The Surprising Life Inside Frozen Soil
In many places across the U.S., winter soil is blanketed with frost and snow—a seemingly lifeless environment. Although the warmth of spring is still months away, beneath the surface is a different story. Matthew Wallenstein, associate professor of ecosystems science and sustainability at Colorado State University, says there is a wealth of activity as microbes exchange nutrients with soil and root systems even in frozen soils. He’s joined by Colleen Iversen, ecosystem ecologist at Oak Ridge National Laboratory, to discuss the life that’s teeming in some of the world’s frostiest soils in the Arctic tundra.
Matthew Wallenstein is an associate professor in the Department of Ecosystems Science and Sustainability at Colorado State University in Fort Collins, Colorado.
Colleen Iversen is a senior staff scientist and ecosystems ecologist at Oak Ridge National Laboratory in Oak Ridge, Tennessee.
IRA FLATOW: This is Science Friday. I’m Ira Flatow. Look outside your window. In many places across the country, what do you see? You see a frozen wasteland. Oh, yeah, it’s been that way for a lot of us in the East and in the Midwest.
And the activity of springtime is still months away. But beneath the soil, it is a different story. My next guest studies some of the frostiest soils in the world or permafrost is– see what I did there– in the Arctic tundra. They are Matthew Wallenstein, associate professor in the Department of Ecosystem Science and Sustainability Colorado State University and Colleen Iversen, senior staff scientist and ecosystems ecologist at Oak Ridge National Laboratory. Welcome, both, to Science Friday.
MATTHEW WALLENSTEIN: Thanks. It’s a pleasure to be with you on such a frigid day.
COLLEEN IVERSEN: Thank you, It’s wonderful.
IRA FLATOW: Well, let me ask you, Matthew, you pick up a shovel full of frozen soil. It looks like there’s nothing going on in there, winter time. But really, it’s bustling with microbial activity, even in the winter, right?
MATTHEW WALLENSTEIN: That’s true. In recent years, we’ve really discovered that, even in soils that are frozen, there are still active microbes, bacteria, and fungi, and other soil organisms that not only can survive those frozen conditions, but actually maintain activity. So we can measure, essentially, their breathing.
We can measure gases like CO2 coming out of the soil. And then we can use other techniques to actually study their ability to reproduce. Through replicating DNA, we can see that they’re actually transforming that soil and making nutrients available so that, when the plants start coming to life in the spring, there’s actually nutrients available for them.
IRA FLATOW: Well, all living life as we know it needs water. So if the soil is frozen, where did the microbes get the water they need to survive from?
MATTHEW WALLENSTEIN: That’s right. So microbes that live in soil are actually really aquatic organisms. They live on water films. And even in soils that appear frozen to use, there are– at the scale of a microbe, if you could shrink yourself down to the microbe level, you would actually be living in these water films.
And one of the reasons that they can maintain these water films well below 0 Celsius, and the point of freezing, is that there’s a lot of salt in that water. And as that soil water concentrates down due to freezing, those solutes become more concentrated. And that lowers the freezing point of water.
IRA FLATOW: That’s kind of interesting. Now Colleen, I know you study what root systems are doing in arctic soil when it looks like there’s nothing going on above the ground. What are the roots doing?
COLLEEN IVERSEN: Yeah, so I could take you to Barrow, Alaska in the summertime.
IRA FLATOW: Let’s go.
COLLEEN IVERSEN: The maximum temperature is about 40 degrees Fahrenheit. So it’s really not that far above freezing in the summertime there. I mean, you can look out over the tundra. And it would look like sort of a green seashore.
You don’t see any trees. And the plants that are there are hugging the ground to sort of avoid the harsh wind and cold. And really, that’s just the tip of the iceberg.
So really, all the action in the tundra is below ground. There can be five times as much plant biomass beneath the ground surface compared to above. And those roots that live there are really adapted to live and survive in cold conditions.
IRA FLATOW: Well, what do you mean really adapted? What’s different about them?
COLLEEN IVERSEN: Yeah, so Barrow is underlain by permafrost. So it can be 1/2 a mile thick, frozen soil. And the part of the soil that the roots live in is just this narrow 12 inches of soil on top of the permafrost that thaws every year.
And so, if you think about it, the further the roots sort of dip their toes into the soil, the closer they are to that freezing permafrost. So there’s some roots that grow along the permafrost surface at 1 degree C really close to freezing. But they take up nutrients, and water and help the plant survive even in those conditions.
IRA FLATOW: Dr. Wallenstein, how do permafrost microbes differ from the microbes we’re going to find in our own backyard soil?
MATTHEW WALLENSTEIN: That’s a great question. So I would have assumed that the microbes that were buried in permafrost tens of thousands of years ago would be long gone. But in recent decades, scientists started finding evidence that there were microbes that were viable, meaning that they could extract those microbes and culture them.
And then we found out that they’re are actually not only viable, but they’re actually living in those conditions, albeit much more slowly than we might see in warmer soils. But they’re doing all the things that microbes do. And indeed, they have some special adaptations.
