Which worm species are the worst culprits is not yet known. ‘We suspect it’s the earthworms in the upper soil layer, the humus eaters, that contribute most to the laughing gas production, because they eat fresh plant remains,’ says Van Groenigen.
As they digest plant remains, the worms produce nitrous oxide, commonly known as laughing gas, in their digestive tract. Laughing gas is also produced in the soil as bacteria and fungi break down organic material. Van Groenigen: ‘Soil contains less oxygen and more nitrogen, so more laughing gas is formed there than above the ground.’
If there are earthworms in the soil, up to eighteen times as much laughing gas can be released as plant remains decay. ‘This can disturb the balance considerably. Not because of the quantity, but because laughing gas is a very strong greenhouse gas. One molecule is comparable to three hundred molecules of carbon dioxide.’
Until now, worms have always been regarded as beneficial for the environment. They take organic material with them into the soil, where it decays more slowly, resulting in lower carbon dioxide emissions. Now it appears that the process of plant decay in the soil is associated with higher levels of laughing gas production. The researchers now need to work out the net effect of earthworms on climate change. Van Groenigen: ‘Our research makes it clear that soil fauna play a role in greenhouse emissions from agriculture. We need to take this into account in our climate models.’