Biologically defined soil organic matter pools as affected by rotation and tillage

The importance of soil organic matter is well recognized; however, changes in C and N fractions are inadequately quantified. The objective of this study was to determine tillage and crop rotation effects on soil organic C and N fractions from a long-term (27-year) study in eastern Kansas. Cropping systems included continuous and rotation sequences of wheat (Triticum aestivum L.), grain sorghum (Sorghum bicolor (L.) Moench), and soybean (Glycine max (L.) Merrill) on a Muir silt loam (fine-silty, mixed, mesic Cumulic Haplustolls). Tillage included conventional (CT), reduced (RT), and no-till (NT). Total C and N (C-T and N-T) were determined on all treatments. Mineralizable C and N (C-o and N-o) and microbial biomass C and N were determined for the NT and CT soybean and sorghum rotations. Cropping systems that included wheat contained the greatest amount of C-T and N-T. Continuous wheat contained 2910 g C m(-2) and 287 g N m(-2), compared to 2225 g C m(-2) and 222 g N m(-2) (0-15 cm) for continuous soybean. No-tillage contained 1128 g C m(-2) and 109 g N m(-2) at 0-5 cm compared to 918 g C m(-2) and 87 g N m(-2) for CT. Sorghum contained 51% more C-o than soybean, and NT accounted for 59% more C-o than CT. More crop residue was produced and retained in rotations that included sorghum. No-tillage increased C 2440 kg ha(-1), while CT increased C 340 kg ha(-1) across all soybean/sorghum rotations. The highest sequestration rate (122 kg C ha(-1) y(-1)) was observed with NT sorghum and was equivalent to similar to 3.2% of the plant material (root and shoot, less gain harvest) remaining in the soil annually.