Effects of temperature and substrate stoichiometry on microbial specific respiration rate, carbon use efficiency, and 13C fractionation

Microbial activity contributes up to 60% of soil respiration. However, uncertainty in microbial respiration with rising temperature has previously prevented better predictions of the amount and the source of carbon (C) respired from soil. Three key variables of microbial C economies are of particular interest for estimating microbially mediated C release with temperature: (1) specific respiration rate (SRR), which is microbial CO2 release per microbial biomass-C, (2) carbon use efficiency (CUE), which determines how much organic C consumed by microbes is transformed into biomass, and (3) changes in the δ13C of respired CO2 with temperature, which suggests the form of organic C mineralized and helps to partition soil respiration in plant- and microbe-derived CO2. However, it is difficult to obtain these variables in intact soils, due to confounding factors that influence the amount and δ13C of respired CO2. Here we present an experimental approach that allows us to grow an isolated microbial population on well-characterized organic substrates and directly measure SRR, CUE and δ13C of respired CO2. We explored the effect of temperature on those variables, and how it changes with C:N of the substrate provided. This is important given various substrates available for microbial decay, and the potential for changing microbial CUE with substrate C:N. This approach thus can help constrain potential microbial C loss with warming as soil organic substrates with varying C:N are decomposed. We hypothesized that (1) increased SRR and declined CUE with warming would be more evident at higher C:N, (2) apparent 13C fractionation between biomass and respired CO2 would decrease with temperature due to C limitation, and (3) this fractionation would be higher for high C:N. Pseudomonas fluorescens (a ubiquitous Gram-negative bacterium) was grown at 0.13 h-1 in a chemostat from 13 to 26.5°C. The concentration of cellobiose, the sole C source with constant δ13C, was adjusted to have either a C:N of 10 or 20. Nitrogen was supplied as NH4Cl and KNO3. At steady-state, the rate and δ13C of respired CO2, and biomass were measured. From 13 to 26.5°C, SRR increased and CUE decreased by 152% and 30%, respectively, and these trends were unaffected by substrate C:N. For all runs, the δ13C was highest for cellobiose, followed by microbial biomass and respired CO2. 13C fractionation between biomass and respired CO2 was positively correlated with temperature at C:N=10, while no such relationship was evident at C:N=20. Additional experiments are currently underway at a C:N of 1 to examine if reducing substrate C:N below that of microbial biomass will alter these microbial responses to temperature. This study suggests that with warming enhanced SRR linked to declining CUE drives an increase in total microbial CO2 release. Furthermore, increased total CO2 releases may be independent of substrate C:N, if substrate C:N is greater than microbial C:N. However, non-uniform 13C fractionation between biomass and respired CO2 with varying C:N suggests that different mechanisms are at play, which may confound interpretations of ecosystem-level δ13C -CO2