Submitted to: Soil & Tillage Research
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/9/2004
Publication Date: 8/1/2004
Citation: Cambardella, C.A., Moorman, T.B., Andrews, S.S., Karlen, D.L. 2004. Watershed-scale assessment of soil quality in the loess hills of southwest Iowa. Soil & Tillage Research. 78(2):237-247. Interpretive Summary: Environmental endpoints in agricultural watersheds are frequently related to soil quality dynamics that are driven by agricultural management practices in these watersheds. The potential for soil quality change within fields and watersheds may vary depending upon landscape position and the spatial distribution of critical soil properties. This study was conducted to evaluate the effects of two tillage systems on soil quality parameters in three watersheds in southwest Iowa. We found that overall soil quality was higher in the watershed under long-term ridge tillage than in the two conventionally tilled watersheds. Soil quality differences under ridge-till were found specifically at the steeper side slope landscape positions, suggesting that soil quality increases in these positions on the landscape are responsible for higher watershed-scale soil quality in the ridge-tilled watershed. This information will be useful to scientists in improving understanding of the effects of tillage systems on soil quality and the impact of spatial variability on watershed-scale assessments of soil quality.
Technical Abstract: Soil quality is a concept that integrates soil biological, chemical and physical factors into a framework for soil resource evaluation. Conventional tillage practices can result in a loss of soil organic matter and decreased soil quality. The potential for soil quality degradation with tillage may vary depending upon landscape position and the spatial distribution of critical soil properties. Information on how to accurately integrate soil spatial information across fields, landscapes and watersheds is lacking in the literature. The primary objective of this study was to evaluate the long-term effect of conventional and ridge tillage on soil quality in three small watersheds at the Deep Loess Research Station near the town of Treynor in southwest Iowa. Soil types included Monona silt loams in summit positions, Ida or Dow silt loams in backslope positions, and Napier or Kennebec silt loams in footslope positions. We removed surface soil cores from transects placed along topographic gradients in each watershed and quantified total soil organic carbon (SOC), total soil nitrogen (TN), particulate organic matter carbon (POM-C) and nitrogen (POM-N), microbial biomass carbon (MB-C), nitrogen mineralization potential (PMIN-N), nitrate nitrogen, extractable phosphorous and potassium, pH, water-stable macroaggregates (WSA), and bulk density (BD). We used terrain analysis methods to group the data into landform element classes to evaluate the effect of topographic position on soil quality. Results indicate that soil quality is higher under long-term ridge-tillage compared with conventional tillage. Soil quality differences were consistently documented among the three watersheds by (1) quantification of soil indicator variables, (2) calculation of soil quality index values, and (3) comparison of indicator variable and index results with independent assessments of soil function endpoints (ie sediment loss, water partitioning at the soil surface, and crop yield). Soil quality differences under ridge-till were found specifically for the backslope and shoulder landform elements, suggesting that soil quality increases on these landform elements are responsible for higher watershed-scale soil quality in the ridge-tilled watershed.