Kari M Finstad, Erin E Nuccio, Katherine E Grant, Taylor A B Broek, Jennifer Pett-Ridge, Karis Jensen McFarlane 

Microbial processing of soil organic matter is a significant driver of C cycling, yet we lack an understanding of what shapes the turnover of this large terrestrial pool. In part, this is due to limited options for accurately identifying the source of C assimilated by microbial communities. Laboratory incubations are the most common method for this; however, they can introduce artifacts due to sample disruption and processing and can take months to produce sufficient CO2 for analysis. We present a biomass extraction method which allows for the direct 14C analysis of microbial biomolecules and compare the results to laboratory incubations. In the upper 50 cm soil depths, the 14C from incubations was indistinguishable from that of extracted microbial biomass. Below 50 cm, the 14C of the biomass was more depleted than that of the incubations, either due to the stimulation of labile C decomposition in the incubations, or the inclusion of biomolecules from non-living cells in the biomass extractions. Our results suggest that measurement of 14C of microbial biomass extracts can be a useful alternative to soil incubations, possibly avoiding some of the drawbacks associated with laboratory incubations.