Aggregate stability in the Palouse region of Washington. Effect of landscape position

An extensive study was conducted to measure spatial patterns in aggregate stability in the steep, hilly, and highly erodible loessial soils of the Palouse region in southeastern Washington. Soil samples were collected at 20-m spacings along four 800-m-long transects within a winter wheat field and analyzed for aggregate stability at a slow and a fast rate of wetting, organic C, amorphous Fe, soil water content, and particle size distribution. Aggregate stability under slow wetting, organic C, and clay content were significantly different at summit, shoulder, toeslope, and footslope positions on the landscape. More specifically, aggregate stability and organic C content were highest in footslope and toeslope positions, and lowest at the summit. Clay content was highest at the summit and lowest in footslope positions. Aggregate stability at a fast rate of wetting had a coefficient of variation (40%) that was nearly double that for slow wetting, and was not well correlated to either landscape position or measured soil properties such as amorphous Fe and water or clay content were only weakly correlated to aggregate stability. Thus, although the spatial patterns in aggregate stability exhibited moderate variability (CV = 22%), the patterns were closely related to changes in a few key properties, namely, organic C content and landscape position. A likely explanation for these findings is that soil erosion removes the topsoil and organic matter from the ridgetops, thus exposing subsoil horizons, which are higher in clay content and lower in aggregate stability.