Will invasive burrowing earthworms affect soil food web and carbon stability in arctic tundra?

The fate of the soil organic matter stored in arctic ecosystems in a future warmer climate is highly debated but remains quite uncertain, especially as most studies do not take into account the combined effect of climate change and simultaneous invasion by non-native fauna. For example, while the impact of climate change on carbon losses from the arctic might be limited due to the strong nutrient limitations restricting microbial activity and decomposition speed in these ecosystems, the current invasion by burrowing earthworms as a result of human activity might alleviate the nutrient limitations and modify the soil food web, which could significantly increase carbon losses. We investigated the effect of burrowing earthworm addition on soil mesofauna and microbial community composition and on associated carbon stability of the arctic tundra by the end of a 4-year-long mesocosm experiment in northern Sweden. The abundance of collembola and oribatid mites was positively affected by earthworm addition in a heath-type tundra ecosystem, while no changes were detectable in a meadow-type tundra. This is surprising as the meadow-type tundra was strongly affected by earthworms in terms of soil structure with a decrease in total carbon stock. We tested the stability of the residual carbon by measuring CO₂ emissions during an incubation of the organic and mineral soil horizons at current and increased temperatures. We found that while carbon stability is not clearly affected by earthworm addition in the heath-type tundra, the stability of the leftover carbon is increased in the presence of earthworms in the meadow-type tundra in the first 10 soil centimeters in both incubation temperatures. This suggests that the ultimate effect on carbon dynamics of earthworm invasion cannot be simply estimated from the immediate carbon loss from the organic layer, as the changes in carbon forms and quality could modify the future organic matter availability to decomposers.