Fall 2007

Living Science

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.

If it’s possible that microbes may release less carbon dioxide, does that mean the current models may overstate the effects of warming temperatures?

Absolutely. That is definitely a possibility. However, temperature is only one of many factors that influences microbial communities in soil. One of the most important things we are doing in my lab is trying to understand how much temperature is actually controlling carbon dioxide release versus the influence of other factors. Microorganisms generally respond to stress by producing more carbon dioxide, and things like extreme soil pH or disturbance from land use changes such as tillage can also result in carbon dioxide feedbacks to the atmosphere.

How are you simulating these changes in your research? Do you physically warm the soil to see how the microbes react?

We are taking soil from northern Wisconsin and moving some of it to middle Wisconsin and some to southern Wisconsin. Basically, we use the change in latitude to simulate climate change.

In another experiment, we’ll be taking soil cores and flipping them upside down. Soils that are deeper don’t experience temperature fluctuation the way that soil on the surface does. So if you want to create artificial climate change, you can take soil that’s deeper in the ground and put it at the surface, where suddenly it’s experiencing temperature fluctuations and stress.

Is this the kind of work you imagined when you set out to be a soil scientist?

I wasn’t born knowing I wanted to study dirt. I kind of stumbled into it, and it turned out to be the perfect thing. In my first soils class I learned that soil is formed from rocks, which brings in a geology aspect, and it’s also formed by vegetation at the surface, which brings in a biology aspect. So soil is this perfect blending of my major interests, geology and biology.

In my junior year of college, I took a field trip to a pine stand on the Dartmouth campus. Back in colonial times, it had been cleared and plowed for agriculture, and if you dig a soil pit there today, you will see a layer of charcoal exactly 30 centimeters below the surface, which is exactly the level where a plow would flip the soil over. I guess they planted potatoes or corn, and they would burn the stubble and then plow it under. So even a hundred years later, there was this layer of charcoal right exactly where they flipped the soil over with a plow. And that to me was just really fascinating. I never expected to see that in the soil.

What’s the most interesting thing you’ve unearthed in your work?

There is a type of soil, called spodosol, that you find in conifer forests, and it’s very dramatic and pretty. It’s dark black at the top, because that’s where high levels of carbon are decomposing. And then it has a bright white layer that is called the e-horizon, for eluviation. Underneath that is this bright orange-red layer that has streaks of brown in it, which is super pretty. And then it gets to be yellow and then cream colored as you go down. It looks like a sunset below the surface of the earth and it’s absolutely beautiful.

People make fun of me all the time for saying that you never forget your first spodosol. But it’s true. Any time I had dug holes in the ground before that, the soil was just brown. I had no idea that soil could be that pretty, and that helped convert me.

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