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\section{Results and Discussion}  In this study, we couple nucleic acid SIP with next generation sequencing (SIP-NGS) to observe microbial community soil C use dynamics. A series of parallel soil microcosms amendeded amended  with an identical C substrate mixture, in which the only difference is the identity of the $^{13}$C-labeled substrate, were incubated for 30 days. The C substrate 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. 5.3 mg of the C substrate mixture per gram soil (including 0.42 mg xylose-C and 0.88 mg cellulose-C g soil$^{-1}$) was added to each microcosm, representing 18\% of the total C present in the soils. Microcosms were harvested at several time points during the incubation period and the isotope assimilation by the microbial community was observed by sequencing 16S rRNA gene amplicons from bulk soil DNA and CsCl gradient fractions (\href{https://www.authorea.com/users/3537/articles/8459/master/file/figures/20140708_ConceptualFig2/20140708_ConceptualFig2.pdf}{Fig. S1}). Xylose degradation was observed immediately, while cellulose degradation was observed after two weeks. \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 ($\geq$1.72 g mL\textsuperscript{-1}) 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}).