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 soil organic C accumulation. In a simulated no‐till experiment, we studied the fate of 14 C‐labeled surface residue and in situ roots during a 1‐yr incubation. Soil samples collected during the incubation were chemically dispersed and separated into five particle size and density fractions. The organic C, 14 C, and total N content of each fraction was determined. Alkali traps were used to measure 14 C losses due to respiration. After 360 d, 66% of the 14 C contained in the surface residue on Day 0 had been respired as 14 CO 2 , 11% remained in residue on the soil surface, and 16% was in the soil. In comparison, 56% of the root‐derived 14 C in the soil was evolved as 14 CO 2 and 42% remained in the soil. The large (500–2000 μm) and small (53–500 μm) particulate organic matter (POM) fractions together contained 11 to 16% of the initial root‐derived 14 C in the soil. In contrast, POM contained only 1 to 3% of the inital surface residue–derived 14 C. These data show clear differences in the partitioning of surface residue– and root‐derived C during decomposition and imply that the beneficial effects of no‐till on soil organic C accrual are primarily due to the increased retention of root‐derived C in the soil.