A mineralisation-based method for estimating soil microbial carbon use efficiency

Microbial carbon use efficiency (CUE) reflects how microorganisms allocate carbon between mineralisation and biosynthesis, with consequences for soil carbon storage. We present a mineralisation-based ¹³C-glucose method that infers substrate-specific CUE from time series of 13C-CO2 alone using a two-component model, thereby avoiding the extraction of microbial biomass 13C and a fixed kEC conversion factor that is a requirement of widely-used 13C-biomass/CO2 partitioning methods. Applied to a temperate sandy loam grassland soil, the approach yielded a CUE of 0.80± 0.005 compared with 0.70 ± 0.023 from the conventional 6h-13C-biomass/CO2 partitioning approach. The 13C-mass balance for the mineralisation-based method revealed a greater 13C recovery (~87%) than for the biomass/CO2 partitioning method (~54%). Because the mineralisation-based estimate integrates intracellular and extracellular biosynthetic products, it provides a less assumption-sensitive CUE under standardised incubations. Furthermore, our method avoids the requirement for labour-intensive fumigation-extraction protocols which currently present a barrier to high throughput CUE estimation of large numbers of soil samples. The reported mineralisation-based CUE value represents a glucose-specific, incubation-condition CUE and should not be interpreted as in situ CUE for native SOC.