ARS | Publication request: Distribution of soil bulk density and organic matter along an elevation gradient in central Oklahoma

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 1, 2010
Publication Date: October 1, 2010
Citation: Daniel, J.A., Northup, B.K. 2010. Distribution of soil bulk density and organic matter along an elevation gradient in central Oklahoma. Transactions of the ASABE. 53(6):1749-1757.

Interpretive Summary: Livestock producers in central Oklahoma graze cattle on different types of pasture. The two most-common types are native rangeland, and winter wheat that is either grazed for its entire growing season, or grazed in the fall and cut for a grain crop in the spring. The soils that support these pastures undergo stress due to the climate and management. Understanding how soils respond to such stress is important to proper pasture management. We compared the response of soils in 4-acre experimental pastures on an upland site in central Oklahoma to 26 years of different forms of management. We included three different forms of management applied to native pasture (light stocking rates over long grazing periods, high stocking rates over short grazing periods, and no management), and grazed-out winter wheat under conventional tillage practices. The response of soil bulk density and organic matter were measured in the upper 10 inches of the soil profile of each pasture, along 150 yd transects situated between a common ridge and slope bottom, at 4.9 ft intervals. We found that soils of grazed wheat pasture were 11% denser, and contained 20% less organic matter, than soils under native prairie. Also, after 26 years, the effects of pasture management were confined to the upper 5 inches of soil, with more management (wheat, high stocking rates) producing soils that were 12% denser, with 15% less organic matter. Management regimes applied to pastures over several decades can change near-surface soil properties, which may alter how landscapes function.

Technical Abstract: Native rangeland and winter wheat are among the primary forage resources used to graze cattle in Oklahoma. Understanding the response of soil characteristics to stressors caused by pasture management is crucial to sustainable use of these resources. This study compared soil responses of 1.6 ha pastures on an upland site in central Oklahoma (35°33’29” N, 98°1’50” W), to 26 years (1977-2003) of different forms of sustained management. Included were grazed, conventionally tilled winter wheat, and three levels of grazing intensity (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 pastures under each form of management at 1.5 m intervals along 150 m transects (n=101 per paddock) situated between a ridge and toe slope, and divided into 3 sections (0-5 cm, 5-10 cm, and 10-25 cm depths). Soil bulk density and organic matter were determined, descriptive characteristics were calculated, and measures of spatial variability were defined. Soil profiles of grazed wheat pasture were 11% denser (1.20±0.11 vs 1.08±0.03 Mg m-3) and contained 20% less organic matter (33.7±6.0 vs 44.8±14.3 g kg-1) than soil profiles under native rangeland. Management effects were confined to the upper 10 cm of soil, with intensive forms of management (wheat, high stocking rates) producing soils that were 12% denser, with 15% less organic matter, than extensive management. Responses indicate that management regimes applied over several decades can change the population of near-surface soil properties, which may alter landscape function.