Acetate fate in organic and conventionally managed soils

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.