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The kernel density estimate (KDE) of BD shifts resulting from \textsuperscript{13}C-assimilation reveal that cellulose responders exhibit a significantly (wilcox rank sum; p\textless...) 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 \textsuperscript{13}C-cellulose or \textsuperscript{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 (\textless0.02 g mL\textsuperscript{-1}) than cellulose utilizers (0.005-0.03 g mL\textsuperscript{-1}), with few exceptions.   Most xylose responders are found at higher rank abundances than cellulose responders, 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 and may be difficult or unable to detect in bulk community sequencing efforts. Additionally, 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 } S5}  \href{https://authorea.com/users/3537/articles/8459/master/file/figures/cellulose_resp_profiles/cellulose_resp_profiles.png}{, S6}). For example, the increase in Bacteroidetes in the xylose treatment at d3 is not observed in the bulk community abundances. In another instance, the increased response from Proteo- and Actinobacterial OTUs at d7 is also observed in the bulk community analysis as a marginal increase, yet it would be difficult to differentiate that change from natural variation or methodological noise. \textbf{Patterns of carbon use vary dramatically within phylum.} Dynamic patterns of \textsuperscript{13}C-assimilation from xylose and cellulose occur at discrete, fine-scale taxonomic units (Figure 4). (\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 (Figure 4). (\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 although within a phylum we observe utilization of both substrates by only six taxa OTUs  (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.This suggests assigning phylum level functionality is not an accurate depiction of soil C utilization though ecological functionality is often assigned at the phylum level (refs).  In this study, we have identified Actinobacteria responders for both substrates with a peak shift of ~0.036 gmL\textsuperscript{-1} g mL\textsuperscript{-1}  for cellulose and ~x gmL\textsuperscript{-1} g mL\textsuperscript{-1}  for xylose, suggesting a strong substrate specificity (Figures Sx and Sz - the substrate utilization charts). (\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}).  Albeit, there are no OTUs within Actinobacteria 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 (Figure 4). (\href{https://authorea.com/users/3537/articles/3612/master/file/figures/bacteria_tree/bacteria_tree.png}{Fig. 4}).  In the same vein, we identified Bacteroidetes responders for both substrates, yet, at a finer taxonomic resolution there is a clear differential response for xylose (Flavobacteriaceae and Chitinophagaceae) and cellulose (Cytophagaceae). Disagreeing correlations of phylum level abundance associated with C availability has been the source of debate for the role of Bacteroidetes in the degradation process \cite{Fierer_2007,Rui_2009,Sharp_2000,L_pez_Lozano_2013,Bastian_2009}. Our results would suggest that both perspectives are correct at a phylum level, but highlights the need of greater resolution to capture the subtleties of substrate utilization. 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 \textsuperscript{13}C-assimilation over time (Figure (\href{https://authorea.com/users/3537/articles/3612/master/file/figures/bacteria_tree/bacteria_tree.png}{Fig.  4, heatmap). heatmap}).  In a study that amended forest soils with single C substrates in the presence of 3-bromodeoxyuridine (BrdU), they determined that more than 500 taxa responded to labile C but occured predominantly in two phlya, Proteobacteria and Actinobacteria \cite{Goldfarb_2011}. On the other hand, the soils amended with polymeric C such as cellulose and lignin were noted for spanning eight phyla, but were limited to a small number of taxa within each of those phyla that were responders \cite{Goldfarb_2011}. It has previously been suggested that all taxa within a phylum are unlikely to share ecological characteristics \cite{Fierer_2007}, and furthermore, within a species population \cite{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 \cite{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 \cite{Schimel_2012}. The traditional phylum level assignment conventions could in part be due to limitations in finer scale taxonomic identifications or methodological limitations (ie sequencing depth), but our data in concert with others \cite{Goldfarb_2011,Fierer_2007,Choudoir_2012,Preheim_2011,Hunt_2008} would suggest that portraying the response of a few OTUs or clades as a phylum level response would be overreaching serves as a strong argument towards moving away from extrapolating substrate utilization to phylum level responses. \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. Traditionally, SIP is performed with a single time point in an attempt to minimize or eliminate cross-feeding of the substrate. 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. Yet, given the ability to resolve partially responsive taxa, the ecology would still be difficult to discern. For example, a generalist utilizing many substrates including \textsuperscript{12}C substrates and the \textsuperscript{13}C-labeled substrate may exhibit the same partial labeling that a specialist utilizing both the \textsuperscript{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.