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\subsection{Conclusion}   % Fakesubsubsection:SOM represents more C than the  We found that $^{13}$C substrate responders changed as much as X-fold in  relative abundance over time (Figure~XX). This is in contrast to  a previous study CITE which suggested cellulose decomposers were found to  be consistent in relative abundance with time.   The succession hypothesis of decomposition predicts a succession from  microbial types that use labile C to those that use recalcitrant polymeric  C over time CITE. Cellulose degraders succeeded labile C degraders as  predicted. But, in response to $^{13}$C-xylose, \textit{Firmicutes}  phylotypes were succeeded by \textit{Bacteroidetes} which were then  succeeded by \textit{Actinobacteria} representing a nested succession  (Figure~XX).   Soluble C more robust to temperature changes because or redundancy? But we  found similar numbers of xylose and cellulose degrader  Microorganisms sequester atmospheric carbon and respire soil organic matter  (SOM) influencing climate change on a global scale but which microbial lineages  transform different soil C components are not established. Molecular tools will  unravel the soil microbial food web and reveal how specific microbial lineages  impact soil C flux. We present a cultivation independent, molecular, high resolution  DNA-SIP method to chart fine-scale  C use into microbial lineages. This approach allows us to resolve discrete OTUs that would otherwise  be missed using fingerprinting techniques or bulk community sequencing efforts.  Our results show physiologically undefined cosmopolitan microbial lineages decompose cellulose. We also show  phylogenetic groups rise and fall and are supplanted by others in activity over  7 days in response to labile C addition. OTUs that assimilate xylose and those that assimilate cellulose are largely mutually exclusive.  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 The succession hypothesis of decomposition predicts  a need to  resolve taxa that are partially responsive which we cannot differentiate succession  from noise with confidence at this time. Although, if we could identify partially  responsive taxa, their contributions microbial types that use labile C  to the C-cycle would still be difficult those that use recalcitrant polymeric  C over time CITE. Cellulose degraders succeeded labile C degraders as  predicted. But, in response  to discern. For example, $^{13}$C-xylose, \textit{Firmicutes}  phylotypes were succeeded by \textit{Bacteroidetes} which were then  succeeded by \textit{Actinobacteria} representing  a generalist utilizing many substrates including $^{12}$C  substrates and the $^{13}$C-labeled substrate may exhibit the same partial  labeling nested succession  (Figure~XX). We found  that a specialist utilizing both the $^{13}$C-substrate and the same $^{13}$C  substrate (unlabeled) that responders changed as much as X-fold in  relative abundance over time (Figure~XX). This  isinherent  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 contrast to  a previous study CITE which suggested  cellulose are labeled primarily after 2-4  weeks. decomposers were found to  be consistent in relative abundance with time.  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 
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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 within a  phylumlevel  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 
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whole community context to better understand how specific community members  function within the whole.   The sensitivity of SIP-NGS HR-SIP  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.