Has the worm turned for climate change?
The earthworm: climate friend or climate foe? Some studies claim their presence in soil leads to an increase in carbon dioxide emissions. But new research argues that over time, worms help soil act as a major carbon sink – locking up more carbon from the atmosphere than it otherwise would.
How do worms lock up carbon?
Earthworms don’t produce much in the way of carbon emissions, so they’re not typically thought of as a key part of the climate system. But soils take up and give out a lot of greenhouse gases – in fact they store twice as much carbon dioxide as the atmosphere. Since worms’ activity affects how much carbon dioxide is produced in the soil, how much is locked away and how much escapes to the atmosphere, their presence plays an important part in the carbon cycle.
Scientists know that in the process of breaking down organic matter like leaves, worms release carbon dioxide (and nitrous oxide) – driving up emissions from soil. But that digestion process also produces a solid from of carbon which remains in the soil, where it is locked up.
Image - Worms (note)
Source: Lubbers et al (2013)
The big question is whether, on balance, the worms help soil lock up more carbon dioxide than it releases into the atmosphere.
Emissions up or down?
A new study out today in the journal Nature Communications concludes that worms’ presence mean soils end up storing more carbon than they release to the atmosphere in the long run.
This conclusion seems at odds with the results of an earlier study, which compiled the results of 237 separate experiments from already published studies. The older research suggested earthworms cause carbon dioxide emissions from soils to increase by around 33 per cent.
We asked the authors of the most recent study why they came to such different conclusions. They told us that many of the 200+ experiments the earlier study had looked at were quite short-lived, lasting only a few weeks. The main problem with that is initially, earthworms do cause greenhouse gas emissions to increase.
But this is a short term effect, the authors say – and one they observed in their own controlled experiment on two different earthworm species. Given a longer time period, however, the temporary blip in emissions disappears. After that, the amount of carbon dioxide being produced by worms goes down, relative to the amount of solid carbon which gets locked up in the soil.
Weixin Zhang, lead author of the new paper, explained:
“In the earthworm gut, the processes of carbon stabilisation [making carbon into a solid form which stays in the soil] and carbon mineralisation [making carbon into a form which is easily lost as carbon dioxide] are enhanced unequally. As a result, a larger proportion of carbon is trapped [in the solid form] when earthworms are present.”
Short-term experiments do not detect the extra carbon that the action of earthworms stores, Zhang says.
We also asked the authors of the earlier study – Ingrid Lubbers and her colleagues – why the results differed. The authors agreed that more carbon could possibly be stored in the soil in the long run, a result of the unequal gut processes described above. But Lubbers added that the experiments in the new study also weren’t long enough to detect that mechanism in practice:
“The problem with this study (and previous studies, including ours as well) is that no long term measurements were actually done. 53 days is simply too short to measure the processes through which earthworms make carbon into a solid form which stays in the soil, a point which Zhang et al. actually make themselves as well.”
They also noted that the new study had looked only at carbon dioxide emissions, and suggested that factoring in other types of greenhouse gases given off by soils, such as nitrous oxide, would be a better measure of how earthworms affect the climate.
Earthworms – climate friend or foe?
The authors of the new study acknowledge that their results may not fully represent earthworms climate role the world over. Their experiments are based only on a few species, and are performed in controlled environments. In reality, different types of earthworm live in different soils, breaking down different types of plants. Without more research looking at a wider range of species, its difficult to know whether worms would always cause soils to lock up more carbon than it releases.
The authors of the earlier study agreed more work was needed, saying:
“It is good to read that Zhang et al. re-emphasise our plea for more studies […] What we really need now are studies that provide hard data on the effect of earthworms on soil carbon in the longer term under more life-like conditions.”
It’s worth considering too that human activities disturb and change most soils in the real world. On top of that, soils are exposed to climate extremes of heat, cold and moisture. Both of these factors could affect how much carbon soils store.
At this stage, worms’ effect on greenhouse gas emission is an interesting part of the carbon cycle that scientists still don’t know much about. The new research identifies a mechanism through which more carbon could be locked up than is released as carbon dioxide, but experiments have yet to prove the concept
In the grand scheme of things, humans’ rising greenhouse gas emissions are still by far the biggest contribution to climate change.