Legacies of native climate regime govern responses of boreal soil microbes to litter stoichiometry and temperature

• Climate legacy governs SOM decay with warming and varying stoichiometry.
• High C:N Oi enhanced a warming-induced increase in CO2 from Oea at warmer site.
• Oi C:N did not alter distinct microbial groups' responses to warming at this site.
• At the cooler site fungal acquisition of low C:N Oi increased with warming.
• C:N did not influence the CO2 source released in response to warming at this site.

Temperature and substrate stoichiometry can influence soil organic matter (SOM) decomposition, but whether their interactive effect is significant remains unclear. This study explores linkages between substrate stoichiometry and warming-induced patterns of microbial carbon (C) mineralization in boreal forest soils via replacements of native litterfall with Oi material possessing distinct δ13C and C:N. Following 120 day incubations of soils from sites similar except for mean annual temperature, we examined exo-enzyme activities, microbial biomass, and δ13C of respired CO2 and individual phospholipid fatty acids (PLFA) to identify microbial community and decay responses to these experimental manipulations. In accordance with stoichiometric theory, we hypothesized that warming-induced increases in microbial use of naturally low C:N Oea materials and fungal incorporation of Oea would be enhanced when available Oi material exhibited relatively high C:N. These hypotheses were only partially supported. At the warmer site, higher C:N Oi was linked to an enhanced, warming-induced decline in Oi-derived CO2, lower C:N Oi prompted an increase in the warming-induced increase in phenol oxidase activities, and the interaction between temperature and Oi C:N had no influence on the relative abundances or activities of distinct microbial groups. In contrast, responses to Oi C:N and temperature at the cooler site were evident only via changes in the structure and substrate preference of the active microbial community; we observed a ∼25% increase in the incorporation of low C:N Oi into fungi relative to bacteria in these soils when they were incubated at a relatively high temperature. The data emphasize the importance of native climate imposing a legacy effect on how these microbial communities respond to temperature and substrate stoichiometry, by governing those communities' adaptive characteristics and the climate-related composition of the SOM on which they depend. That legacy effect is reflected in patterns of decay and in the relative abundances of microbial groups that both decay and generate SOM.