8 February 2013

Africa: Climate Conversations - Do Earthworms Contribute to Climate Change?

We often hear about livestock being a major cause of greenhouse gas emissions. Picture them: Formidable herds of flatulent quadrupeds munching their way - figuratively speaking - through millions of hectares of rainforest.

But it seems we need to look below the surface, literally, to find another climate change culprit: earthworms.

That's because a new study published in the scientific journal Nature Climate Change shows that worms may be a major cause of greenhouse gas emissions, creating what it describes as the "earthworm dilemma".

The findings could soil the reputation of a creature which, as one of nature's ugliest, has beaten all the odds to find its way into the hearts and minds of gardeners and ecologists the world-over, by virtue of its much-championed role as a custodian of soil fertility.

In the study, scientists from Wageningen University, the International Center for Tropical Agriculture (CIAT) and the University of California-Davis reviewed existing literature and found that earthworms, through the supposedly benevolent act of breaking down organic matter and boosting soil health, could be responsible for as much as one-third of the carbon dioxide emissions from soil, and 42 percent of soil-based emissions of nitrous oxide, a greenhouse gas 300 times more potent than carbon dioxide.

Crucially, they also bring into question the long-established idea that earthworms help trap carbon dioxide in the soil, thought to have at least partially negated their greenhouse gas footprint (well, drilosphere). Instead the study found that worms may contribute to global warming by helping release carbon dioxide from the soil into the atmosphere.

And that's not all: The study also questions the increasingly popular "no-till" farming practices that eschew ploughing in order to protect soil structure. That helps preserve earthworm habitats, enabling them to thrive. Couple that with the increasing use of organic fertiliser - a veritable banquet for earthworms - and their greenhouse gas emissions could be set to rise further still.


But according to CIAT soil scientist Steven Fonte, one of the authors of the study, when it comes to dishing the dirt on earthworms, the creatures still have a significant amount of, well, wriggle room.

"These are really important findings that challenge a long-held consensus of the precise role of worms in climate change mitigation," he said.

"But earthworms should definitely not be seen as pests. They're still vital to farm productivity and food security. They help to move nutrients through the soil, providing food for plants, and by improving soil fertility they can also reduce the need for chemical fertilisers.

"They can help quickly restore seriously degraded land to make it productive again, which, for a smallholder farmer, can mean the difference between a failed harvest and a bountiful one."

For Fonte, the jury is definitely still out: "While our findings are provocative, they are mostly based on laboratory studies and largely ignore the potential for earthworm benefits to plant growth and nitrogen use, which could counteract the negative trends observed here".

While further research might help vindicate the humble earthworm, it could be some time before they grow back their good reputation.

Neil Palmer is a photographer and writer for the International Center for Tropical Agriculture. This blog first appeared on the CIAT website.

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