Earthworms lock up more carbon in soils than they release as carbon dioxide (CO2), according to a study published in Nature Communications this week. This finding contradicts recent claims that earthworm presence in soils leads to a substantial increase in CO2 emissions.
Globally, soils contain twice as much CO2 as the atmosphere and earthworms are known to affect how much CO2 is produced in the soil, how much is locked away as stabilized carbon and how much is released to the atmosphere. Weixin Zhang and colleagues now suggest that that the presence of earthworms is more likely to create a carbon sink as the carbon stabilized by earthworms outweighs that converted to CO2 during carbon mineralization. Their experiments show that the initial increase of CO2 release from soils containing two Asian and European invasive earthworm species can be offset in the longer term by enhanced carbon storage and that the total amount of CO2 that can potentially escape the soil differs little from soil containing no earthworms. The authors propose a mathematical formula that could help to quantify the effects of earthworms on soil carbon storage and lead to more accurate predictions of soil carbon flux.
The team conclude that the suggestion that earthworm presence in soils leads to a 33% increase in CO2 emissions is likely a severe overestimate. However, there are over 3,000 species of earthworm living in many different soil types and more experiments are needed before we can definitely determine whether the humble earthworm is a global friend or foe.