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\section{Results and Discussion}  In this study, we couple nucleic acid SIP with next generation sequencing  (SIP-NGS) to observe C use dynamics by the soil microbial community. A series  of parallel soil microcosms all amended with a C substrate mixture were  incubated for 30 days. The substrate mixture was identical for each bottle  except in one series of bottles the cellulose was $^{13}$C-labeled in another  the xylose was $^{13}$C-labeld and in the last no sustrattes were labeled. The  C substrate mixture was designed to approximate freshly degrading plant  biomass. Xylose or cellulose carried the isotopic label so we could examine C  assimilation dynamics for labile, soluble C versus insoluble, polymeric C. 5.3  mg total mass of 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 soil C. Microcosms were harvested at several time points  during the incubation period and $^{13}$C assimilation was observed by  sequencing 16S rRNA gene amplicons from bulk soil DNA and CsCl gradient  fractions. Assimilation of $^{13}$-C from Xylose degradation peaked immediately,   while cellulose $^{13}$C assimilation peaked after two weeks of incubation  (Figure~\ref{fig:ord}).  \subsection{$^{13}$C from cellulose assimilated by canonical  cellulose-degrading and uncharacterized microbial lineages in many phyla  including Chloroflexi and Verrucomicrobia}   Only 2 and 5 OTUs were found to have incorporated $^{13}$C from labeled  cellulose at days 3 and 7, respectively. At days 14 and 30, however, 42 and 39  OTUs were found to incorporate $^{13}$C from cellulose into biomass. An average  16\% of the $^{13}$C-cellulose added was respired within the first 7 days, 38\%  by day 14, and 60\% by day 30. A \textit{Cellvibrio} and  \textit{Sandaracinaceae} OTU assimilated $^{13}$C from cellulose at day 3. Day  7 responders included the same \textit{Cellvibrio} responder as day 3, a  \textit{Verrucomicrobia} OTU and three \textit{Chloroflexi} OTUs. 50\% of Day  14 responders belong to Proteobacteria (66\% Alpha-, 19\% Gamma-, and 14\%  Beta-) followed by 17\% \textit{Planctomycetes}, 14\% \textit{Verrucomicrobia},  10\% \textit{Chloroflexi}, 7\% \textit{Actinobacteria} and 2\%  \textit{Cyanobacteria}. \textit{Bacteroidetes} OTUs begin to incoporate  $^{13}$C from cellulose at day 30 (13\% of day 30 responders). Other day 30  responding phyla include \textit{Proteobacteria} (30\% of day 30 responders;  42\% Alpha-, 42\% Delta, 8\% Gamma-, and 8\% Beta-), \textit{Planctomycetes}  (20\%), \textit{Verrucomicrobia} (20\%), \textit{Chloroflexi} (13\%) and  \textit{Cyanobacteria} (3\%). \textit{Proteobacteria},  \textit{Verrucomicrobia}, and \textit{Chloroflexi} had relatively high numbers  of responders with heavy response across multiple time points  (Figure~\ref{fig:l2fc}).  \textit{Proteobacteria} represent 46\% of all cellulose responding OTUs  identified. \textit{Cellvibrio} accounted for 3\% of all Proteobacterial  responding OTUs detected. \textit{Cellvibrio} was one of the first identified  cellulose degrading bacteria and was originally described by Winogradsky in  1929 who named it for its cellulose degrading abilities  \citep{boone2001bergeys}. All $^{13}$C-cellulose responding  \textit{Proteobacteria} share high sequence identity with 16S rRNA genes from  sequenced type straints (Table XX) except for OTU.442 (best type strain match  92\% sequence identity in the \textit{Chrondomyces} genus) and OTU.663 (best  type strain match outside \textit{Proteobacteria} entirely,  \textit{Clostridium} genus, 89\% sequence identity). Some  \textit{Proteobacteria} responders share high sequence identity with type  strains for genera known to posess cellulose degraders including  \textit{Rhizobium}, \textit{Devosia}, \textit{Stenotrophomonas} and  \textit{Cellvibrio}. One \textit{Proteobacteria} OTU shares high sequence  identity with the \textit{Brevundimonas} type strain. \textit{Brevundimonas}  has not previously been identified as a cellulose degrader, but has been shown  to degrade cellouronic acid, an oxidized form of cellulose  \citep{Tavernier_2008}.  \textit{Verrucomicrobia}, a cosmopolitan soil phylum often found in high  abundance \citep{Fierer_2013}, are implicated in polysaccaride degradation in  many environments \citep{Fierer_2013,Herlemann_2013,10543821}.  \textit{Verrucomicrobia} comprise 16\% of the total cellulose responder OTUs  detected. 40\% of \textit{Verrucomicrobia} responders belong to the uncultured  FukuN18 family originally identified in freshwater lakes \citep{Parveen_2013}.  The \textit{Verrucomicrobia} OTU with the strongest \textit{Verrucomicrobial}  response to $^{13}$C-cellulose shared high sequence identity (97\%) with an  isolate from Norway tundra soil \citep{Jiang_2011} although growth on cellulose  was not assessed for this isolate. Only one other $^{13}$C-cellulose responding  verrucomicrobium shared high DNA sequence identity with a sequenced type  strain, OTU.638 with \textit{Roseimicrobium gellanilyticum} (100\% sequence  identity) and \textit{Roseimicrobium gellanilyticum} grows on soluble celluose  \citep{Otsuka_2012}. The remaining $^{13}$C-cellulose \textit{Verrucomicrobia}  responders did not share high sequence identity with any type strains (maximum  sequence identity with any type strain 93\%).   \textit{Chloroflexi} are traditionally known for their metabolically dynamic  lifestyles ranging from anoxygenic phototropy to organohalide respiration  \citep{Hug_2013}. Recent studies have focused on \textit{Chloroflexi} roles in C  cycling \citep{Hug_2013, Goldfarb_2011,Cole_2013} and several members of this  phylum demonstrated cellulose utilization \citep{Goldfarb_2011, Cole_2013,  Hug_2013}. Four closely related OTUs in an undescribed \textit{Chloroflexi}  lineage (closest matching type strain for all four OTUs: \textit{Herpetosiphon  geysericola}, 89\% sequence identity) responded to $^{13}$C-cellulose in this  microcosm experiment. One additional OTU also from a poorly characterized  lineage (closest type strain match a proteobacterium at 78\% sequence identity)  responded to $^{13}$C-cellulose (Figure~\ref{fig:trees}).  Other notable $^{13}$C cellulose responders include a \textit{Bacteroidetes}  OTU that shares high sequence identity (99\%) to \textit{Sporocytophaga  myxococcoides} a known cellulose degrader \citep{Vance_1980}, and three  \textit{Actinobacteria} OTUs that share high sequence identity (100\%) with  sequenced type strains. One of the three \textit{Actinobacteria}  $^{13}$C-cellulose responders is in the \textit{Streptomyces}, a genus known to  possess cellulose degraders, while the other two closely match the type strains  \textit{Allokutzneriz albata} \citep{Labeda_2008, Tomita_1993} and  \textit{Lentzea waywayandensis} \citep{LABEDA_1989, Labeda_2001} that do not  decompose cellulose in culture. Nine \textit{Plantomycetes} OTUs responded to  $^{13}$C-cellulose but none are within described genera (closest type strain  match 91\% sequence identity) (Figure~\ref{fig:trees}). Interestingly, one  responder is annotated as belonging in the \textit{Cyanobacteria}. The phylum  annotation is misleading as the OTU is not closely related to any oxygenic  phototrophs (closest type strain match \textit{Vampirovibrio chlorellavorus},  95\% sequence identity). A sister clade to the oxygenic phototrophs that does  not itself possess known phototrophs has recently been proposed to constitute  its own phylum ("Melainabacteria", \citet{Di_Rienzi_2013}) although its  phylogenetic position is debated \citep{Soo_2014}. The catalog of metabolic  capabilities associated with \textit{Cyanobacteria} (or candidate phyla  previously annotated as \textit{Cyanobacteria}) are quickly expanding  \citep{Di_Rienzi_2013, Soo_2014}. Our findings provide evidence of cellulose  degradation within a lineage closely related to but apart from oxygenic  phototrophs. Notably, polysaccharide degradation is suggested by the analysis  of a \textit{Melainabacterial} genome \citep{Di_Rienzi_2013}. Although we  highlight $^{13}$C-cellulose responders that share high sequence identity with  described genera, by and large $^{13}$C-cellulose responders uncovered in this  experiment are not closely related to isolates (Table~XX).  \subsection{Putative spore-formers in the Firmicutes assimilate $^{13}$C from  xylose within first day after soil amendment followed by Bacteroidetes and then  Actinobacteria OTUs}   Within the first 7 days of incubation an average 63\% of $^{13}$C-xylose was  respired and only an additional 6\% more was respired between days 7 and 30. At  the end of the 30 day experiment 30\% of the original $^{13}$C from xylose  remained in the soils. The $^{13}$C remaining in the soil from $^{13}$C-xylose  addition has likely been stabilized by assimilation into microbial biomass  and/or microbial conversion into other forms of organic matter, though it is  possible that some $^{13}$C-xylose remains unavailable to microbes due to  abiotic interactions in soil \citep{Kalbitz_2000}. All xylose  responders were first responsive in first 7 incubation days.   At day 1, 84\% of xylose responsive OTUs belong to Firmicutes, 11\% to  \textit{Proteobacteria} and 5\% to \textit{Bacteroidetes}. At day 3 (d3),  Firmicutes responders decreased to 5\% (from 16 OTUs to 1) while  \textit{Bacteroidetes} increased to 63\% (from 1 to 12 OTUs) of day 3  responders. The remaining day 3 responders are members of the  \textit{Proteobacteria} (26\%) and the \textit{Verrucomicrobia} (5\%). Day 7  responders were 53\% Actinobacteria, 40\% Proteobacteria, and 7\% Firmicutes. A  substantial amount (75\%) of xylose responders for day 7 had not previously  been identified as responders at earlier time points.   We observe dynamic changes in $^{13}$C-xylose assimilation with time at the  phylum level. The numerically dominant xylose responder phylum shifts from  \textit{Firmicutes} to \textit{Bacteroidetes} and then to \textit{Actinobacteria}  across days 1, 3 and 7 (Figure~\ref{fig:l2fc}). We also see strong and significant  correlation between estimated rRNA operon genome copy numbers per $^{13}$C-xylose  responder OTU genome and time (p-value, Figure~\ref{fig:copy}). $^{13}$C-xylose responder  rRNA operon geneome copy number is inversely related to time. That is, OTUs  that first respond at later time points have fewer estimated rRNA operons per  genome than OTUs that first respond earlier. rRNA operon copy number estimation  is a recent advance in microbiome science \citep{Kembel_2012} and the relationship of rRNA  operon copy number per genome with ecological strategy is well established  \citep{Klappenbach_2000}. Specifically, microorganisms with a high number of rRNA operons per  genome tend to be fast growers specialized to take advantage of boom-bust  environments whereas a low rRNA operon copy number per genome tends to occur in  microorganisms that favor slower growth under lower and more consistent  nutrient input \citep{Klappenbach_2000}. At the beginning of our incubation, OTUs with estimated  high rRNA operon copy numbers per genome or ``fast-growers'' assimilate xylose  into biomass and with time slower growers (lower rRNA operon number per genome)  begin to respond to the xylose addition. Further, $^{13}$C-xylose responders  have fewer estimated rRNA operon copy numbers per genome than  $^{13}$C-cellulose responders suggesting xylose respiring mircrobes are  generally faster growers than cellulose degraders.  All of the $^{13}$-xylose responders in the \textit{Firmicutes} phylum are closely related  (at least 99\% sequence idetity) to cultured isolates from genera that are known to form   endospores (Table XX). Each responder is closely related to strains annotated  as members of \textit{Bacillus}, \textit{Paenibacillus} or  \textit{Lysinibacillus}. \textit{Bacteroidetes} $^{13}$C-xylose responders are  predominantly closely related to \textit{Flavobacterium} species (5 of 8 total  responders). Only one \textit{Bacteroidetes} responder is not closely related  to a cultured isolate, ``OTU.183'' (closest LTP BLAST hit, \textit{Chitinophaca  sp.}, 89.5\% sequence identity). OTU.183 shares high sequence identity with  environmental clones derived from rhizosphere samples (accession AM158371,  unpublished) and the skin microbiome (accession JF219881, CITE). Other  \textit{Bacteroidetes} responders share high sequence identities with canonical  soil genera including \textit{Dyadobacer}, \textit{Solibius} and  \textit{Terrimonas}. Six of the 8 \textit{Actinobacteria} $^{13}$C-xylose responders  are in the \textit{Micrococcales} order. One $^{13}$C-xylose responding \textit{Actinobacteria}  OTU shares 100\% seqeunce identity with \textit{Agromyces ramosus} (Table~XX).   \textit{Agromyces ramosus} is a known predatotry bacterium CITE but is not dependent  on a host for growth in culture CITE. It is not possible to determine the specific origin  of assimilated $^{13}$C in a DNA-SIP experiment. The isotopically labeld C can be passed   down through trophic levels CITE. It's possible that the $^{13}$C labeled \textit{Agromyces}   OTU assimilating $^{13}$C by predataion on alreadly $^{13}$C labeled OTUs.  \subsection{Cellulose degrader DNA exhibits greater bouyant density shifts upon $^{13}$C incorporation than xylose degrader DNA}  \subsection{DNA BD shifts due to $^{13}$C-asimilation differ across phylogenetic types}  \textbf{Temporal dynamics of C-assimilation in soil.}   The dynamics of $^{13}$C-xylose and $^{13}$C-cellulose assimilation varied  dramatically. Isotope incorporation increases the bouyant density (BD) of DNA  and labeled DNA is enriched in 'heavy' fractions of the density gradient.  Isotope incorporation by an OTU is revealed by enrichment of the OTU in heavy  CsCl gradient fractions containing $^{13}$C labeled DNA relative to heavy  fractions from control gradients (no $^{13}$C labeled DNA). Variation in 16S  rRNA gene amplicon pool composition in fractions of $^{13}$C-labeled samples  and their corresponding controls is readily observed in 'heavy' gradient  fractions  (\href{https://www.authorea.com/users/3537/articles/3612/master/file/figures/ordination_all1/ordination_all1.png}{partitioning  along axis 2, Fig. 1}). The amplicon pool composition of 'heavy' fractions of  $^{13}$C-xylose and $^{13}$C-cellulose samples vary from corresponding controls  and from each other, indicating that the substrates were assimilated by  different members of the microbial  community(\href{https://www.authorea.com/users/3537/articles/3612/master/file/figures/ordination_all1/ordination_all1.png}{Fig.  1A}).  The $^{13}$C-incorporation reveals temporal dynamics of C  degradation demonstrated by $^{13}$C-xylose incorporation at days  1, 3, and 7 and $^{13}$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}), as expected \citep{Amelung_2008}. The microbial community changed  significantly (pval) with time in the bulk community supporting the temporal  dynamics observed in the gradient fraction amplicons  (\href{https://authorea.com/users/3537/articles/8459/master/file/figures/bulk_ordination/bulk_ordination.png}{Fig.  S2}). Although within a single time point, the bulk community demonstrated no  significant difference between treatments.   'Heavy' fraction amplicon pools from samples that received  $^{13}$C-xylose diverged from corresponding controls on days 1  through 7  (\href{https://www.authorea.com/users/3537/articles/3612/master/file/figures/ordination_all1/ordination_all1.png}{Fig.  1}). Furthermore, amplicon pool composition varied across these days  (\href{https://www.authorea.com/users/3537/articles/3612/master/file/figures/ordination_all1/ordination_all1.png}{Fig.  1B}) 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.   $^{13}$C-cellulose incorporation isn't detected until day 14 and  amplicon composition is consistent for both days 14 and 30  (\href{https://www.authorea.com/users/3537/articles/3612/master/file/figures/ordination_all1/ordination_all1.png}{Fig.  1}). The consistency of amplicon composition for cellulose degradation over  time compared to xylose suggests a wider array of microorganisms utilize  xylose, whereas, cellulose utilization occurs in a select few. This is  consistent with long standing notions that more microorganisms are capable of  utilizing simple carbohydrates than complex C substrates.   \textbf{Differential C utilization by taxa.} Individual OTUs that assimilated  $^{13}$C-substrates were identified using the DESeq framework  \citep{Anders_Huber_2010} to analyze differential representation in 'heavy'  fractions  (\href{https://www.authorea.com/users/3537/articles/3612/master/file/figures/l2fc_fig1/l2fc_fig.pdf}{Fig.  2}). There were 43 and 35 unique OTUs that significantly (false discovery rate  corrected \textit{P}-values \textless 0.10,  \href{https://authorea.com/users/3537/articles/8459/_show_article}{SI})  assimilated $^{13}$C-xylose and $^{13}$C-cellulose,  respectively; herein called 'responders'  (\href{https://www.authorea.com/users/3537/articles/8459/master/file/figures/OTU_screening_schematic/OTU_screening_schematic.pdf}{Figs.  