Environ Exp Biol (2020) 18: 61–75

The potential of earthworms in soil carbon storage: a review

Abstract

Earthworms are known to modify the soil structure and alter the physico-chemical as well as biological regimes of the inhabiting soil. Numerous studies focussing on the role earthworms play in soil carbon (C) mineralization and stabilization have been conducted, with recent reviews portraying earthworms as greenhouse-gas emitters. The present work aims (i) to assess all possible earthworm mediated C dynamics by compiling results of the past several years of research, and (ii) to identify gaps in knowledge and evaluate the need for further research. The review noted the role of earthworms in soil C sequestration to be site-specific; i.e., dependent on the ecosystem. It also noted that although the presence of earthworms may not increase the soil organic C stocks, over the long term they are capable of improving the resilience of C in mineral soils. The aggregates formed as a result of earthworms feeding and casting behavior serves as a mechanism for the physical stabilization of C trapped within. The calcite granule formation by certain Lumbricid species, possibly in an attempt to regulate CO₂ levels, is one of the many ways earthworms promote the chemical stabilization of C. Humification of organic matter by earthworms renders a biochemical stabilization mechanism for soil C. Nevertheless, we also address the ways by which earthworms emit CO₂, even though we found much of this emission to be indispensable, it being a living organism. The lack of long-term studies in the area is a drawback that leaves room for debate. Earthworms occurring globally in almost every ecosystem are in fact biochemical reactors that transform the labile C components into more stable forms. The review then sheds light on the so-called “soil carbon-earthworm dilemma” and finally concludes by stating that earthworms, while facilitating C input to the soil, simultaneously also support soil biological activities and the corresponding CO₂ emission, all of which may be seen as an integral part of the natural C cycle.