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Soil microbes burp carbon dioxide after drought-breaking rain -

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Robyn Williams: And this is The Science Show on RN, where a couple of weeks ago we heard from Catherine Osborne on the plight of PhD people and postdocs. This time she's been thinking about Beethoven and burping bacteria.

Catherine Osborne: One of the most unpredictable sources of carbon emissions in places with sporadic rainfall like Australia is the CO2 released from soil when it rains after a drought. Why can't we predict it? Because the little masters of soil carbon turnover, the soil microbes, have just been too bloody difficult to study. Up until very recently.

When I worked at the University of California, Berkeley, I teamed up with my French mate Romain, with his wicked soil CO2 skills (complementing my own wicked soil microbiology skills) to try to understand which microbes burp a whole lot of CO2 into the atmosphere after a drought. That's right, the soil (or more specifically the microbes in soil) burp quite significant amounts of CO2 into the air with the first rains after drought. And generally, the longer the drought period before the rain, the more CO2 is released. But the amount of CO2 is very difficult to predict accurately, and therefore it's very difficult to include in any carbon emission calculations. Hence, why Romain and I were trying to get a better understanding of which of the little microbes in soil were most important to these soil CO2 burps.

You're probably wondering what Beethoven has to do with all of this. Beethoven's 6th Symphony, also known as the Pastoral Symphony, re-creates the atmosphere of an idyllic European countryside during the first three movements. Then the fourth movement is the dramatic storm movement, with its vivid depiction of sheets of rain and thunder. And then the final fifth movement Beethoven titled 'Thankful feelings after the storm'.

Quite often I have thought about how the work that Romain and I did in California on soil carbon emissions mirrored Beethoven's 6th Symphony, written in Austria more than 200 years ago. We collected soil three times over a summer season, as the soils dried out. So our first three time-points in our study were a bit like Beethoven's first three movements, with serene soil sampling in picturesque countryside, and everything in the soil drying out and slowing down.

Then comes the fourth movement, or the dramatic storm. In our study, we carefully watered our dry soils in a controlled way to mimic the first rainfall after drought. And then our last time-point in our study was two hours after the 'storm event', and these were the key samples for discovering which soil microbes have 'thankful feelings after the storm' and were therefore those most likely to be happily burping CO2 out into the atmosphere.

What did we find? We found that the soil fungi don't seem to change much over summer or with the 'storm', but the soil bacteria were a different story. And what was fascinating was that in three very different soils, it was always the same small group of bacteria, known as the acidobacteria, that were responding very rapidly after the rainfall, or in that fifth movement. They seemed to be the soil microbes that were ramping up their activity and had the most 'thankful feelings after the storm', and therefore are probably most likely responsible for the huge soil CO2 burps that occur when it rains after drought.

Why haven't we been able to know this before? You know that awesome smell of soil that normally accompanies rain? For a very long time, we have known that soil smell is caused by a small group of bacteria called actinomycetes, because we've been able to grow and study actinomycetes in the lab. That smell is the actinomycetes releasing their spores, but it doesn't seem to have anything to do with the CO2 burps. In contrast, acidobacteria are very difficult to grow in the lab, and we only know about their ability to ramp up their activity after rain because of exciting new DNA sequencing technologies, which we used in this study.

In fact, during my Honours year at Melbourne Uni, I actually grew a number of acidobacteria and our lab was one of the first labs in the world to grow and study any acidobacterial cultures. The next logical thing to investigate would be to take the info that Romain and I have about the acidobacteria and their response to drought and rainfall and then study acidobacteria in the lab to determine what factors affect their CO2 burping, which in turn could then help make this important part of the carbon cycle more predictable.

In New Zealand they invest a lot in trying to understand cows burping methane, because that's a significant contribution to their national carbon emissions. So you'd think that understanding the source and scale of soil CO2 burps when drought breaks would be worth studying in Australia. But that's a story for another time. Or perhaps another Beethoven symphony!

Robyn Williams: Catherine Osborne in Melbourne. You may recall that two weeks ago on The Science Show she talked of short-term contracts and other blights on the early and mid careers of young scientists in Australia.