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In this study, we couple DNA-SIP with high-throughput sequencing in order to better understand microbial community C use dynamics in soil. A series of parallel soil microcosms amendeded with an identical C mixture, in which the only difference is the identity of the \textsuperscript{13}C-labeled substrate, were incubated for 30 days. The C mixture was designed to approximate freshly degrading plant biomass and either xylose or cellulose were isotopically labeled to examine the dynamics of C assimilation for labile soluble C and insoluble polymeric C. A total of 5.3 mgC g\textsuperscript{-1} soil (including 0.42mg xylose-C g\textsuperscript{-1} soil and 0.88mg cellulose-C g\textsuperscript{-1} soil) was added to each microcosm and this represented 18\% of the total C present in the soils. Microcosms were harvested at discrete time points during the incubation period and the temporal and isotope assimilation dynamics of the microbial community were measured by sequencing 16S rRNA gene amplicons in the bulk microbial community and fractions from CsCl gradient fractionation (\href{https://www.authorea.com/users/3537/articles/8459/master/file/figures/20140708_ConceptualFig2/20140708_ConceptualFig2.pdf}{Fig. S1}). Xylose degradation was observed immediately within the first 7 days, while cellulose degradation is observed after 14 days.  \textbf{Temporal dynamics of C-assimilation in soil.}   The dynamics of \textsuperscript{13}C-cellulose and \textsuperscript{13}C-xylose assimilation varied dramatically within the microbial community. Isotope incorporation into DNA was revealed by analyzing variation in 16S rRNA amplicons across gradient fractions (n = 20) from control samples in relation to identical experimental samples that differed by a single substitution of \textsuperscript{12}C-cellulose or \textsuperscript{12}C-xylose with their \textsuperscript{13}C equivalents (\href{https://www.authorea.com/users/3537/articles/8459/master/file/figures/20140708_ConceptualFig2/20140708_ConceptualFig2.pdf}{Fig. S1}). Isotope incorporation changes amplicon composition relative to control in increases  the gradient fractions bouyant density (BD) of DNA  and this effect can be visualized in ordination by divergence causes the relative abundance  of experimental samples OTU to increase in amplicon pools  from 'heavy' fractions of the density gradient. As a result, isotopic incorporation into DNA will cause variation in amplicon pool composition in 'heavy' fractions containing isotopically-labeled DNA relative to  corresponding control points (\href{https://www.authorea.com/users/3537/articles/3612/master/file/figures/ordination_all1/ordination_all1.png}{Fig. fractions (\href{https://www.authorea.com/users/3537/articles/3612/master/file/figures/ordination_all1/ordination_all1.png}{axis 1, Fig.  1}). Primary variation of amplicon composition in gradient fractionsalong axis 1  is attributed to varying bouyant densities of genomes due to G+C content (\href{https://www.authorea.com/users/3537/articles/3612/master/file/figures/ordination_all1/ordination_all1.png}{Fig. 1}). Divergence due to isotope incorporation can be seen Variation  in high-buoyant density amplicon pool composition between  fractions that partition of \textsuperscript{13}C-labeled samples and their corresponding controls is readily observed in 'heavy' gradient fractions (partitioning  along axis 2 in Figure 1. 2, Fig. 2).  The differential divergence amplicon pool composition  of high density 'heavy'  fractions in the of  \textsuperscript{13}C-xylose treatment compared to the and  \textsuperscript{13}C-cellulose treatment is indicative of a difference in samples vary dramatically from corresponding controls and from each other, indicating that  the \textsuperscript{13}C-assimilating OTUs for microbial community has distinct responses to  each of the these  substrates (\href{https://www.authorea.com/users/3537/articles/3612/master/file/figures/ordination_all1/ordination_all1.png}{Fig. 1A}). Had the isotope incorportation from \textsuperscript{13}C-xylose and \textsuperscript{13}C-cellulose occured in the same community members, the divergence of the high-buoyant density fractions of these two treatments relative to control would have coincided in the ordination space. The \textsuperscript{13}C-incorporation reveals temporal dynamics of C degradation demonstrated by \textsuperscript{13}C-xylose incorporation at days 1, 3, and 7 and \textsuperscript{13}C-cellulose incorporation at days 14 and 30 (\href{https://www.authorea.com/users/3537/articles/3612/master/file/figures/ordination_all1/ordination_all1.png}{Fig. 1B}). In support of this, the bulk community sequencing demonstrates significant (pval) microbial community changes over time. Although within a single time point, the bulk community demonstrated no significant difference between treatments (Fig Sx). The temporal dynamics reveal the composition of \textsuperscript{13}C-xylose assimilating amplicons are different for each of the days the label is detected based on their separate distributions for each of the time points (\href{https://www.authorea.com/users/3537/articles/3612/master/file/figures/ordination_all1/ordination_all1.png}{Fig. 1B}). The disappearance of the \textsuperscript{13}C-xylose incorporation signature (relative to control) for days 14 and 30 result from loss of \textsuperscript{13}C-label in DNA over time. This occurs by dilution of \textsuperscript{13}C-label out of the DNA when a switch from \textsuperscript{13}C to \textsuperscript{12}C substrate utilization takes place during biomass turnover and/or predation. \textsuperscript{13}C-cellulose incorporation isn't detected until day 14 and amplicon composition is consistent for both days 14 and 30 (\href{https://www.authorea.com/users/3537/articles/3612/master/file/figures/ordination_all1/ordination_all1.png}{Fig. 1B}). The consistency of amplicon composition for cellulose degradation over time compared to xylose suggests a wider array of microorganisms utilize xylose, whereas, cellulose utilization occurs in a select few. This is consistent with long standing notions that more microorganisms are capable of utilizing simple carbohydrates than complex C substrates. Overall patterns of C degradation observed in this study demonstrate different microbial community members are responsible for the consumption of these two substrates; xylose is consumed quickly, whereas, cellulose decomposition takes longer. This suggests a pattern of microbial community transition accompanying the decomposition process. This is consistent with \cite{Engelking_2007} who observed as much as 75\% of labile C respired or converted into microbial biomass in the first 5 days of decomposition, whereas, cellulose degraders take longer to respond \cite{Hu_1997} with less than 42\% of cellulose metabolized over the first 5 days of incubation \cite{Engelking_2007}.