Andreas Späth

Are earthworms global warming villains?

2013-11-18 09:04

Andreas Wilson-Späth

Charles Darwin was fascinated by earthworms. He carefully observed them as a young man and his last book - The Formation Of Vegetable Mould Through The Action Of Worms, With Observations Of Their Habits - was published in the year before his death and really put oligochaetology (the study of earthworms) on the map.

Earthworms have long been known to improve soil quality by increasing fertility and aeration, and in recent years they've become a favourite pet to the committed treehugger who employs them in worm bins and similar vermicomposting systems designed to convert household vegetable scraps into high-grade compost.

These days, the humble yet slimy creatures (fat chance you'll ever use that combination of attributes when describing a human being!) have become embroiled in a controversy over whether or not they are implicated in climate change.

We've all heard about the methane farting ways of cows, but earthworms. Really!?

A review

Tempers were raised in February, when a review of all 57 previous experiments on the subject suggested that soils that contain earthworms emit 42% more nitrous oxide (N2O, a very powerful greenhouse gas) and 33% more CO2 than those that are worm-free.

Earthworms have this effect because they enhance the mixing and decomposition of plant material in the soil, which results in the formation of CO2, because their guts and droppings offer the perfect growth environment for N2O-belching microbes, and because their networks of burrows allow greenhouse gasses to escape from the soil into the atmosphere more easily.

The authors of the paper concede that it is still unclear to what extent the boost earthworms give to plants by enhancing soil fertility counteracts the greenhouse gas emissions for which they are responsible, but they crucially claimed to have found no evidence that earthworms trap carbon in the soils they munch through.

Their conclusion: earthworms increase the global warming potential of soils by 16% overall.

If that was a cruel squish of the boot heel for our wiggly subterranean friends, a new paper published in October has gone some way towards exonerating their green credentials.

It shows that, contrary to the earlier study's claims, earthworms do, in fact, contribute significantly to sequestrating carbon in soils by means of an "earthworm-mediated 'carbon trap'".

Greenhouse gasses

When earthworms chew through dirt, the authors content, microbes in their guts turn organic carbon into a solid and stable form which becomes locked up underground. In the long run, they argue, soils teeming with earthworms end up retaining more carbon than they release.

"We believe the recent estimation of earthworms increasing soil [CO2] emissions by 33% is likely a severe overestimate," says lead author Weixin Zhang of the South China Botanical Garden in Guangzhou.

So earthworms both release and trap greenhouse gasses in soils. The question of their net effect remains controversial, but the scientists in both camps agree that more research is necessary to arrive at a conclusive answer. Darwin would approve.

Until then I suggest that you don’t throw away your worm composting bin or start the task of plucking all the earthworms out of your garden topsoil quite yet, but keep things in perspective: at worst, greenhouse gas emissions from earthworms have been estimated to cause about 1% of global warming; human activities like burning fossil fuels and deforestation are responsible for 60 to 70%.

- Andreas is a freelance writer with a PhD in geochemistry. Follow him on Twitter: @Andreas_Spath
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