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\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 63\% on average of $^{13}$C-xylose was  respired and only an additional 6\% more was respired from day 7 to 30. At  the end of the 30 day incubation 30\% of the $^{13}$C from added xylose  remained in the soils. The $^{13}$C remaining in the soil from $^{13}$C-xylose  addition was likely stabilized by assimilation into microbial biomass  and/or microbial conversion into other forms of organic matter. It is also  possible that some $^{13}$C-xylose remains unavailable to microbes due to  abiotic interactions in soil \citep{Kalbitz_2000}.   At day 1, 84\% of $^{13}$C-xylose responsive OTUs belong to  \textit{Firmicutes}, 11\% to \textit{Proteobacteria} and 5\% to  \textit{Bacteroidetes}. At day 3, \textit{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\% \textit{Actinobacteria}, 40\% \textit{Proteobacteria},  and 7\% \textit{Firmicutes}. The identities of $^{13}$C-xylose responders  change with time. The numerically dominant $^{13}$C-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},  Figure~\ref{fig:xyl_count}).   All of the $^{13}$C-xylose responders in the \textit{Firmicutes} phylum are  closely related (at least 99\% sequence identity) to cultured isolates from  genera that are known to form endospores (Table~\ref{tab:xyl}). Each  $^{13}$C-xylose responder is closely related to isolates 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)  (Table~\ref{tab:xyl}. Only one \textit{Bacteroidetes} $^{13}$C-xylose  responder is not closely related to a cultured isolate, ``OTU.183'' (closest  LTP BLAST hit, \textit{Chitinophaca sp.}, 89.5\% sequence identity,  Table~\ref{tab:xyl}). OTU.183 shares high sequence identity with environmental  clones derived from rhizosphere samples (accession AM158371, unpublished) and  the skin microbiome (accession JF219881, \citet{Kong_2012}). 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\% sequence identity with  \textit{Agromyces ramosus} (Table~\ref{tab:xyl}). \textit{A. ramosus} is a  known predatory bacterium but is not dependent on a host for growth in culture  \citep{16346402}. It is not possible to determine the specific origin of  assimilated $^{13}$C in a DNA-SIP experiment. $^{13}$C can be passed down  through trophic levels although heavy isotope representation in C pools  targeted by cross-feeders and predators would be diluted with depth into the  trophic cascade. It's possible, however, that the $^{13}$C labeled  \textit{Agromyces} OTU was assimilating $^{13}$C primarily by predation if the  \textit{Agromyces} OTU was selective enough with respect to its prey that it  primarily attacked $^{13}$C-xylose assimilating organisms and that those  $^{13}$C-xylose assimilating organisms utilized $^{13}$C-xylose as a sole  carbon source.