DISTRIBUTION OF SOIL BULK DENSITY AND ORGANIC MATTER ALONG AN ELEVATION GRADIENT IN CENTRAL OKLAHOMA

Native prairie and winter wheat are among the primary forage resources used to graze cattle in Oklahonza. Understanding the spatial distribution of soil characteristics is crucial to understanding the function of managed landscapes. This study compared soil bulk density (BD) and organic matter (OM) of experimental paddocks on an upland site in central Oklahoma after 26 years (1978-2004) of different forms of sustained management. Included were grazed, conventionally tilled winter wheat, and three levels of grazing (light stocking rates over long periods, high stocking rates over short periods, and no grazing) applied to tallgrass prairie. Soil cores (25 cm depth) were collected from paddocks under each management regime at 1.5 m intervals along 150 m transects situated between a common ridge and toe slope, and divided into three sections (0-5, 5-10, and 10-25 cm depths). Soil BD and OM were determined, means were tested, and measures of variability in spatial distributions were defined. Soils in the grazed wheat paddock were denser (1.35 +/- 0.11 vs. 1.21 +/- 0.03 Mg m(-3)) and contained less OM (35 +/- 6 vs. 45 +/- 14 g kg(-1)) than soils under native prairie. Management effects on BD and OM were largely confined to the upper 10 cm of soil, with intensive management forms (wheat, high stocking rates) producing denser soils with less OM. Both OM and BD had highly variable distribution patterns in all paddocks, and exhibited patterns that indicate the presence of multiple catenae that comprised the common slope of the study site, with little similarity in distribution of BD or OM across paddocks. Results indicated that developed paddocks should be relatively homogeneous in terms of topography and soils, to improve management and develop effective experimental designs.