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Greenhouse gas emissions from soils increased by earthworms: opening a can of worms

  • 12 Feb 2013, 10:15
  • Ingrid Lubbers and colleagues

The results of a new study on the effect of earthworms on greenhouse gas emissions attracted a fair bit of attention last week. Here, the researchers respond to a few of the most popular comments and misconceptions about their findings.

Last week, our study on the effect of earthworms on soil greenhouse emissions appeared in Nature Climate Change. Although we were pleased about all the attention focused on our work in Carbon Brief and the Guardian, almost inevitably some misunderstandings showed up in online discussions. Here are our answers to six of the most frequently posted comments and questions:

1: Earthworms cause global warming, not humans, so we can all stop worrying now

This comment, which also comes in another form - that we're making excuses for human-caused global warming - is without a doubt the most serious misunderstanding of our work.

Humans cause global warming, not earthworms. Since earthworms have been around for hundreds of millions of years, it would be naïve to assume they suddenly started to cause global change somewhere in the previous century.

In the case of carbon dioxide, the largest cause of increasing atmospheric concentrations is burning fossil fuels and land-use change. Whereas with nitrous oxide, the largest cause is the enormous amount of nitrogen fertilizer that mankind applies to agricultural soils.

Our study shows earthworms are an important actor through which humans can cause nitrous oxide emissions. It also suggests that, due to the increasing habitat for earthworms over the coming decades, earthworm-induced emissions may increase. But these emissions are largely conditional on humans adding fertilizer to agricultural systems.

2: The authors ignore all the positive things earthworms do in the soil

We certainly do not - not in our Nature Climate Change article, and not in the many studies on earthworms that we published in the past. Agriculture would be much worse off without earthworms. They have many positive effects on soil fertility, drainage, and on overall soil quality.

In our article we just show that we cannot give them credit for decreasing greenhouse gas emissions. Rather, they appear to increase greenhouse gas emissions as an unfortunate side-effect of their positive effects on soil fertility.  

3: How much do earthworms contribute to greenhouse gas emissions, compared to fossil fuels?

This is an understandable question - but unfortunately the answer is very complex. It is estimated that carbon dioxide from fossil fuel burning causes around 60 to 70 per cent of global warming, and soil nitrous oxide emissions cause around five to seven per cent. It is difficult to say what part of that five to seven per cent is due to earthworms, as detailed data on global earthworm populations is lacking.

But a rough guess based on our findings is that earthworm-induced emissions cause around one per cent of global warming. This may not seem much at first glance, but these emissions are only marginally less than all fossil fuel used in global air travel.

                              Earthworms1 

The team of scientists found earthworms increase carbon dioxide emissions from soil by 33 per cent and nitrous oxide emissions by 42 per cent. 

4: The study shouldn't have been published, as the results could be misused by special interest groups

The results of scientific studies can be selectively misinterpreted. But we believe that should not keep anyone from publishing good and important science, provided they are careful in explaining its implications and limitations.

We strongly feel that, if we would choose to not publish our work because it might be misused, science would have lost.

5: You are not helping the cause of organic agriculture

The purpose of our study was to determine the role of earthworms in the soil greenhouse gas balance, not to endorse any particular form of agriculture. We don't agree that our results reflect badly on organic agriculture.

It is true that organically farmed soils generally contain more earthworms than conventionally managed soils. It is also true that our results therefore suggest that earthworms will have a relatively larger impact on nitrous oxide emissions from organic agriculture.

That said, recent studies suggest that emissions of nitrous oxide from organic systems might be lower than in conventional farming systems and more carbon is stored in the topsoil of organic systems, despite higher earthworm densities. This might partly be because, as we have pointed out in our article, none of the studies that we could use in our meta-analysis accounted for the fact that - especially in organic systems - earthworms can increase crop growth.

This might in turn lead to increased soil organic matter storage and increased nitrogen uptake, and therefore in lower greenhouse gas emissions. This needs to be studied further in the future.

6: Does this mean that we should now kill all earthworms?

Certainly not! And we couldn't if we wanted to. Any farmer knows that earthworms are beneficial for agricultural productivity and soil quality. But the evidence suggests that we should not delude ourselves into thinking that they can reduce greenhouse gas emissions for us. We will have to do that ourselves.

Can of worms

We hope that this has shed some more light on the main outcomes and limitations of our study, and that we succeeded in handling the can of worms that we opened.

We would like to thank the Netherlands Organization for Scientific Research, the funding source for our project.

By Ingrid Lubbers, Kees Jan van Groenigen, Steve Fonte, Johan Six, Lijbert Brussaard and Jan Willem van Groenigen

 

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