The importance of bacterial metabolism contribution to soil organic carbon revealed by Monte Carlo simulations

Soil is the largest terrestrial carbon reservoir and processes leading to carbon sequestration play a crucial role in quantifying size as well as changes of this important pool. Microorganisms transform plant residues to smaller organic compounds, and often necromass is assumed to be the main stable end product. The turnover of microbial biomass at end of life, however, is only one pathway by which microorganisms contribute to soil organic matter. As a proof-of-concept, we use a mechanistic modeling approach with Monte Carlo simulation of 10,000 iterations, where input parameters vary according to values derived from literature. Bacterial growth follows a Monod kinetic, and biomass is further transformed to exudates, waste, and exo-enzymes, which vary in their C:N ratios. Assuming abundant N-resources, bacterial necromass contributes with 23.2% (median) of organic carbon only a minor portion to microbially-derived soil organic matter at the end of the simulation of 72 days. Most of the microbially derived organic carbon originates as part of metabolism by a combination of exudation (median 39.0%), wastes such as for osmotic regulation (median 22.4%), and exoenzyme production (median 10.3%). The organic product yields vary by about 300% between anabolic stages six days after substrate additions compared to catabolic stages at the end of the simulation. Predictions and management of soil organic carbon sequestration should therefore be based on carbon input through microbial metabolism rather than assumptions of carbon input solely at end of life.