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\textbf{Soil samples} Collection and Preparation}  Soils were collected from an organic farm in Penn Yan, New York. These soils are characterized as Honoeye/Lima, a silty clay loam on calcareous bedrock. To get a field average, 10cm cores were collected (in duplicate) from six different sampling locations around the field using a slide hammer bulk density sampler (coordinates: (1) N 42° 40.288’ W 77° 02.438’, (2) N 42° 40.296’ W 77° 02.438’, (3) N 42° 40.309’ W 77° 02.445’, (4) N 42° 40.333’ W 77° 02.425’, (5) N 42° 40.340’ W 77° 02.420’, (6) N 42° 40.353’ W 77° 02.417’) on November 21, 2011. Cores were all sieved through a 2mm sieve, homogenized by mixing, and stored at 4°C until use (within 1-2 week of collection). Carbon and nitrogen content , as well as, water holding capacity were previously measured for these soils as blah blah blah.   \textbf{Cellulose production} Production} 
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\textbf{Soil microcosms} \textbf{Microcosm}  A subset of soil was dried at 105°C overnight to determine soil moisture content. Microcosms (35 total) were created with the equivalent of ten grams dry soil weight of the sieved soil in a 250mL Erlenmeyer flask capped with a butyl rubber stopper to preventing drying of soils. Microcosms were kept at 25°C for 2 weeks for bottle conditioning (better phrasing: pre-incubation?) and stoppers were removed for 10min every 3 days to prevent anoxia of soils. A complex carbon mixture (38\% cellulose, 23\% lignin, 20\% xylose, 3\% arabinose, 1\% galactose, 1\% glucose, 0.5\% mannose) was designed based on switch grass biomass composition. This complex carbon mixture was added to microcosms along with an amino acid (in-house made replica of teknova Cat#C0705) and basal salt mixture (Murashige and Skoog, Sigma M5524) at 52.6mg^{-1}10g soil; representative of natural concentrations. Cellulose (20mg) and lignin (12mg) were sprinkled over the surface of the soil in each microcosm as dry additions due to their insolubility to ensure same concentration in every flask. The remaining carbons, amino acids, and basal salts were added by pipetting evenly over soil surface as a liquid addition until 50\% water holding capacity is achieved. Three parallel treatments were performed; (1) control, all \textsuperscript{12}C-carbons, (2) cellulose, all \textsuperscript{12}C-carbons except \textsuperscript{13}C-cellulose, (3) xylose, all \textsuperscript{12}C-carbons except \textsuperscript{13}C-xylose. Replicate microcosms were harvested (stored at -80°C until nucleic acid processing) at days 1 (control and xylose only), 3, 7, 14, and 30. Microcosms remained stoppered throughout incubation, except for every third day when flasks were flushed with fresh air for 2 minutes. A subset of microcosm soil for each treatment and time point were isotopically analyzed at Cornell University Stable Isotope Laboratory to determine amount of \textsuperscript{13}C that remained at each time point.  
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\textbf{Nucleic acid processing} Acid Processing}  DNA was extracted from 0.25g soil using a modified Griffiths procotol (Griffiths 2000 Rapid Method for Coextraction of DNA and RNA from Natural Environments for Analysis of Ribosomal DNA- and rRNA-Based Microbial Community Composition, 10.1128/AEM.66.12.5488-5491.2000). Soils were bead beat in 2mL lysis tubes containing 0.5g silica/zirconia beads (treated at 300°C for 4 hours to remove RNases), 0.5mL extraction buffer (240 mM Phosphate buffer  0.5\% N-lauryl sarcosine), and 0.5mL phenol-chloroform-isoamyl alcohol (25:24:1) for 1 min at 5.5 m s ^{-1}. After beat beading, 85uL 5M NaCl and 60uL 10\% hexadecyltriammonium bromide (CTAB)/0.7M NaCl were added to lysis tube, vortexed, chilled for 1min on ice, and centrifuged at 16,000 x g for 5min at 4°C. The aqueous layer was transferred to a new tube and reserved on ice. To increase DNA recovery, a double extraction was performed by adding 85uL 5M NaCl and 0.5mL extraction buffer to same lysis tube, vortexed, and centrifuged same as before. New aqueous layer was added to previously collected aqueous layer and washed with 0.5mL chloroform:isoamyl alcohol (24:1), vortexed, and centrifuged same as before. Aqueous layer was transferred to a new tube and nucleic acids were precipitated using 2 volumes polyethylene glycol solution (30\% PEG 8000, 1.6M NaCl) on ice for 2hrs, followed by centrifugation at 16,000 x g, 4°C for 30min. Supernatant was discarded and pellets were washed with 1mL ice cold 70\% EtOH; each wash being a vortex followed by centrifugation at 16,000 x g, 4°C for 10min. Pellets were air dried, resuspended in 50uL TE, and replicate extractions pooled. Nucleic acids were stored at -20°C.   To prepare nucleic acid extracts for DNA-stable isotope probing (DNA-SIP), DNA was size selected (>4kb) using 1\% low melt agarose gel and $\beta$- agarase I enzyme extraction per manufacturers protocol (New England Biolab, M0392S). Final resuspension of DNA pellet was in 50$\mu$L TE. \textbf{Isopycnic centrifugation and fractionation}  Isopycnic gradients were established as described previously (35, 48), for five \textsuperscript{12}C-control, five \textsuperscript{13}C-xylose, two [13C]cellulose- amended  and four \textsuperscript{13}C-cellulose samples (one gradient per treatment two unamended microcosms  per time point). A soil type. All gradients used identical buffer solution to minimize methodological differences. Briefly, a  density gradient solution of 1.762g 1.599 g  cesium chloride/ml in gradient of 1􏰉 Tris-EDTA (TE)  buffersolution  (pH 8.0 15mM Tris-HCL, 15mM EDTA 15mM KCl) 8.0) with ethidium bromide  was used to separate \textsuperscript{13}C-enriched [13C]cellulose-enriched  and 12C-nonenriched DNA. Approximately Triplicate tubes per treatment containing approximately  10 to 25 􏰌g of DNA were loaded into a Beckman Polyallomer bell-top quick-seal centrifuge tube (part no. 344625; Beckman, Palo Alto, CA) containing the cesium chlo- ride-1􏰉 TE solution. Samples were centrifuged on a Beckman Optima ultracentrifuge using a TLA 120.2 fixed-angle rotor for 69 h at 57,000 rpm and 14°C. Fractions of ca. 100 􏰌l were collected manually by displacing the DNA-cesium chloride-1􏰉 TE-ethidium bromide solution with sterile distilled water. The refractive index of the fractions was measured on a Bausch and Lomb Abbe-3L refractometer. The buoyant density was cal- culated from the refractive index using the equation 􏰐 􏰊 a􏰑 􏰈 b, where 􏰐 is the density of the CsCl (g/ml), 􏰑 is the measured refractive index, and a and b are coefficient values of 10.8601 and 13.497, respectively, for CsCl at 25°C (6). Triplicate tubes per treatment were pooled with the appropriate range of buoyant density. The enriched and unenriched DNA fractions were purified by an ethanol precipitation, and DNA was quantitated using a NanoDrop 1000 (Thermo Scientific, Wilmington, DE). Fractions con- taining the 13C-enriched and 12C-nonenriched DNA were determined by relating the buoyant density and DNA concentration (for information on total microcosm-extracted DNA along with fraction buoyant densities and their associated DNA concentrations for all soils and replicates, see Table S1a, Fig. S1b, and Table S1c in the supplemental material). \textbf{Post-Sequencing Analysis}