Bacterial metabolism rather than necromass dominates input to soil organic carbon

Soil organic carbon (OC) sequestration is presumed to rely to a large extent on microbial transformation of plant residues into microbial necromass. Necromass formation, however, represents only one pathway by which microorganisms contribute to soil organic matter, while OC released through metabolism is often neglected. Using a dynamic modeling approach, we show that exudates and waste products contribute about equally to bacterially derived OC inputs to soil with median contributions of 10% each (95% CI of 0.5 to 73% and 0.6 to 71%, respectively). Exoenzymes contribute an additional 15% (5 to 41%) and necromass contributes 49% (5 to 84%) to bacterial products. Overall, 6% (2 to 27%) of the organic input is released into the soil as bacterial metabolites (exoenzymes, exudates, and waste products), and the same amount as bacterial necromass 6% (8 to 20%). Exudates and waste products are typically composed of small reactive compounds that differ greatly from necromass in their molecular properties and will therefore likely contribute disproportionally to long-term soil OC accrual.