Earthworm impacts on soil carbon storage: the importance of quantifying all drilosphere compartments

Earthworms play a key role in soil organic carbon (SOC) dynamics. Yet few studies have quantified SOC accumulation in specific drilosphere compartments as affected by different earthworm species and their interaction. The aim of this study was to determine the effects of the tropical species Amynthas zenkevichi (burrower) and Pontoscolex corethrurus (shallow bioturbator) on SOC accrual in the drilosphere, comprising surface casts, belowground casts, and burrow walls, over a three-month period under controlled laboratory conditions. Buffalo dung was provided as a surface food source. X-ray computed tomography combined with bulk density measurements were used to estimate the mass of burrow walls and belowground casts. Drilosphere compartments and surrounding soil were collected manually at the end of the experiment. Their SOC stocks and the incorporation of dung-derived carbon were quantified. Total SOC stocks in earthworm mesocosms were compared to those of control mesocosms without earthworms. Although the drilosphere mass was similar between species, A. zenkevichi produced a higher mass of surface casts, whereas in P. corethrurus treatments more mass was allocated to burrow walls and belowground casts. Dung-derived carbon was more incorporated into casts than into burrow walls, with both species showing similar incorporation rates. For both species, SOC stocks in the soil mesocosms increased by 7 % relative to the control, with the majority of SOC stored in surface casts and burrow walls. In mixed species treatments, synergistic effects were observed with SOC stocks exceeding additive expectations derived from single species treatments by 3 %. Our findings highlight that two earthworm species belonging to contrasting functional groups have similar positive effects on SOC accrual in the drilosphere compartments and that their interspecific interactions may further enhance SOC stocks. • Both species ( A. zenkevichi and P. corethrurus ) increased SOC storage by 7 %. • Higher rates of SOC accrual in surface and belowground casts than burrow walls • Synergy of mixed species enhanced SOC stocks by 3 % compared to expected additive effects.