Dynamics and turnover of soil organic matter as affected by tillage

Greater knowledge of the dynamics of organic matter in different locations in the soil matrix can provide valuable information for implementing tillage practices that may favor C sequestration and improve soil quality. The objective of this study was to investigate the effect of tillage practices on the dynamics and turnover of organic matter located outside (free light fraction [FLF]) and inside (occluded light fraction [OLF]) aggregates, and in intimate association with soil minerals (heavy fraction [HF]). Composite soil samples from 11-yr-old corn (Zea mays L.) plots under no-till (NT) and conventional tillage (CT) practices, and from >60-yr--old tobacco (Nicotiana tabacum L.)-rye (Secale cereale L.) plots under CT were fractionated into these fractions, and changes in the amounts of total soil organic C (SOC), corn-derived (C-4) C, and native (C-3) C and the turnover of native C as a consequence of tillage were estimated. Adoption of NT increased (P < 0.05) standing SOC and C-4-C stocks in whole soil and all its density fractions in the 0- to 5-cm depth, but it did not alter these in the plow layer (0-20-cm soil depth), indicating that NT primarily redistributed C within the profile without necessarily increasing SOC storage. Relative to CT NT slowed the turnover of native C by 1. 5 times and marginally (P = 0. 1) increased the sequestration of C-4-C by 23% in the plow layer in OLF only; however, C-3 and C-4-C of OLF accounted for only small (6-10%) proportions of the soil's C-3- and C-4-C. Therefore, NT did not result in increased SOC sequestration in the plow layer.