G05_P01

What are soil microbes eating? Novel methods for determining the age of microbially utilized soil carbon

Finstad K¹,  Nuccio E¹, Grant K¹, Broek T², Pett-Ridge J¹, McFarlane K¹

¹Lawrence Livermore National Laboratory, Livermore, United States, ²Woods Hole Oceanographic Institution, Falmouth, 02543

Soils represent a large component of the global C cycle, storing more C than plants and the atmosphere combined. Microbial processing of organic matter in soil is a main driver of soil C cycling, yet we lack robust methods for accurately identifying the age of C respired by microbial communities, hindering our ability to predict how disturbance or climate change affects soil C persistence. The most common method for identifying the age of microbially respired C is through laboratory incubations where homogenized soils are incubated in sealed jars until sufficient CO₂ has accumulated for ¹⁴C measurement. However, comparison of lab incubation ¹⁴CO₂ to in situ field collections suggests that soil sampling and processing can cause incubated microbes to respire older C than they would under natural conditions. We therefore seek to develop a method to determine the age of microbially utilized C more accurately, such as the extraction and dating of microbial biomass. We find that in general, the Δ¹⁴C of CO₂ from lab incubations in surface soils is indistinguishable from the Δ¹⁴C of chloroform extracted microbial biomass, but the values diverge at depth, with the lab incubations often producing depleted values relative to the extracted biomass. These findings corroborate previous suspicions that laboratory incubations may bias the results and falsely suggest the consumption of older soil C than occurs under natural conditions. Future work is being conducted to refine this method and investigate the utility of ¹⁴C measurements on nucleic acids extracted from soil.