For example, they’re able to produce antifreeze compounds to reduce the freezing point. And one of the critical things is that they have to be able to maintain flexibility. So all of the molecules inside of a microbial body, DNA, proteins need to be specially adapted so that they’re not rigid at those cold conditions. So we do see special adaptations. But in fact, those microbes can also live at temperatures above 0, we often find.
IRA FLATOW: Interesting. Let’s go to the phones, 844-724-8255, to Suzanne in Grinnell, Iowa. Hi, Suzanne.
SUZANNE: Oh, hello, yes. My question was similar to that. Do you find that there are sort of species, adaptations that– so for instance, in Iowa, are my microbes doing some things different? Bears hibernate. Other animals do other things. So are my microbes– what are they doing, as opposed to the– I mean, that’s a part of it.
MATTHEW WALLENSTEIN: Yes, that’s a great question. And most of the work in frozen soils has occurred in regions like the Arctic where they’re permanently frozen. I think we know less about the microbes in places like Iowa that experience more. Seasonality.
We do know that the microbes there are active in the winter. And we detect respiration. But there’s less known about whether those microbes are as well-adapted. Certainly, they all survive the winter or they wouldn’t be there.
So they have to be adapted at least to survive it. But there are different strategies. They may go dormant during winter.
IRA FLATOW: Now I heard something recently that sort of blew my mind when we were talking about doing this thing. And I’m a gardener. And I deal with soil. Well, I recently learned that scientists are starting to think differently about what soil is actually made of. It’s not the leaf mold, and the compost, and all that kind of stuff, Matthew?
MATTHEW WALLENSTEIN: That’s right. So we really used to think that most of the organic matter in soil was really just kind of the leftover parts of plants that weren’t easily decomposed by microbes and other organisms. And instead, what we’re now finding is that most of the organic matter in soils, that brown and black stuff that you see, is actually dead microbial bodies. So when your roots die, if you let the leaves fall to the ground, and they decompose, all of that plant material, that is the original source, the original carbon that becomes soil organic matter.
But it’s all transformed. It’s utilized by microbes and transformed it in different chemical compounds. And then, in order for it to be stabilized, it actually has to interact with minerals. So the soil particles and minerals form chemical bonds with the organic matter that protect it for the long term from being decomposed and lost from the soil.
IRA FLATOW: So it’s dead microbes I’m picking up when I grab a hand of soil?
MATTHEW WALLENSTEIN: That’s right, dead microbes and their waste products. We have a great term for it. We call it necromass.
IRA FLATOW: Necromass. Necro, is that Latin for dead?
MATTHEW WALLENSTEIN: Exactly, yeah.
IRA FLATOW: I’m just still in shock thinking about all my– Colleen, did you know that? I mean, is that well-known?
COLLEEN IVERSEN: I have been following that work for some time. That is well-known. And the partnership between microbes and roots is really important, as well. So the fact that the microbes are transferring or transfiguring that organic matter that goes into the soil– both for their own sakes, but also releasing nutrients that plants can get– that’s the reason the ecosystem works.
IRA FLATOW: Now this is on a scale of when I learned that all the dust under my bed is actually my skin cells. So this is all about microbes. I go into the plant store and ask, give me a bag of dead microbes, I’m not sure that they’re going to know what I’m talking about. That’s great.
Now Colleen, let’s talk more about the adaptations that the roots need to have for growing in the permafrost soil. What are they making? What kind of adaptations do they need?
COLLEEN IVERSEN: So similar to what Matt was talking about, in terms of the microbes producing sort of an antifreeze, roots will concentrate sugars in their roots to sort of lower the freezing temperature and keep themselves from freezing. And also, one of the big adaptations that we see is that these plants put quite a lot of biomass below ground. And so that’s not just in roots. But it’s in below-ground stems and rhizomes.
And that’s for storage. So there’s not much plant biomass above ground. And so the plants are keeping the nutrients and the carbon they need for the next year in these below-ground storage organs, so they can perk right up the minute spring comes. And sometimes spring to these plants can mean poking a flower through the snow.
IRA FLATOW: That’s interesting. I know this is not your specialty, but you’re a plant scientist dealing with frozen soil. What is there about perennials and roots– is it the roots– that they can survive being frozen in the soil over the winter?
COLLEEN IVERSEN: It’s this sort of antifreeze, getting rid of some of the water. But the other thing is that your listeners might not know is there are lots of different kinds of roots. They might think of roots below ground as all the same thing, because we call them roots.
But if you think about the forest in your backyard, there’s big, woody roots that you might trip over when you’re walking around in the forest. And those are there to hold the tree in the soil to provide structure and transport. But then there are what we call fine roots. And those are roots that are narrower in diameter than the charger for your iPhone.
And those are really the roots that are important for water and nutrient uptake for the plants. But they don’t live very long. So the plant could make them at the beginning of the summer and they could die by the end of the summer or in the fall. Oftentimes, sort of the maximum lifespan for those fine roots can be a year.
