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Analysis of the sequenced bulk community DNA demonstrates Proteobacteria (26-35\%), Actinobacteria (19-26\%), and Acidobacteria (12-21\%) as the most dominant phyla throughout the duration of the experiment. This is consistent with previous observations \cite{Goldfarb_2011, Fierer_2007, Rui_2009}. We found trends of Proteobacteria and Actinobacteria decreasing and Acidobacteria increasing as C availability declines (TableS1). This is congruent with findings in soils sampled from a wide range of ecosystems in the US \cite{Fierer_2007}. At days 1, 3, and 7 the bulk community was composed of ~12-18\% Bacteriodetes and Firmicutes combined. At days 14 and 30, these phyla declined to a combined 7-9\% of the whole community accompanied with an increase in Plancktomycetes, Verrucomicrobia, and Chloroflexi (2-3\% at day 1 to 5-7\% at day 30). There has been conflicting evidence about the correlation of Bacteriodetes abundance with C availability \cite{Fierer_2007, Rui_2009, Sharp_2000}, however, our data is consistent with Fierer (2007). Additionally, the abundance of Planctomycetes, Verrucomicrobia, and Chloroflexi at later stages of decomposition are in accord with findings in wheat straw degradation \cite{Bastian_2009}. The rank abundance of the community depicts transitions we observe in high ranking phyla abundance beginning at day 7 (Fig S2A). Despite the flucations we observe at the phylum level, the biological variability observed over time is low (FigS2B) demonstrating community stability.   Twenty fractions from a CsCl gradient fractionation for each treatment at each time point were sequenced (Fig. S1). Using NMDS analysis from weighted unifrac distances, the relationship between microbial communities at each buoyant density from all treatments and time points are plotted (Fig 1). Each point on the NMDS represents the microbial community based on 16S sequencing from a single fraction where the size of the point is representative of the denisty of that fraction and the colors represent the treatments (Fig1A) or days (Fig1B). The high-density fractions that are differentiating from the control along NMDS2 correspond to fractions that control \textsuperscript{13}C-labeled OTUs (herein called 'responders'). The differential separation of high density fractions in the \textsuperscript{13}C-xylose treatment compared to the \textsuperscript{13}C-cellulose treatment is indicative of a difference in the responders for each of the substrates (Fig 1A). There is an observable time signature of responders at days 1,3, 1, 3,  and 7 for the xylose treatment and days 14 and 30 for the cellulose treatment (Fig1B). This demonstrates that different microbial community members are responsible for the consumption of these two substrates; xylose is consumed quickly, whereas, cellulose decomposition takes longer. This supports the hypothesis of a microbial community succession during the decomposition process. Furthermore, this demonstrates the sensitivity of this technique by being able to detect \textsuperscript{13}C-label incorporation in samples with low C additions (2.18mgC g\textsuperscript{-1} soil). \textbf{Differential taxa C utilization.} Using fractions from within a denisty range of 1.7125-1.755 g/ml, relative abundances of phyla in the experimental treatments were compared to the respective relative abundances in the control treatment to calculate the log\textsubscript{2} fold change (Fig2). The log\textsubscript{2} fold change demonstrates the boom and bust of phyla with time. More notably, it shows that phylum level inferences are misrepresentative, as only a few OTUs within a phylum are responders. To portray the response of a few OTUs or clades as a phylum level response would be overreaching. For OTUs passing a conservative threshold of \textit{p}-value = <0.10 for log\textsubscript{2} fold change (FigSx), we measured the density shift in the experimental treatment compared to the control (Fig Sx). Those OTUs with a center of mass shift greater than zero were considered 'responders'.