Soil organic C and N distribution for wheat cropping systems after 20 years of conservation tillage in central Texas

Long-term conservation tillage and cropping intensity may alter the depth distribution of soil organic C and N. The objectives of this study were to investigate the impacts of conventional tillage (CT), no tillage (NT), and wheat cropping sequences on the depth distribution of dissolved organic C (DOC), soil organic C (SOC), and total N in a central Texas soil after 20 years. Soil was sampled for six depth intervals ranging from 0 to 105 cm. Conventional tillage consisted of disking, chiseling, ridging, and residue incorporation into soil, while residues remained on the soil surface for NT. The depth distribution of DOC was similar to SOC. Tillage impacts on DOC, SOC, and total N were primarily observed in surface soil (0-5 cm) under continuous wheat but also in subsurface soil depth intervals down to 55 cm for more intensive cropping sequences. On average, NT increased SOC, DOC, and total N compared to CT by 28, 18, and 33%, respectively. Soil organic C and,e tal N were highest at 0-5 cm and decreased with depth to 30-55 cm, below which few tillage or cropping sequence effects were observed. The depth distribution of SOC and total N indicated treatment effects below levels of the maximum tillage depth, while intensive cropping increased SOC and total N for NT compared to CT to a greater depth than for monoculture wheat. High intensity cropping sequences, coupled with NT, resulted in the highest soil organic matter levels in subsurface soils, demonstrating the importance of subsurface C and N storage for potential mitigation of greenhouse gases. (c) 2006 Elsevier B.V. All rights reserved.