Carbon dynamics of surface residue- and root-derived organic matter under simulated no-till

No-till practices have the potential to increase soil organic C, but little is known about the relative contribution of surface residue and roots to s oil organic C accumulation, In a simulated no-till experiment, we studied t he fate of C-14-labeled surface residue and in situ roots during a 1-yr inc ubation. Soil samples collected during the incubation were chemically dispe rsed and separated into five particle size and density fractions. The organ ic C, C-14, and total N content of each fraction was determined, Alkali tra ps were used to measure (14C) losses due to respiration. After 360 d, 66% o f the C-14 contained in the surface residue on Day 0 had been respired as ( CO2)-C-14, 11% remained in residue on the soil surface, and 16% was in the soil. In comparison, 56% of the root-derived C-14 in the soil was evolved a s (CO2)-C-14 and 42% remained in the soil. The large (500-2000 mu m) and sm all (53-500 mu m) particulate organic matter (POM) fractions together conta ined 11 to 16% of the initial root-derived C-14 in the soil, In contrast, P OM contained only 1 to 3% of the inital surface residue-derived C-14. These data show clear differences in the partitioning of surface residue- and ro ot-derived C during decomposition and imply that the beneficial effects of no-till on soil organic C accrual are primarily due to the increased retent ion of root-derived C in the soil.