Technical Abstract: A combination of biological analysis, such as incubation and microbial biomass determination, 13C and 14C tracers, soil fractionation, and matrix analysis (LF, POM, silt, and clay) with molecular structure analysis (py-MBMS) on long-term sites with a C3 ' C4 crop switch provided the tools for determining the interacting controls in SOC dynamics. Mean residence times vary from a few months to 4967 yr. Long-term incubation, in which the soil microbiota decomposed 33% of the SOC and 58% of its N, produced microbial metabolites and transferred materials between pools. Our use of py-MBMS is shown for a cultivated, Colorado Grassland soil before and after an 853 dy incubation (Figure 1). The positive values for the ion intensity represent m/z species present in higher concentrations at day zero than after 853 days. Peaks for amino acids dropped significantly and show the source of some of the 58% of the soil N that was mineralized relative to the 33% of the soil C lost during incubation. The peaks representing carbohydrates and lignin also dropped during the incubation reflecting the drop in both C3 and C4 constituents of the LF and POM during incubation. Our work has shown microbial production and transfers of materials between pools.