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\textbf{Temporal microbial succession during C degradation.} With the rapid advancement and declining costs of high throughput sequencing, it has become increasingly easy to investigate microbial communities. In this study, we couple stable-isotope probing with 454 pyrosequencing in order to better understand organic matter decomposition dynamics as a function of soil microbial community C utilization. We ran a temporal series of parallel microcosms and measured the changes in the microbial community as result of the addition of a complex carbon mixture using 454 pyrosequencing of the bulk microbial community and fractions from CsCl gradient fractionation (Fig. S1). Overall, temporal changes in microbial community composition are consistent with C decomposition being accompanied by a microbial community succession.   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}. 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). Abundance There has been conflicting evidence about the correlation  of Bacteriodetes have been shown to be positively correlated abundance  with C availability \cite{Fierer_2007}.   Despite these changes, \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 fluctuates minimally over time depicts transitions we observe in high ranking phyla abundance beginning at day 7  (Fig S2A). accompanied with Despite the flucations we observe at the phylum level, the biological variability observed over  time associated is low (FigS2B) demonstrating  community shifts (FigS2B). 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 NMDSrepresents 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, 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 (blah mg g\textsuperscript{-1} soil).