transects during 2002 and 2003, including four dates with sampling in both watersheds. Soil properties (bulk density, organic and inorganic carbon, texture) and terrain attributes (elevation, slope, curvature, contributing area, wetness index) were determined. W3 had more organic carbon (2.1 versus 1.7% OC) and smaller bulk densities (1.16 versus 1.25 Mg m-3) than W1 (p<0.001), showing differences found in 1995 persisted until 2003. Greater water contents occurred in W3 when moisture exceeded 33% (p<0.001). OC was related to slope in both watersheds (R2 0.21 - 0.25), but soil and terrain attributes explained more of the variation in water content in W3 than W1. Most notably, up to 40% of the variation in water content was accounted by OC in W3, but in W1 no such relation was significant. Even in these deep and uniform loess soils, long-term erosion under conventional tillage in W1 decreased soil water storage, and obscured influences of soil and terrain on its spatial pattern.