S3},  \href{https://www.authorea.com/users/3537/articles/8459/master/file/figures/l2fc_fig_pVal/l2fc_fig_pVal.png}{S4},\href{https://authorea.com/users/3537/articles/8459/master/file/figures/manhattan/manhattan.png}{S5}).  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  (\href{https://authorea.com/users/3537/articles/8459/master/file/figures/resp_table/resp_table.png}{Table  S1}); Cellvibrio (d3) and Planctomyces (d14).   \textit{Responder Characteristics}. We found xylose responders were from  higher rank abundances than cellulose responders, however, cellulose responders  exhibited a greater shift in BD (i.e. assimilated more $^{13}$C)  than xylose responders in response to isotope incorporation  (\href{https://authorea.com/users/3537/articles/3612/master/file/figures/shift_and_rabund2/shift_and_rabund2.png}{Fig.  3}).   The kernel density estimate (KDE) of BD shifts resulting from  $^{13}$C-assimilation reveal that cellulose responders exhibit a  significantly (\textit{p}\textless 0.01) greater BD shift than xylose  responders  (\href{https://authorea.com/users/3537/articles/3612/master/file/figures/shift_and_rabund2/shift_and_rabund2.png}{Fig.  3A}). A density profile for each responder is generated for the experimental  and control treatment at each of the sampling time points using relative  abundances from sequence libraries  (\href{https://authorea.com/users/3537/articles/8459/master/file/figures/xylose_resp_profiles/xylose_resp_profiles.png}{Figs.  S5}\href{https://authorea.com/users/3537/articles/8459/master/file/figures/cellulose_resp_profiles/cellulose_resp_profiles.png}{,  S6}). The difference in center of mass for each set of density profiles  (control and experimental) is measured (supp. MM) and each KDE curve represents  the collection of density shifts calculated for all responders in the  $^{13}$C-cellulose or $^{13}$C-xylose treatment  (\href{https://authorea.com/users/3537/articles/3612/master/file/figures/shift_and_rabund2/shift_and_rabund2.png}{Fig.  3A}). We observe xylose utilizers having a smaller density shift (0.008 $\pm$  0.008 g mL$^{-1}$) than cellulose utilizers (0.015 $\pm$ 0.009 g  mL$^{-1}$), with few exceptions.   Most xylose responders are found at higher rank abundances than cellulose  responders (0.01 \textless \textit{p} \textless 0.05), which fall among the  rarer taxa in the tail of the RA curve (Fig 3B). This demonstrates that many  taxa important to cellulose cycling are present in the rarer fraction of the  overall microbial community. Yet, the transitions in abundances of responders  is difficult to discern in the bulk community abundances  (\href{https://authorea.com/users/3537/articles/8459/master/file/figures/xylose_resp_profiles/xylose_resp_profiles.png}{Figs.  S5}  \href{https://authorea.com/users/3537/articles/8459/master/file/figures/cellulose_resp_profiles/cellulose_resp_profiles.png}{,  S6}) or may not be detected with bulk community sequencing efforts. For  example, the increase in Bacteroidetes in the xylose treatment at d3 is not  observed in the bulk community abundances. Other instances may result in subtle  changes in bulk community abundance that would be difficult to differentiate  from natural variation or methodological noise.  \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  (\href{https://authorea.com/users/3537/articles/3612/master/file/figures/bacteria_tree/bacteria_tree.png}{Fig.  4}). Responders for xylose and cellulose are widespread across 6 and 7 phyla,  respectively  (\href{https://authorea.com/users/3537/articles/3612/master/file/figures/bacteria_tree/bacteria_tree.png}{Fig.  4}). 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  (\href{https://authorea.com/users/3537/articles/8459/master/file/figures/xylose_resp_profiles/xylose_resp_profiles.png}{Figs.  S5}  \href{https://authorea.com/users/3537/articles/8459/master/file/figures/cellulose_resp_profiles/cellulose_resp_profiles.png}{,  S6}). 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  (\href{https://authorea.com/users/3537/articles/3612/master/file/figures/bacteria_tree/bacteria_tree.png}{Fig.  4}). 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  (\href{https://authorea.com/users/3537/articles/3612/master/file/figures/bacteria_tree/bacteria_tree.png}{Fig.  4, heatmap}). 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.