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\section{Results and Discussion}  \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, 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 Planctomycetes, 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}. Our bulk data demonstrates a positive correlation between labile C availability and Bacteroidetes abundance which is consistent with the findings of 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 (RA) 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 contain \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 (2.18mgC g\textsuperscript{-1} soil).   \textbf{Differential taxa C utilization.} Using fractions from within a density range of 1.7125-1.755 gmL\textsuperscript{-1}, relative abundances of taxa in the experimental treatments were compared to their 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 taxa with time. 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'.   For the \textsuperscript{13}C-xylose treatment at day 1 OTUs within Firmicutes demonstrate the strongest response. This is not surprising as it has been demonstrated that Firmicutes maintain a metabolically-ready state \cite{Jenkins_2010}\cite{Griffiths_1998}\cite{Brookes_1987}\cite{De_Nobili_2001}. \cite{Jenkins_2010,Griffiths_1998,Brookes_1987,De_Nobili_2001}.  Additionally, Proteobacteria, Bacteroidetes, and Actinobacteria contain responder OTUs at day 1. Genomic analysis of fast growing bacteria, specifically Proteobacteria and Firmicutes, have a higher number of total transporters enabling them to import or export a broad range of compounds \cite{Barabote_2005}. The low affinity of these transporters facilitates fast growth in times of high nutrient conditions \cite{Trivedi_2013}. Day 3 exhibits a strong increase in Bacteroidetes response and the onset of Verrucomicrobia reponders. Notably, this pronounced response by Bacteroidetes is not captured by the bulk community abundances. Day 7 demonstrates an increased response from Proteo- and Actinobacterial OTUs. While there is a slight increase in their abundances in the bulk community analysis at day 7, it would be difficult to differentiate that change from natural variation or methodological noise. All OTUs have a decreasing log\textsubscript{2} fold change at by  days 14 and 30, with only a single Firmicutes OTU passing the 'responder' criteria. For the \textsuperscript{13}C-cellulose treatment only one Proteobacteria passes the 'responder' criteria at day 3 and two OTUs (Proteobacteria and Chloroflexi) at day 7. This is expected since competition for a limited resource typically results in the dominance of one or a few populations with the highest growth rates \cite{Fontaine_2003}. By day 14, responders are detected in Proteobacteria, Verrucomicrobia, Chloroflexi, Planctomycetes, and Actinobacteria. The degradation of cellulose by Verrucomicrobia is consistent with recalcitrant carbon degradation by Verrucomicrobia in soil, aquatic, and anoxic rice patty soils \cite{Fierer_2013}\cite{Herlemann_2013}\cite{10543821}. \cite{Fierer_2013,Herlemann_2013,10543821}.  Fierer found Verrucomicrobia to be more than 50\% of their bulk community sequences which has strong implications of the importance of this taxa in soil carbon cycling \cite{Fierer_2013}. In the \textsuperscript{13}C-cellulose treatment the same responders were detected at day 30 as with earlier time points, with the exception of Actinobacteria and the addition of Bacteroidetes. The cellulose degrader trends are more readily observable in the bulk community abundances than discerned with xylose responders. This is likely due to the low abundance of these phlya, where changes in bulk community abundance are more pronounced and easier to detect. Comparatively, phlya of consistently high abundance mask response changes unless they present changes of grand proportions. Kernel density estimates (KDE) of the CsCl density shifts measured for responders in \textsuperscript{13}C-xylose were compared to those of \textsuperscript{13}C-cellulose responders (Fig 3A). An organism with 100\% 13C-labeling of DNA would exhibit a density shift of 0.04gmL\textsuperscript{-1}. Xylose utilizers have a smaller density shift (<0.02 gmL\textsuperscript{-1}) than cellulose utilizers (0.005-0.03 gmL\textsuperscript{-1}), with few exceptions. This suggests a greater substrate specificity among cellulose degraders than xylose degraders. Partial \textsuperscript{13}C-labeling (<0.04gmL\textsuperscript{-1} density shift) could be a result of various lifestyles ('trophic strategy' better word choice?) such as (1) assimilation of C from multiple substrates (both \textsuperscript{12}C and \textsuperscript{13}C in this instance) or (2) \textsuperscript{13}C-label dilution as it cascades through trophic levels via consumption of \textsuperscript{13}C-labeled organisms or waste products from organisms that are metabolizing the \textsuperscript{13}C-substrate.