Unreported role of earthworms as decomposers of soil extracellular polymeric substance

Soil microbial extracellular polymeric substances (EPS) are important components of soil organic carbon (SOC), which are thought to promote the formation of soil structures and induce the stabilization of SOC. Earthworms may not only stimulate the production of EPS through the promotion of microbial activity and microbial biomass, but might also accelerate the decomposition of EPS via ingestion, resulting in SOC sequestration or loss. However, the exact impacts of earthworms on EPS in soils have never been evaluated. Here, a pioneering experiment involving two species of endogeic earthworms (Drawida gisti and Metaphire guillelmi) with different body sizes was conducted to investigate their impacts on soil EPS. It was found that the D. gisti and M. guillelmi earthworms reduced the soil EPS-protein concentrations by 12.6 % and 17.6 %, respectively; however, they did not affect the EPS-polysaccharide concentration. Moreover, the presence of these endogeic earthworms increased soil microbial biomass, and elevated microbial respiration, microbial metabolic quotient, dissolved organic carbon, and soil pH. The results of this study provide the first evidence for the decoupling effects of earthworms on extracellular residual components. This suggests that earthworms may alter the molecular composition of SOM, thereby influencing its persistence and soil-atmosphere carbon exchange.