Surface-Soil Properties and Water Contents across Two Watersheds with Contrasting Tillage Histories

Soil properties and water contents (u) vary spatially, but management effects on spatial patterns are poorly understood. This study’s objective was to compare surface-soil properties and u in two small watersheds (30–43 ha) in Iowa’s loess hills. Both watersheds were in continuous corn (Zea mays L.) from 1972 through 1995, one (CW1) under conventional tillage and the other (RW3) under ridge tillage. In 1996, CW1 was converted to no-till. Surface-soil (0–0.2 m) samples were collected along hillslope transects during 2002 and 2003, including four dates with water-content measurements by gravimetry in both watersheds. Soil bulk density (rb), organic carbon (OC), and texture were determined, along with terrain attributes (elevation, slope, surface curvature, contributing area, andwetness index). After accounting for landscape-position effects, RW3 had more OC (2.1 versus 1.7%) and smaller rb (1.16 versus 1.25Mgm) than CW1 (P, 0.001). Larger u values occurred in RW3 (P , 0.002) when u was .33%. Landscape position and terrain attributes better explained variation in u in RW3 than CW1. Also, OC was correlated with u in RW3, but not in CW1. Soil textures were similar (within 2%,) but finer in CW1 (P , 0.05). Pedotransfer functions confirmed that differences in soil properties between watersheds resulted in greater u in RW3 than CW1, particularly at low soil-water potential, and that more distinct patterns of u should occur in RW3. Results indicate long-term conventional tillage in CW1 affected soil properties and water-holding characteristics in ways that decreased water retention and muted spatial patterns of u. VARIATION IN SOILS across landscapes influences crop productivity and watershed hydrology. Coupled with landscape effects on soils, agricultural management systems also affect soil properties, including organic matter content (Cambardella et al., 2004; Rhoton, 2000), aggregation (Cambardella and Elliott, 1993), and rb (Logsdon et al., 1999b). Management systems can affect u (Johnson et al., 1984) and the efficiency of soil-water uptake by plants (Hatfield et al., 2001; Varvel, 1994). Management also affects off-site impacts of agriculture by influencing infiltration, runoff, and erosion (Rhoton et al., 2002). Yet, our understanding of management impacts includes little about their effects on spatial variations in soil properties and u across landscapes. Variation in soil properties has been evaluated in relation to landscape position (Ruhe and Walker, 1968), and terrain characteristics obtained from digital elevation data (Moore et al., 1991). A number of soil properties can differ according to landscape position, including horizon thickness, texture, organic matter, aggregate stability, carbonates, Mn, redoximorphic features, pH, and exchangeable cations (Brubaker et al., 1993; Cambardella et al., 2004; Cassel et al., 2002; Pierson and Mulla, 1990; Young and Hammer, 2000). Impacts of erosion on soil properties at different landscape positions have also been examined (Kreznor et al., 1989). Terrain characteristics including slope, aspect, contributing area, profile curvature, wetness, and stream-power indices (Moore et al., 1991) can be used to predict andmap soil attributes such as A-horizon thickness and color, profile thickness, organic matter content, pH, and texture (Gessler et al., 2000;Moore et al., 1993; Thompson et al., 1997). Park and Burt (2002) showed the predictive capacity of terrain features depends on depth and the soil property of interest. At broad spatial scales, Bell et al. (1994) showed terrain attributes can predict soil drainage class across large areas. The spatial distribution of u has also been studied in relation to landscape position and terrain characteristics. Increases in plant-available soil water in toeslope and/or footslope positions have been attributed to infiltration of runoff that originates upslope and/or shallow water tables (Afyuni et al., 1993; McGee et al., 1997). Topographic attributes reflecting lateral flows and accumulations at the landscape scale have been statistically related to u (Tomer and Anderson, 1995; Western et al., 1999) and water retention characteristics (Pachepsky et al., 2001). The prediction of hydraulic parameters using pedotransfer functions (Wösten et al., 2001) can be improved if terrain attributes (e.g., slope, aspect) are included among the independent variables (Romano and Palladino, 2002). The capacity of terrain attributes to account for spatial patterns of umay depend on the temporal stability of these patterns. Temporal stability in u patterns can be associated with the arrangement soil types and textures on the landscape (da Silva et al., 2001). In other instances, temporal stability has been found in sandy and/or flat terrain, (Tomer and Anderson, 1995; Wendroth et al., 1999) where runoff and shallow lateral flows are rare. However, on landscapeswhere runoff and/or lateralmovement of shallow soil water do occur, temporal instability in patterns of soil moisture has been reported (Grayson et al., 1997). This instability occurs because topographically driven lateral flows dominate spatial patterns when there is surplus moisture (precipitation . evapotranspiration), but spatially random variation prevails when dry conditions restrict lateral water movement. Accordingly, Western et al. (1999) found the proportion of variation in u accounted by terrain indices varied from 22 to 61%, USDA-ARS, National Soil Tilth Lab., 2150 Pammel Dr. Ames, IA50011. Received 10 Nov. 2004. Mention of trade names does not constitute or imply endorsement by USDA. *Corresponding author