Earthworm Pontoscolex corethrurus stimulated soil CO2 emission by enhancing substrate availability rather than changing microbiota community structure

Abstract

Earthworms may regulate carbon (C) mineralization through a top-down process by altering microbiota community and/or a bottom-up process by providing basic conditions such as mineralizable substrates. However, these two roles performed by earthworms have not been separately quantified. Here, we focused on how the pantropical widespread earthworm, Pontoscolex corethrurus, affected CO₂ emission by changing soil microbiota community and substrate availability in infertile subtropical soils. Two experiments were performed. Firstly, we conducted a 3-year field experiment wherein P. corethrurus population was manipulated by electrical shocking. The two treatments were earthworm reduction (ER) and earthworm addition (EA). Thereby, we tried to understand CO₂ emission pattern through the earthworm-induced changes in soil microbiota community and C-related enzyme activities in field plots. Secondly, a cross-soil inoculation microcosm experiment was conducted to partition the contributions of earthworm-regulated microbiota community and substrate quality to CO₂ emission. The four treatments were 1) autoclaved ER soil + ER microbiota, 2) autoclaved EA soil + ER microbiota, 3) autoclaved ER soil + EA microbiota, and 4) autoclaved EA soil + EA microbiota. We found that, in the field experiment, earthworm addition changed soil microbiota community structure, but increased CO₂ emission despite decreasing soil β-glucosidase activity by 12%. In the cross-inoculation experiment, the β-glucosidase activity and CO₂ emission (1-day incubation) in EA soils was 65.5% and 35.5% greater than that in the ER soils, respectively; the cumulative CO₂ emission (30-day incubation) in EA soils was also significantly greater than that in ER soils. However, no significant effects of microbiota inoculation on soil CO₂ emissions were observed. These results suggested that the earthworm-enhanced substrate availability, rather than the earthworm-induced changing in the microbiota community structure and enzymatic activities, played a key role in C mineralization. This study implies that P. corethrurus occupies a "bottom niche" in infertile subtropical soils.

Keywords: Earthworm; Ecological niche; Soil C mineralization; Soil enzyme activity; Soil microbiota community; Soil substrate quality.