Quantifying the role of trophic guilds in soil organic carbon mineralization

Soil organic carbon (SOC) is a critical carbon pool on the planet, essential for soil functions and services such as climate regulation through C sequestration. SOC is dynamically recycled within microbial biomass and channelled into long term storage in the soil (the 'microbial carbon pump'). Understanding the biotic processes that drive SOC mineralization is essential for predicting the climate warming-carbon cycle feedback. The microbial pump is influenced by trophic interactions in the soil food web and SOC mineralization is a result of complex biotic interactions.
Here, we combined Tree-of-life sequencing (TOLseq; metatranscriptomic sequencing of ribosomal RNA for three-domain profiling of soil biota) with quantitative conversion factors which links transcript abundance to biomass, using standard parameters of microbial cell stoichiometry and physiology. We show how this can form the basis for energetic food web models, an approach we refer to as TOLmodel. The novel approach was applied on soil samples originating from natural grassland sites in Iceland (‘Forhot’ sites), where geothermal activity has created soil warming for more than 60 years, with soil warming gradients up to +6 °C used in this study. Field observations showed that warming reduced SOC stocks, but after years of warming SOC mineralisation had acclimated. 
Our TOLmodel approach allowed the quantification of soil biota, aligning with laboratory measurements of microbial biomass carbon, and their SOC mineralization rates, aligning with measured CO2 efflux from the field site. Furthermore, the food web model revealed how decimated soil fauna under soil warming relaxed the top-down control of microbial growth increasing SOC mineralisation rate per unit microbial biomass six-fold during summer.