Dissolved and soil organic carbon after long-term conventional and no-tillage sorghum cropping

Distribution of dissolved (DOC) and soil organic carbon (SOC) with depth may indicate soil and crop-management effects on subsurface soil C sequestration. The objectives of this study were to investigate impacts of conventional tillage (CT), no tillage (NT), and cropping sequence on the depth distribution of DOC, SOC, and total nitrogen (N) for a silty clay loam soil after 20 years of continuous sorghum cropping. Conventional tillage consisted of disking, chiseling, ridging, and residue incorporation into soil, while residues remained on the soil surface for NT. Soil was sampled from six depth intervals ranging from 0 to 105cm. Tillage effects on DOC and total N were primarily observed at 0-5cm, whereas cropping sequence effects were observed to 55cm. Soil organic carbon (C) was higher under NT than CT at 0-5cm but higher under CT for subsurface soils. Dissolved organic C, SOC, and total N were 37, 36, and 66%, respectively, greater under NT than CT at 0-5cm, and 171, 659, and 837% greater at 0-5 than 80-105cm. The DOC decreased with each depth increment and averaged 18% higher under a sorghum-wheat-soybean rotation than a continuous sorghum monoculture. Both SOC and total N were higher for sorghum-wheat-soybean than continuous sorghum from 0-55cm. Conventional tillage increased SOC and DOC in subsurface soils for intensive crop rotations, indicating that assessment of C in subsurface soils may be important for determining effects of tillage practices and crop rotations on soil C sequestration.