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'Heavy' fraction amplicon pools from samples that received $^{13}$C-xylose  diverged from corresponding controls on days 1 through 7 . Furthermore,  amplicon pool composition varied across these days indicating dynamic changes  in $^{13}$C-xylose assimilation with time. At days 14 and 30 heavy fractions  from $^{13}$C-xylose labeled samples are no longer differentiated from  corresponding controls indicating that $^{13}$C is no longer detectable in DNA.  The decline in $^{13}$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.   There were 6 shared responders among all unique responders identified in both  the xylose and cellulose treatments (n = 72); Stenotrophomonas, Planctomyces,  two Rhizobiaceae, Comamonadaceae, and Cellvibrio. Of these, Stenotrophomonas  and Comamonadaceae are the only taxa that are among the top ten l2fc responses  measured in both treatments. On the other hand, the only shared responder that  is not among the top ten responders for either the cellulose or xylose  treatment is Rhizobiaceae. Two of the shared responders corresponded in time  between the two treatments  many taxa important to cellulose cycling are present in the rarer fraction of the  overall microbial community.  \textbf{Patterns of carbon use vary dramatically within phylum.} Dynamic  patterns of $^{13}$C-assimilation from xylose and cellulose occur at discrete,  fine-scale taxonomic units . Responders for xylose and cellulose are widespread  across 6 and 7 phyla, respectively . There are 5 phyla containing responders  for both treatments; of all the responder OTUS detected within those phyla for  either xylose or cellulose, there are only six OTUs that respond to both xylose  and cellulose (discussed previously). This result suggests that phyla do not  represent coherent ecological units with respect to the soil C-cycle, that is,  taxa within phyla exhibit differences in substrate use, level of substrate  specialization, and dynamics of incorporation.   In this study, we have identified Actinobacteria responders for both substrates.   Although there were no shared Actinobacteria OTUs that responded to both  xylose (Microbacteriaceae, Micrococcaceae, Cellulomonadaceae, Nakamurellaceae,  Promicromonosporaceae, and Geodermatophilaceae) and cellulose  (Streptomycetaceae and Pseudonocardiaceae). This information may suggest that  while Actinobacteria exhibit an ability to utilize an array of carbon  substrates, substrate use may be more clade specific and not widespread  throughout the phylum . Similarly, Bacteroidetes responders were identified for  both substrates, yet, at a finer taxonomic resolution there is a clear  differential response for xylose (Flavobacteriaceae and Chitinophagaceae) and  cellulose (Cytophagaceae).   Whole phylum responses were not detected for xylose or cellulose yet  utilization of these substrates spanned many phylogenetically diverse groups.  Within each phylum we observed substrate utilization at the clade or single  taxa level with each exhibiting a unique pattern of $^{13}$C-assimilation over  time. It has previously been suggested that all taxa within a phylum are  unlikely to share ecological characteristics \citep{Fierer_2007}, and  furthermore, within a species population  \citep{Choudoir_2012,Preheim_2011,Hunt_2008}. Habitat traits of coastal Vibrio  isolates were mapped onto microbial phylogeny revealing discrete ecological  populations based on seasonal occurrence and particulate size fractionation  \citep{Preheim_2011,Hunt_2008}. Yet, it has been proposed that the microbial  community functionality responsible for soil C cycling appear at the level of  phlya rather than species/genera \citep{Schimel_2012}. The traditional phylum  level assignment conventions could in part be due to limitations in finer scale  taxonomic identifications or methodological limitations (\textit{i.e.}  sequencing depth). Our data in concert with others  \citep{Goldfarb_2011,Fierer_2007,Choudoir_2012,Preheim_2011,Hunt_2008} would  suggest that assigning substrate utilization of a few OTUs or clades as a  phylum level response is not accurate.  \textbf{Conclusions.} We have demonstrated how next generation  sequencing-enabled SIP gives an OTU level resolution for substrate utilization.  Using this technique, we are able to resolve discrete OTUs that would otherwise  be missed using bulk community sequencing efforts. Additionally, this technique  provides greater taxonomic resolution than previous techniques (cloning, TRFLP,  ARISA) used to determine substrate utilizing community members. While we are  currently able to resolve highly responsive OTUs, there is still a need to  resolve taxa that are partially responsive which we cannot differentiate from  noise with confidence at this time. Although, if we could identify partially  responsive taxa, their contributions to the C-cycle would still be difficult to  discern. For example, a generalist utilizing many substrates including $^{12}$C  substrates and the $^{13}$C-labeled substrate may exhibit the same partial  labeling that a specialist utilizing both the $^{13}$C-substrate and the same  substrate (unlabeled) that is inherent in the soil. Additionally, partially  labeled taxa could be further down the trophic cascade including predators or  secondary consumers of waste products from primary consumer microbes that were  highly labeled.   OTUs that assimilate xylose and those that assimilate cellulose are largely  mutually exclusive. Those OTUs that assimilate xylose are labeled within 1-7  days, while those that assimilate cellulose are labeled primarily after 2-4  weeks. The xylose responders demonstrate a smaller change in BD than the  cellulose responders suggesting that xylose responders assimilate multiple C  sources (labeled and unlabeled) consistent with a generalist response, while  cellulose responders are more heavily labeled suggesting that cellulose is  their main source of C, a response more consistent with a specialist lifestyle.  Xylose responders include many taxa, such as spore-fomers, known for the  ability to respond rapidly to an influx of new nutrients while cellulose  responders include many OTUs that are common uncultivated soil organisms.  Finally, xylose responders are more abundant in the community while cellulose  responders are, on average, more rare as indicated by their rank abundance  within the soil community. These results indicate that different bacteria in  soil have distinct physiological and ecological responses which govern their  interactions with soil C pools.   We did not observe consistent C utilization at the phylum level although both  xylose and cellulose utilization were observed across 7 phyla each revealing a  high diversity of bacteria able to utilize these substrates. The high taxonomic  diversity may enable substrate metabolism under a broad range of environmental  conditions \citep{Goldfarb_2011}. Other studies of microbial communities have  observed a positive correlation with taxonomic or phylogenetic diversity and  functional diversity  \citep{Fierer_2012,Fierer_2013,Philippot_2010,Tringe_2005,Gilbert_2010,Bryant_2012}.  The data presented here supports that specific functional attributes can be  shared among diverse, yet distinct, taxa while closely related taxa may have  very different physiologies \citep{Fierer_2012,Philippot_2010}. This  information adds to the growing collection of data suggesting that community  membership is important to biogeochemical processes. Furthermore, it highlights  a need to examine substrate utilization by discrete microbial taxa within a  whole community context to better understand how specific community members  function within the whole.   The sensitivity of SIP-NGS provides a means to elucidate substrate utilization  by discrete microbial taxa with the hope that we can begin to construct a  belowground C food web. We obtained enough information to conclusively  determine isotope incorporation for 61\% of the more than 6,000 OTUs detected.  For those OTUs with enough information (n = 3,825), approximately 2\% (n = 72)  significantly assimilated $^{13}$C from either xylose or cellulose. In the  future deeper sequencing will enable us to increase coverage and assess C use  by more community members. Using the informations we gain from SIP-NGS, we can  expand our knowledge of specific C-cycling OTUs by taking a targeted  metagenomic approach in the nucleic acid pools of 'heavy' fractions.  Furthermore, we can now expand our knowledge of soil C use dynamics to a wide  array of C substrates and increase our grasp on specific community member  contributions. Illuminating these microbial contributions associated with  decomposition in soil are important because as environments change, there are  measurable and functional changes in soil C \citep{Grandy_2008} which could  cumulatively have large impacts at a global scale.