Huggins, David (USDA-ARS, 215 Johnson Hall, Washington State University, Pullman, WA, 99164; Phone: 509-335-3379; Fax: 509-335-3842; Email: dhuggins@wsu.edu)

Tillage and Corn-Soybean Sequence Effects on Carbon Dynamics Estimated from Natural C-13 Abundance

D.R. Huggins *, R.R. Allmaras, C.E. Clapp, J.A. Lamb

Tillage and crop rotation are major regulators of soil organic carbon (SOC). Our objectives were to assess tillage and crop sequence effects on SOC storage and dynamics using natural ¹³C abundance. A randomized, split-plot design (four replications) with main plots consisting of tillage treatment: moldboard plow (MP), chisel plow (CP) and no-tillage (NT) and subplots of crop sequence: continuous corn (CC), continuous soybean (SS) and alternating corn-soybean (CS) was established in 1981. Soil samples were collected in each treatment after 14 years and analyzed for bulk density, pH, delta ¹³C values and SOC. Crop sequence effects on total SOC (0 to 45 cm) occurred in CP and NT. The CP treatment had 20% greater SOC in CC than SS, while NT had 15% greater SOC in CC than SS. Tillage effects on SOC were greatest in CC where CP had 30% and NT 20% more SOC than MP. Continuous soybean was the only crop sequence where tillage had no influence on SOC. Interaction between tillage and crop sequence on SOC resulted in CP treatments with corn and NT continuous corn to have the most SOC (averaging 165 Mg ha^-1), while all MP and SS treatments and NT treatments with soybean, the least SOC (averaging 137 Mg ha^-1). These results underscore the importance of both crop sequence and tillage in controlling SOC storage and dynamics.