Digging Dirt

TERI BALSER, assistant professor of soil science, studies microbes in the soil, focusing on their role in releasing carbon dioxide into the atmosphere. Balser believes this release, rarely accounted for in climate models, may be a crucial piece of the climate-change puzzle. She recently received a Faculty Early Career Development Award from the National Science Foundation–one of the most prestigious honors beginning researchers can receive–to advance her research and teaching.

What does soil have to do with climate change?

The soil has more carbon stored in it than the atmosphere and vegetation combined, and bacteria and fungi living in the soil are responsible for decomposing that carbon. Basically, they eat it, if it’s something they like. They chew it up, decompose it and turn it into carbon dioxide.

As the climate warms, it’s predicted that soil carbon is going to be decomposed faster by microorganisms. So there’s a real possibility that you might get this vicious cycle set up, where increased warming causes more carbon dioxide to be released from the soil, which causes more warming.

Ninety percent of soil carbon is in a fairly stable form. But it’s such a large amount of carbon that even a tiny change in that pool size can really influence atmospheric carbon dioxide levels.

Yet we don’t often hear about soil in the discussion of global warming. Why not?

So far, the people who have been studying climate change have typically been climate modelers. They look at the large scale and try to make models that will predict the impact of increasing atmospheric carbon dioxide. They tend to leave out details about the soil microorganisms, which are like the valve that controls the release or storage of carbon in soil.

What I’m really studying is the extent to which the microbial community controls that release and how we include it in models. I’m asking: Are we getting ourselves into trouble by ignoring the microbial community when we’re talking about climate warming?

Are we? What have you found so far?

I found that when we look at the physiology of the organisms, we sometimes get the opposite result than what the modelers predict.

Warming the soil a little is great for microbes; it makes them more active. If you’re just thinking like a chemist, then an increase in temperature equals an increase in reaction rate, and that means more carbon dioxide. For a really stable molecule, like complex carbon, you have to have a lot of energy input before it will break down, and so the higher the temperature, the faster and easier those big complex molecules will break.

That works great in a test tube. But this is where the biology comes in. Microorganisms are not just simple reactors. Microorganisms are biological entities, and although it’s not really accurate to say they make choices, they do have survival strategies. What we have seen happen is that when we increase the temperature, the microbes did not increase the breakdown of complex carbon. We saw the opposite. When the temperature went up, the utilization of simple carbon increased, and the utilization of complex carbon decreased. So, they’re effectively making a choice to use simple carbon and not complex carbon.

What does this mean for climate change models?

It means that microbes may not make a continuous contribution to atmospheric carbon dioxide as the climate warms. It could be that after all the simple carbon is used up, they won’t release any more carbon dioxide no matter how much the temperature heats up. It’s just a big unknown. It’s something that could turn out to be important in the grand scheme of things.