Organic management practices and mixed-crop rotations have beneficial impacts on biologically active soil organic matter (SOM); however, the specific impacts of these practices on SOM composition and turnover characteristics are not known. The effect of organic and conventional management on soil C cycling was assessed using soils collected from Rodale’s Farming Systems Trial experiment, which included organic-manure amended, organic-legume cover cropped, and conventionally managed-inorganically fertilized farming systems. Soil respiration rates, atom %13C, and cross polarization 13C nuclear magnetic resonance (CP-MAS 13C NMR) were used to trace the fate of 13C-labeled acetate added to soils incubated for 2 and 13 wk. Microbial activity and C turnover rates were initially greater in the manure-amended soil than in the other two soils. Acetate mineralization lagged in the cover-cropped soil during the first 2 wk. Low atom %13C in the conventional soil showed that, even though its soil respiration rates, and therefore overall microbial activity, equaled rates in the cover-cropped soil, microorganisms present in this soil metabolized significantly more of the applied acetate. By the end of the experiment, C cycling characteristics of the cover-cropped soil were similar to the manure-amended soil; NMR spectra indicated 13C was incorporated into new SOM in both of these soils. Soil respiration and 13C assimilation rates in the conventional soil remained relatively low. Microorganisms in the organic soils mineralized and respired more native C and assimilated more 13C than the conventional soil. Conversely, the conventional soil mineralized more and assimilated less 13C than the organic soils. Differences observed in this laboratory experiment may or may not reflect on C cycling occurring in the field.