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\section{Results and Discussion}  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 mg C mixture  g soil\textsuperscript{-1} (including 0.42 mg xylose-C and 0.88 mg cellulose-C g soil\textsuperscript{-1}) was added to each microcosm, representing 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-xylose and \textsuperscript{13}C-cellulose assimilation varied dramatically within the microbial community. Isotope incorporation into DNA was revealed by analyzing variation in 16S 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-xylose or \textsuperscript{12}C-cellulose 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 increases the bouyant density (BD) of DNA and this causes the relative abundance of an 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 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 fractions 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}).