CROP-ROTATION AND TILLAGE EFFECTS ON ORGANIC-CARBON SEQUESTRATION IN THE SEMIARID SOUTHERN GREAT-PLAINS

Limited information is available regarding soil organic carbon (SOC) d istribution and the total amounts that occur in dryland cropping situa tions in semiarid regions. We determined crop rotation, tillage, and f ertilizer effects on SOC distribution and mass in the semiarid souther n Great Plains. A cropping system study was conducted for 10-years at Bushland, TX, to compare no-till and stubblemulch management on four d ryland cropping systems: continuous wheat (CW) (Triticum aestivum L.); continuous grain sorghum (CS) (Sorghum bicolor [L.] Moench.); wheat/f allow/sorghum/fallow (WSF); and wheat/fallow (WF). Fertilizer (45 kg N ha(-1)) was added at crop planting to main plots. Subplots within eac h tillage and cropping treatment combination received no fertilizer. T en years after treatment initiation, soil cores were taken incremental ly to a 65-cm depth and subdivided for bulk density and SOC determinat ion. The no-till treatments resulted in significant differences in SOC distribution in the soil profile compared with stubblemulch tillage i n all four crop rotations, although differences were largest in the co ntinuous cropping systems. Continuous wheat averaged 1.71% SOC in the surface 2 cm of soil compared with 1.02% SOC with stubblemulch tillage . Continuous sorghum averaged 1.54% SOC in the surface 2 cm of soil in no-till compared with 0.97% SOC with stubblemulch tillage. Total SOC content in the surface 20 cm was increased 5.6 t C ha(-1) in the CW no -till treatment and 2.8 t C ha(-1) in the CS no-till treatment compare d with the stubblemulch treatment. Differences were not significantly different between tillage treatments in the WF and WSF systems. No-til l management with continuous crops sequestered carbon in comparison to stubblemulch management on the southern Great Plains. Fallow limits c arbon accumulation.