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and activity of key microbial players. Bur first, we must identify which  microbial phylogenetic types decompose different SOM C components.  We %We  found that $^{13}$C substrate responders changed as much as X-fold in relative %relative  abundance over time (Figure~XX). This is in contrast to a %a  previous study CITE which suggested cellulose decomposers were found to be %be  consistent in relative abundance with time. Although presence in heavy fractions %fractions  can indicate $^{13}$C labeling, not all DNA in heavy fractions is %is  $^{13}$C-labeled. Some DNA is heavy due to high G+C. With %With  lower resolution fingerprinting techniques the banding pattern of SSU rRNA %rRNA  gene sequences can look similar across the entire density gradient CITE, %CITE,  however, high throughput sequencing of density gradient fractions shows %shows  light and heavy fractions are statistically different even when input %input  DNA is entirely unlabeled (Figure~\ref{fig:time_class}, and CITE). Hence, %Hence,  DNA-SIP studies that do not incorporate controls wherein amendments contain %contain  only $^{12}$C substrates, may confuse high G+C organisms with organisms %organisms  that incorporated $^{13}$C into biomass. %The succession hypothesis of decomposition predicts a succession from  %microbial types that use labile C to those that use recalcitrant polymeric