So it happens that the greatest rates of root production are often in the summer into the fall. And then the plants will often let those roots die over the winter, so they don’t have to maintain them, they don’t have to respire over the winter. And then it can grow them new in the spring, because it has all these stories of carbon and nutrients saved up in below-ground stems.
IRA FLATOW: That reminds me, I know– because I have trees and I’ve been watering them for years– they always say to water a shrub or a tree at the drip line, right? Is that where these small roots are? When you when you speak about these fine roots, is that what we’re talking about?
COLLEEN IVERSEN: Yeah, they stretch out very far into the soil, sometimes past the canopy of these big trees, and are interconnected with other trees, and signaling, and go down deeper into the soil profile. And I think they’re sort of beautiful, the sort of fractal nature. One of the things that I like about winter is seeing the trees without leaves, because you can sort of see the fractal nature of the branches. And that’s what it looks like below ground, as well.
IRA FLATOW: This is Science Friday from PRI, Public Radio International, talking about the roots in the wintertimes with Matthew Wallenstein and Colleen Iversen. Go back to that thought again, the fractal nature of the roots are like the fractal nature of the branches above ground.
COLLEEN IVERSEN: Yeah, so– sorry were you going to ask a question?
IRA FLATOW: No, go ahead. No, no.
COLLEEN IVERSEN: So one of the interesting developments over the last few decades is thinking about roots not sort of as a monolith– they all do the same thing– but even within fine roots, there are differences in what the roots do, depending on where they’re located. So if you think about streams coming down from a mountain system, the first-order streams would be the highest and the smallest. And then they join to form a second-order stream and a third-order stream.
Roots work the same way. So the roots that are furthest out, the little fingertips of roots, those are the first-order roots. And those are most important for acquiring water, and nutrients, interacting with soil fungi, providing carbon for microbes. And as you get further in and closer to the plant, those roots are more and more woody and more used to transport those nutrients and water to the rest of the plant.
IRA FLATOW: All right, interesting. Yeah, it is a whole different kind of thing than we think about. Matthew, how does the changing arctic seasons– because we know the Arctic is probably for global warming is changing faster than any other place on Earth.
MATTHEW WALLENSTEIN: Yeah, I’ve seen it myself. I’ve been working up in the north slope of Alaska for about 15 years. And it’s such a rapid and clear increase in the length of the season.
So the spring is coming earlier. The fall is coming later and in the overall temperatures. We end up wearing shorts out there in the winter when we’re doing fieldwork sometimes.
And so it’s clear that the seasonality is changing. And that’s something we don’t talk about it a lot. But the length of that growing season, which historically, has been very short. So the plants have to complete their full life cycle within that short season. The microbes have had that longer, because the soils are still either unfrozen or in that range where the microbes can still be active well past the point where plants freeze.
But what’s interesting is that there’s different processes happening. So during the winter at the sites that I work at, the microbes are still active. The plants are largely inactive. And the microbes are slowly processing organic matter.
And one of the things that they do is they release nitrogen into the soil into the forms that plants can utilize. So in the spring when the plants pop out of the ground and are ready to go, there’s all this available nitrogen that the microbes have made available over the winter. And then the plants and microbes start competing for that nitrogen and other nutrients. But as we see the length of the summer increase, we might expect to see that nitrogen actually becomes more limiting, because that period of time where the microbes are making it available over the winter may be getting shorter.
IRA FLATOW: So we’re affecting the microbe population and the plant population. In the minute I have left, do you see that, as the permafrost is defrosting, that there is more carbon dioxide and methane coming out of the ground? Do you study that, also?
MATTHEW WALLENSTEIN: We do. And collectively across the region, it’s a real concern. I mean, the amount of carbon stored in our arctic soils is as much carbon as there is in the vegetation and atmosphere globally combined. So there’s this huge amount of carbon that’s been locked away. And indeed, if it suddenly becomes thawed and the microbes begin to process it and produce not only CO2, but methane– which is a more powerful greenhouse gas– that’s a real concern that could cause a further increase in CO2.
IRA FLATOW: Did you see the movie Downsized?
MATTHEW WALLENSTEIN: No.
IRA FLATOW: Watch the end of that movie. I’m not going to give it away.
MATTHEW WALLENSTEIN: I will.
IRA FLATOW: But is has a whole lot to do with the defrosting of the Arctic regions and the release of methane. Yeah, it doesn’t sound like it from the title. We’ve run out of time. I’m babbling here.
I want to thank you both for taking time to be with me today. Matthew Wallenstein, associate professor in the Department of Ecosystem Science and Sustainability at Colorado State University, Colleen Iverson, senior staff scientist and ecosystems ecologist at Oak Ridge National Laboratory, thank you both for taking time. And have a great weekend.
COLLEEN IVERSEN: It was a pleasure. Thank you.
IRA FLATOW: You’re welcome.