Warming alters routing of labile and slower-turnover carbon through distinct microbial groups in boreal forest organic soils

Our understanding of the mechanisms driving the response of soil organic carbon (SOC) pools to warming, though critical for predicting climate feedbacks, remains limited. Here we report results from a warming experiment using O-horizon soils from two mesic, boreal forest sites with contrasting climate regimes. We replaced extant Oi soil horizons, or litterfall C, with another coniferous Oi possessing a distinct δ13C signature, and tracked the net incorporation of the replaced Oi and, by difference, Oea-C into soil microbial phospholipid fatty acids (PLFA) following 120-day incubations at 15 °C and 20 °C. We demonstrate how regional climate (site effects) and experimental warming (temperature effects) influence microbial incorporation of Oi versus slower-turnover Oea SOC pools. Microbial biomass, estimated from total PLFA, increased by 32–60% with temperature and was 20–42% higher within soils from the warmer versus cooler site, congruent with increased mineralization in those soils. The proportion of Gram-positive bacterial PLFA-C derived from Oi-C more than doubled and coincided with a reduction in the incorporation of Oi-C into fungal relative to bacterial PLFA with warming and in soils from the warmer site. Mirroring the relative decrease in fungal incorporation of Oi-C, warming led to an increase of 22–31% in the proportion of fungal PLFA-C derived from the Oea-C, consistent with the increased incorporation of this slower-turnover SOC pool in soils from the warmer site. The increase in microbial biomass and shift in routing of Oi and Oea pools through PLFA indicate that warming preferentially increases fungal mineralization of more slow-turnover C pools in these boreal organic soils. Shifts in microbial substrate routing and biomass increases with warming observed here underscore the potential importance of changing proportions of microbial biomass remnant contributions to SOC pools with climate warming. ► Fungal incorporation of slower turnover SOC increased by up to 31% with warming. ► Incorporation of faster turnover SOC into Gram+ bacteria doubled with warming. ► Warming altered decay patterns and increased microbial biomass by as much as 60%. ► Warming thus may influence composition of SOM via decay and necromass formation.