Microbial soil carbon sequestration pathways

Microbial sequestration of Carbon in soil can be compared to a microbial carnival. Each microbe have a definite role to play to give shape to the entire event. Bacteria & fungi are the major players, constituting over 90% of soil microbial biomass although protists & other small eukaryotes also play a pivotal role. Soil microbes directly sequesters C by assimilating above ground plant C inputs, plant litter & associated leachates (e.g. dissolved organic carbon) for growth & release C as Extracellular Polymeric Substances (EPS) & CO2 - the phenomenon is known as microbial carbon use efficiency. The below ground C inputs sources includes rhizodeposition, root litter & exudates which are further broken down by soil microbes, microarthropods & nematodes to finally form Dissolved Organic Matter (DOM), Fine & Coarse Particulate Organic Matter (FPOM & CPOM) and Mineral Associated Organic Matter (MAOM). All these gradually contributes together to soil labile C pool & stable C pool. POMs have lifetime of 1-50 yrs (quick cycling) whereas for MAOMs it is 10-1000 years (slow cycling).
EPS, root exudates, hyphal exudates, fungal mycelia, Arbuscular Mycorrhizal Fungi (AMF) hyphae, glycoproteins acts as adhesive which along with soil microbiome, DOM, FPOM, CPOM & MAOM contributes towards soil recalcitrant C pools formation (including soil macroaggregates) by Microbial Carbon Pump (MCP). MCP transfers labile organic matter into recalcitrant organic matter via microbial activity causing long-term C storage.
Microbial biomass & necromass, soil aggregate stability & soil C sequestration are also interrelated. The synthesis & assimilation of plant-derived C materials in the microbial biomass & necromass accounts for between 50-80% of stable Soil Organic Carbon (SOC). Better stability of soil aggregates is imperative for higher stabilized C stocks. It is facilitated by plant leaf or root litter having phenols & lignin & also by certain microbes enhancing soil aggregation, thus protecting SOC from decomposition & promoting C sequestration. Again, soil microbial turnover (i.e. death & necromass formation) & necromass reuse through recycling & destabilization are climate sensitive & ecosystem specific. Further, necromass generation may be a rate-limiting step for stabilization of soil C. Lastly, fungal to bacterial ratio is pivotal towards the soil C flux. A fungal-dominated soil system (e.g. forest) have higher amount of C sequestered in soil.
The following poster is a small step to visualize these intricate process.