Soil contains more carbon than the atmosphere and all plant biomass combined. The question of whether soil is a net source or sink of carbon depends on microbial activity in the rhizosphere — the thin zone surrounding plant roots where root exudates feed soil microorganisms.
The microbial carbon pump concept describes how soil microbes convert labile plant-derived carbon into stable forms: microbial necromass (dead cell walls) and mineral-associated organic matter that persists for decades to centuries. The pump works because microbial cell walls are chemically more resistant to further decomposition than the root exudates they consumed.
Recent work reveals the counterintuitive role of fungi. In the rhizosphere, fungi — not bacteria — dominate soil organic matter formation, potentially via fine-root decomposition and mycelial production. The fungal hyphae themselves become the stable carbon. The organism that decomposes organic matter is simultaneously the organism that stabilizes it, because its own biomass is harder to decompose than its food source.
This creates a strange accounting problem. Carbon sequestration in soil is partially a function of microbial growth rate — more growth means more necromass means more stable carbon. But more growth also means more respiration, which releases CO₂. Whether the net effect is sequestration or emission depends on the ratio of biomass production to respiration, which varies with soil temperature, moisture, and nutrient availability.
The engineering implications for agriculture: soil management that promotes fungal-dominated communities over bacterial-dominated ones should increase carbon sequestration, because fungi convert a larger fraction of consumed carbon into persistent biomass. No-till farming, reduced fertilizer, and diverse cover crops all favor fungi. The carbon fix is also a farming practice fix — the interventions that sequester carbon are the same ones that improve soil health.