Global worming: are earthworms contributing to climate change?
- 04 Feb 2013, 10:30
- Roz Pidcock
It may not be all about us humans - earthworms could be
contributing to climate change too, according to a new study.
What's more, the research warns worm populations are set to boom in
the next few decades. So should we be worrying about worm-induced
warming?
Well, probably not in the grand scheme of things - but the
humble earthworm does have more to do with greenhouse gas emissions
than you might think. Earthworms don't produce much in the way of
emissions themselves. But the soil they live in does - and worms
play a big part in soil.
In the
new study, published in Nature Climate Change, researchers in
Holland, the United States and Colombia compiled the results of 237
separate experiments from other published studies to explore
earthworms' role in global greenhouse gas emissions.
Gut feeling
About
20 per cent of global carbon dioxide emissions and
two thirds of nitrous oxide emissions come from soil. Emissions
are produced by a number of natural biological processes involving
plant roots and the microorganisms that live in the ground.
The authors of the study refer to earthworms as soil ecosystem
engineers. This is in part because they affect the physical
structure of the soil by burrowing - making it more porous.
Earthworms also interact with the microbes that produce the bulk of
carbon dioxide emissions.
The presence of worms affects how much carbon dioxide is
produced in the soil and how much escapes to the atmosphere.
Scientists are concerned that earthworms increase greenhouse gas
emissions - and that earthworm numbers are on the rise.
Nitrous oxide is another powerful greenhouse gas. Bacteria in
the earthworms' gut produce nitrous oxide and emissions from
worm-infested soil can be three times as high as from soil without
any worms, the paper says.
Earthworms increase carbon dioxide and nitrous oxide
emissions from soil. Adapted from
Lubber et al., (2013)
"Earthworm dilemma"
For a while, scientists have faced a problem. They know
earthworms can increase emissions from soil. But worms can also
help the soil store carbon more efficiently, permanently locking it
away.
For a while it was unclear whether worms increase or decrease
the total carbon emissions from soil - what scientists rather
affectionately called the earthworm dilemma.
The scientists in the new study combined all the results they
could find to study this question. Overall, they found that the
presence of earthworms in soil increased nitrous oxide emissions by
42 per cent and carbon dioxide emissions by 33 per cent.
Worm-induced warming
But it's more complicated than that. Worms can increase
emissions of one greenhouse gas while reducing emissions of the
other, the study says. Ideally, to work out the overall impact,
scientists need experiments that look at both gases at the same
time.
When they did this, they found earthworms increased the global
warming potential of soils by 16 per cent overall. The study
concludes:
"Earthworms play an essential part in
determining the greenhouse-gas balance of soils worldwide … Our
results suggest that although earthworms are largely beneficial to
soil fertility, they increase net soil greenhouse-gas
emissions."
Earthworm invasion
Earthworms' influence on global climate is likely to get bigger,
say the scientists - although in the grand scheme of things it will
remain relatively small. According to the paper:
"Over the next few decades, earthworm
presence is likely to increase in ecosystems worldwide. For
example, large parts of North American forest soils are now being
invaded by earthworms for the first time since the last
glaciation".
The growing use of organic fertilisers increases will provide
more food for earthworms, the study says. On top of that, the move
away from conventional land cultivation could also boost
numbers.
On the other hand, changing environmental conditions like
habitat degradation and invasions by non-native species could
reduce worm numbers. So laboratory studies like the ones this paper
analyses are useful up to a point but reality may be more complex,
as the researchers note.
Despite the efforts of the humble earthworm, the vast majority
of carbon dioxide in the atmosphere - which is responsible for
60 per cent of the total warming from greenhouse gases - is
caused by human activities such as burning fossil fuels and
deforestation.
Still, it's perhaps a reminder that there's more to climate
change then what's happening above ground.
Lubbers et al., (2013) Greenhouse-gas emissions from soils
increased by earthworms. Nature Climate Change, doi:
10.1038/nclimate1692