Thursday, February 8, 2007

Soil Carbon Sequestration in Kansas: Long-Term Effect of Tillage, N Fertilization, and Crop Rotation.

Karina Fabrizzi1, Charles W. Rice1, and R. CÚsar Izaurralde2. (1) Kansas State University, 2004 Throckmorton Plant Sciences Center, Manhattan, KS 66506-5501, (2) Joint Global Change Research Institute, College Park, MD 20740

Soil C sequestration in agricultural provides an option to mitigate increases in the atmospheric CO2. Management practices to reduce C loss include reduced tillage intensity, a reduction in bare fallow, and enhanced rotations. However, determining rates of soil C sequestration vary with previous history, soil, and climate.  The objectives of this study were to determine the influence of long-term management practices on soil C content and estimates of C sequestration rates. Four long-term experiments in Kansas were sampled for soil organic carbon (SOC). All sites evaluated (Tribune, Manhattan, Parsons, Hays) included tillage as a variable: conventional tillage (CT), reduced tillage (RT) and no-tillage (NT). Nitrogen (N) fertilization was evaluated two sites (Parsons and Hays). Crop rotations, studied at Manhattan, included combinations of wheat, sorghum, and soybean. Soil samples were taken at depths of 0-5, 5-15 and 15-30 cm for bulk density and soil organic C. Rates of C sequestration were calculated by two methods, reference to initiation of the practice and reference to the standard management practice.  Soil organic C was significantly greater under NT at 0-5 cm and tended to be greater under NT at 0-30 cm.  Bare fallow negatively affected C sequestration even under NT. Nitrogen fertilization increased C sequestration. Rotations that contained wheat or sorghum had the greatest C sequestration rates, while continuous soybean had the lowest rates. We found a factor of 1.14 for change from CT to NT, and 1.08 from CT to RT.  Choosing the appropriate reference is critical to documenting changes in soil C.  Depending on state of the conventional systems, degrading, aggrading, or at equilibrium the C sequestration rate changes.  Site variability influences SOC dynamics and the resulting calculation of C sequestration; thus, both the baseline and the change in practice should be reported.