Novel methods for determining the 14C age of microbially assimilated soil carbon

Soils are a significant component of the Earth’s carbon (C) cycle, yet a mechanistic understanding of what controls the turnover of this large C pool remains elusive. Microbial respiration of organic C accounts for roughly half of the total CO₂ production from soils, though limited options exist for accurately identifying the source of C assimilated by microbial communities. Currently, radiocarbon (¹⁴C) analysis of evolved CO₂ from soil incubations is the most common laboratory 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 novel extraction methods which allow for the direct ¹⁴C analysis of microbial biomolecules and compare the results to laboratory incubations. Preliminary results suggest that in the upper 50 cm soil depths, the Δ¹⁴C from incubations is indistinguishable from that of extracted microbial biomass. Below 50 cm, the Δ¹⁴C of the microbial biomass is 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. Work is ongoing to identify the source of the extracted biomass pool and additional methods for isolating specific, short-lived biomolecules such as RNA, are underway to unambiguously determine the Δ¹⁴C of organic molecules being assimilated by active microbial communities.