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Although presence in heavy  fractions can indicate $^{13}$C labeling, not all DNA in heavy fractions  is $^{13}$C-labeled. Some DNA is heavy due to high G+C.  With lower resolution fingerprinting techniques the banding pattern of SSU  rRNA gene sequences can look similar across the entire density gradient  CITE, however, high throughput sequencing of density gradient fractions  shows light and heavy fractions are statistically different even when  input DNA is entirely unlabeled (Figure~\ref{fig:time_class}, and CITE).  Hence, DNA-SIP studies that do not incorporate controls wherein amendments  contain only $^{12}$C substrates, may confuse high G+C organisms with  organisms that incorporated $^{13}$C into biomass.  marine and atmospheric SOM represents more C than the marine and atmospheric  C reservoirs combined and approximately 80\% of SOM flux is mediated by  microorganisms CITE. SOM decomposition is more sensitive to temperature changes