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\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 increases the bouyant density (BD) of DNA and this causes the relative abundance of 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}). Variation in amplicon pool composition between fractions of \textsuperscript{13}C-labeled samples and their corresponding controls is readily observed in 'heavy' gradient fractions (partitioning along axis 2, Fig. 2). The amplicon pool composition of 'heavy' fractions of \textsuperscript{13}C-xylose and \textsuperscript{13}C-cellulose samples vary dramatically from corresponding controls and from each other, indicating that the microbial community has distinct responses to each of 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}). 1B}), as expected (cite).  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). Heavy fraction amplicon pools from samples that received \textsuperscript{13}C-xylose diverged from corresponding controls on days 1 through 7. Furthermore, amplicon pool composition varied across these days (Fig x) indicating dynamic changes in \textsuperscript{13}C-xylose assimilation with time. At day 14 and 30 heavy fractions from \textsuperscript{13}C-xylose labeled samples are no longer differentiated from corresponding controls indicating that \textsuperscript{13}C is no longer detectable in DNA. The decline in \textsuperscript{13}C-labelling of DNA is likely due to isotopic dilution resulting from assimilation of unlabeled C and/or due to cell turnover resulting from mortality (\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.