Long term tillage regime alters bacterial assimilation of xylose and cellulose.

Microbial growth dynamics determine carbon fate in soil by transforming carbon inputs into microbial products available for stabilization on soil surfaces. Management practices such as tillage disturb microbial communities and promote C loss, but the degree to which tillage alters bacterial metabolism of soil C remains poorly described. We conducted a multi-substrate DNA stable isotope probing experiment using soil from a long-term field experiment with a 42-year legacy of either no-till or annual moldboard plowing. We predicted that this land use history would alter C assimilation dynamics due to differences in bacterial growth responses. We incubated soil from each tillage regime with ¹³C-xylose and ¹³C-cellulose, substrates that differ in bioavailability and which favor different bacterial life history strategies in soil. We identified 730 ¹³C-labeled bacterial taxa and tracked their abundance in bulk soil over a 30 day period. Carbon addition to soil rapidly altered bacterial community structure and function. ¹³C-labeling dynamics differed substantially between tilled and no-till soils with respect to both xylose and cellulose. Bacterial xylose metabolism in tilled soils exhibited substantial lag relative to no-till soils, and this lag corresponded with lower mineralization rates for xylose. In addition, bacterial cellulose metabolism was mediated primarily by specialist taxa in no-till soils, while dual incorporators dominated tilled soils. Differences in carbon assimilation corresponded to lower cellulose mineralization rates and cumulative cellulose mineralization in tilled soils. We show that soil management practices shape the path of carbon through bacterial communities by altering dynamic growth responses associated with bacterial life history strategies.IMPORTANCEWe applied DNA stable isotope probing in a microcosm experiment to understand the role of soil management (till vs no-till) in shaping bacterial carbon cycling. Our hypothesis was that a legacy of disturbance through tillage would exert a selective influence on bacterial growth dynamics, thereby altering bacterial processing of added carbon substrates. We found that lagged growth in tilled soil resulted in delayed bacterial assimilation of xylose and a streamlined, single carbon "channel" characterized by the co-metabolism of xylose and cellulose. In no-till soil, temporally distinct bacterial assimilation of xylose and cellulose by separate carbon "channels" was associated with higher carbon mineralization rates and total mineralization relative to tilled soil. Our findings indicate that soil management practices altered the growth dynamics of active carbon cycling bacteria. Lagged growth associated with a history of disturbance resulted in reduced carbon mineralization.