Earthworm abundance and functional group diversity regulate plant litter decay and soil organic carbon level: A global meta-analysis

A previous review of earthworm impacts on greenhouse-gas emissions concluded that earthworms elevated soil CO2 emissions with no apparent influence on soil organic carbon (SOC), especially in laboratory incubations and in agroecosystems. This conclusion suggests that the elevated soil CO2 emissions may come from enhanced plant litter decomposition. Despite the known important role of earthworms on regulating ecosystem processes, a quantitative analysis of the relationship between earthworms and decomposition in global terrestrial ecosystems is still missing. Here, we present a quantitative synthesis of earthworm effects on plant litter decomposition and SOC based on 340 observations from 69 independent studies. We found a positive correlation between earthworm density and the rate of plant litter decay, and that the presence of earthworms doubled the amount of litter mass loss on average. The presence of all three (anecic, epigeic and endogeic) earthworm functional groups was associated with higher litter mass loss than when either one or two functional groups were present. Anecic earthworms caused the strongest effect on litter mass loss, followed by epigeic earthworms, and there was no apparent influence by endogeic worms. Although the effect of earthworms on SOC was not significant based on all observations, the presence of any two of the three functional groups alone or two (epigeic and endogeic, or anecic and endogeic) and three (anecic, epigeic and endogeic) functional groups together decreased SOC concentrations. Our results indicate that the effect of earthworms on litter and SOC decay depends strongly on earthworm functional groups and diversity, and that a high diversity of earthworm functional groups accelerates litter mass loss and SOC decay. We anticipate that changes in land management practices are likely to alter ecosystem carbon cycling through alteration of earthworm abundance and diversity.