Microbial growth and carbon use efficiency in soil: Links to fungal-bacterial dominance, SOC-quality and stoichiometry

Microbial decomposers are responsible for the breakdown of organic matter (OM) and thus regulate soil carbon (C) stocks. During the decomposition of OM, microorganisms can use the assimilated C for biomass production or respire it as CO2, and the fraction of growth to total assimilation defines the microbial carbon-use efficiency (CUE). As such, CUE has direct consequences for how microbial decomposers affect the balance of C between atmosphere and soil. We estimated fungal and bacterial growth in C units in microcosm systems with submerged plant litter. We established conversion factors between bacterial and fungal growth to biomass and applied this to a dataset representing 9 different sites in temperate forest soils, temperate agricultural soils, and subarctic forest soils, to estimate growth rates of fungi and bacteria in units of C, to estimate the dominance of the two decomposer groups, and to compare these values to respiration to estimate the microbial CUE. We observed that fungal-to-bacterial growth ratios (F:B) ranged from 0.02 to 0.44, and that the fungal dominance was higher in soils with lower C:N ratio and higher C-quality. We found a negative exponential relationship between the dominance of fungi and the microbial CUE. CUE ranged from 0.03 to 0.30, and values clustered most strongly according to site rather than level of soil N. CUE was higher in soil with high C:N ratio and high C-quality. However, within each land-use type, a high mineral N-content did result in lower F:B and higher resulting CUE. In conclusion, a higher soil C-quality coincided with lower F:B and higher CUE across the surveyed sites, while a higher N availability did not. A higher N availability resulted in higher CUE and lower F:B within each site suggesting that site-specific differences such as the effect of plant community via e.g. plant litter and rhizosphere input, overrode the influence of N-availability.