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\section{Discussion}   % Fakesubsubsection: We highlight to key results  We highlight two key results with implications for understanding structure-function  relationships in soils, and for applying DNA-SIP in future studies of the soil-C  cycle. First, cellulose responders were members of physiologically undescribed 
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comprehensively describe which microorganisms consume $^{13}$C-labeled  substrates in nucleic acid SIP incubations.  % Fakesubsubsection: Microorganisms that consumed  Microorganisms that consumed $^{13}$C-cellulose were not closely related to any  physiologically characterized cultured isolates but were members of  cosmopolitan phylogenetic groups in soil including \textit{Spartobacteria}, 
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plant biomass cellulose. These structural differences might select for different   decomposers and caution should be taken when extrapolating our results.  % Fakesubsubsection: In addition to taxonomic identity, we quantified  In addition to taxonomic identity, we quantified several ecological  properties of microorganisms that were actively engaged in labile and  structural C decomposition in an agricultural soil over time. Labile C was consumed 
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to changes in community composition CITE (see SI note 2 for further discussion  with respect to soil-C modelling).  % Fakesubsubsection: We propose that the temporal fluctuations  We propose that the temporal fluctuations in $^{13}$C-labeling in the  $^{13}$C-xylose treatment are due to trophic exchange of $^{13}$C.  Alternatively, the temporal dynamics could be caused by microorganisms tuned to  different substrate concentrations and/or cross-feeding. Only trophic exchange,  however, can account for the precipitous drop in abundance of  \textit{Paenibacillus} with subsequent $^{13}$C-labeling of \textit{Bacteroidetes} at day 3 followed by Micrococcales at day 7. Furthermore, \textit{Bacteroidetes} types have been shown to become $^{13}$C labeled after the addition of live $^{13}$C-labeled \textit{Escherichia coli} to soil CITE and a member of the \textit{Micrococcales}, \textit{Agromyces ramosus} (with which one of the $^{13}$C-xylose responders shared 100\% SSU rRNA gene sequence identity), is a known predator that preys on many microorganisms including yeast and \textit{Micrococcus luteus} CITE. \textit{Agromyces} are abundant microorganisms in many soils and the most abundant xylose responder in our experiment -- the fourth most abundant OTU in our pooled dataset -- shares 100\% SSU rRNA gene sequence identity to \textit{Agromyces ramosus}. ramosus} (see SI note 3 for further discussion of trophic C  exchange).  Climate change is expected to enhance the availability of labile C in soil and soil predators may mitigate the response of microorganisms to increased labile C CITE though the extent of bacterial predatory activity in soil is unknown. Elucidating the identities of bacterial predator in soil will assist in assessing the implications of climate change on global soil-C storage.\subsection{Implications for soil C cycling models}  % Fakesubsubsection: Models of soil  Functional niche characterization for soil microorganisms is necessary to  predict whether and how biogeochemical processes vary with microbial community  composition. Functional niches are defined by soil microbiologists and have  been successfully incorporated into biogeochemical process models (E.g.  \citep{wieder_2014a,Kaiser2014a}). In some C models ecological strategies such  as growth rate and substrate specificity are parameters for functional niche  behavior \citep{Kaiser2014a}. The phylogenetic breadth of a functionally  defined group is often inferred from the distribution of diagnostic genes  across genomes \citep{Berlemont2013} or from the physiology of isolates  cultured on laboratory media \citep{Martiny2013}. For instance, the wide  distribution of the glycolysis operon in microbial genomes is interpreted  as evidence that many soil microorganisms participate in glucose turnover  \citep{McGuire2010}. However, the functional niche may depend less on the  distribution of diagnostic genes across genomes and more on life history  traits that allow organisms to compete for a given substrate as it occurs  in the soil. For instance, fast growth and rapid resuscitation allow  microorganisms to compete for labile C which may often be transient in  soil. Hence, life history traits may constrain the diversity of microorganisms  that metabolize a given C source in the soil under a given set of  conditions.  % Fakesubsubsection: Biogeochemical processes  Biogeochemical processes mediated by a broad array of taxa are assumed  insensitive to community change relative to  processes mediated by a narrow suite of microorganisms  \citep{Schimel_1995,McGuire2010}. In addition, the diversity of  a functionally defined group engaged in a specific C transformation is  expected to correlate positively with C lability \citep{McGuire2010}.  However, the diversity of labile C and structural C decomposers in soil  has not been quantified directly. We found comparable numbers of OTUs  responded to $^{13}$C-cellulose and $^{13}$C-xylose (63 and~49,  respectively). Cellulose responders were phylogenetically clustered  suggesting that the ability to degrade cellulose is phylogenetically  conserved. The clade depth of cellulose responders, 0.028 SSU rRNA gene  sequence dissimilarity, is on the same order as that observed for  glycoside hydrolases which are diagnostic enzymes for cellulose  degradation \citep{Berlemont2013}. Xylose responders clustered in terminal  branches indicating groups of closely related taxa metabolized xylose but  xylose responders also clustered phylogenetically with respect to time of  response (Figure~\ref{fig:tiledtree}, Figure~\ref{fig:xyl_count}).  For example, xylose responders on day~1 are dominated by members of  \textit{Paenibacillus}. Thus, microorganisms that degraded labile C and  structural C were both limited in diversity. Although the genes for xylose  metabolism are likely widespread in the soil community, it's possible only  a limited diversity of organisms had the ecological characteristics  required to degrade xylose under experimental conditions. Therefore it's  possible that only a limited number of taxa actually participate in the  metabolism of labile C-sources under a given set of conditions, and hence  changes in community composition may alter the dynamics of structural  \textit{and} labile C-transformations in soil.  % Fakesubsubsection: Broadly, we observed  Broadly, we observed labile C use by fast growing generalists and structural  C use by slow growing specialists. These results agree with the MIMICS model  which simulates leaf litter decomposition by modeling microbial decomposers  as two functionally defined groups, copiotrophs or oligotrophs  \citep{wieder_2014a}. Including these functional types improved predictions  of C storage in response to environmental change. We identified  microbial lineages engaged in labile and structural C decomposition that  can be defined as copiotrophs or oligotrophs, respectively. We highlight two  additional considerations for soil-C process models based on our results.  First, soil-C may travel through multiple trophic levels within the bacterial  community where each C transfer represents an opportunity for C stabilization  in association with soil minerals or C loss by respiration. And second,  although labile C consumption is generally considered to be a broad process in  terms of microbial participants, we observed that only a small number of  related OTUs conclusively consumed xylose-C (see SI for additional discussion) and  that fast growth, as opposed the ability to use xylose, may constrain the diversity  of microorganisms that process labile-C \textit{in situ} which may often be  pulse delivered and transient. The diversity of microbial participants in a  biogeochemical process is thought to determine how robust process rates are to  changes in community composition. Our understanding of soil C dynamics will  likely improve as we develop a more granular understanding of the ecological  diversity of microorganisms that mediate C transformations in soil.  \subsection{Conclusion}   % Fakesubsubsection: Microorganisms sequester atmospheric C