Increased tundra root biomass offset invasive earthworm effects on SOC decomposition

Arctic soils store nearly half of the global soil organic carbon, but what will happen with this large carbon pool if soil macrofauna, able to ingest organic matter accumulated at depths in the soil, establish in the Arctic? The question is justified as emerging evidence suggests that low soil organic carbon (SOC) turnover rates in high latitude ecosystems could, in addition to abiotic factors, partly be due to the current lack of larger detritivores that can stimulate the breakdown of organic matter. Earthworms are large detritivores increasing their distribution into arctic ecosystems. With potential to both increase stabilisation and decomposition of SOC, but also enhance plant productivity, it has been difficult to determine what net effect earthworms have on ecosystem C storage. The scientific debate around this ‘earthworm dilemma’ has however primarily focused on the fate of SOC in response to earthworms, leaving cascading effects on plant productivity largely undiscussed.

Here, we use a four-year outdoor experiment to study the effects of introducing earthworms to tundra vegetation types on both plant biomass (above and belowground) and SOC. We found that earthworms invasive to the Arctic: i) reduced the SOC pool beneath herb dominated tundra while they increased the SOC pool under dwarf-shrub dominated tundra; ii) increased the below ground biomass in both vegetation types; and iii) increased the total plant biomass C to the degree that it offset the SOC losses from the herb dominated soil. In the dwarf-shrub vegetation, earthworms increased both the plant C pool and the SOC pool resulting in a net increase of the ecosystem C stock. We highlight that the effect on root growth seems of great importance when predicting how ecosystem C sequestering responds to invasive earthworms. Both through increased plant biomass C but also through increased deposition of persistent root-derived organic matter.