Carbon in the Ground 4: Serengeti of the North

Buckle up for a trip back in time to the last ice age. We touch down in the mammoth steppe, and discover that this was a surprisingly productive ecosystem--it supported a high density and diversity of animals, including mammoths and other big hairy herbivores. So productive, it has been called the 'Serengeti of the North'.  And it turns out that the very nature of this ecosystem--the interactions between herbivores, plants, microbes in the guts of animals, microbes in the soil, little digging animals and even mineral grains--can help explain why so much carbon ended up stored in the permafrost ground of the Arctic. 

Tune in to explore the Pleistocene grasslands, and discover how large herbivores acted as 'bioreactors', enabling nutrient cycling to occur even in the frigidly cold conditions of the last ice age. Discover how plants swap carbon with microbes in return for nutrients in an elaborate trading scheme. Find out how decomposition can chop that spaghetti bolognese of organic matter we learned about in episode one into small pieces, which can then bind with minerals in the soil, becoming a very stable form of carbon in the ground.

Our experts on this show are Prof. Marc Macias-Fauria and Dr Jeppe Kristensen.

Co-hosts: Roberta Wilkinson and Sam Cornish
Reporting, editing, mixing and original music: Sam Cornish
Sound design: Jihad Zgheib

Transcript

INTRO 

R: Hello, welcome back to Polar Pod and our mini-series on Carbon in the Ground

S: I’m Sam Cornish

R: I’m Roberta Wilkinson

S: And we are your co-hosts. 

R: Each episode, I send Sam out to talk to experts, and he reports back. So far in this series, we’ve learned how carbon gets into the ground, we’ve learned about permafrost, the great freezer of the north, and in the last episode, we heard about how permafrost thaw can transform a landscape.

S: That’s right, we closed on this image of the Batagay thaw slump, which is the biggest thaw slump in the world.

R: And a thaw slump is like where the permafrost has melted and the ground is collapsing underneath it

S: Exactly, and Loeka Jongejans who we spoke to last time told us how, as the thaw slump has retreated, mammoth tusks, and even fully intact animals like the foal she saw, were being exposed in the wall of the thaw slump, known to the locals as the gateway to the underworld. 

R: Sam you promised that we would step through that gateway and explore the past?

S: I did indeed, and this is not a mere flight of fancy Roberta, we’re going to explore an ice age environment called the mammoth steppe, and we’ll see how much of the carbon that we find today in the permafrost owe their origin to the ecosystem that thrived there.

THEME MUSIC

S: Imagine looking at the northern hemisphere from space during the last ice age. We can big ice sheets sprawled across large parts of the northern latitudes. But, there are also quite extensive areas of land where there is no ice. Ice sheets require significant snowfall to grow, and they only flow downhill, so these ice-free areas were quite dry areas, and often relatively high. Let’s rotate the globe so we’re looking down on the part of the Arctic where Russia and Alaska meet. The Bering Strait separates Asia and North America today, but as we’re looking down on this ice age world, we can see that they actually did meet–sea levels were lower and there is a land bridge between the two. And there’s a region of unglaciated land stretching across from Siberia into Alaska. From Alaska, we can see an ice-free corridor extending southwards through Yukon in Canada between the massive bulk of the Laurentide ice sheet to east, and the glaciers of the mountain ranges to the west. This unglaciated land is called Beringia, and was home to a biome known as the Mammoth Steppe.   

So let’s bring our hybrid time / space craft down to Earth now, somewhere in Beringia, and open the door. 

[SFX: open space craft door]

Marc Macias Fauria, professor of physical geography at the University Oxford, told us what would greet us. 

[SFX: wind through grasses, ambient sound]

Marc: Given the density of fossils, and given their ages, you could sit on top of a hill and look over a vast area of grassland, with a huge diversity of very large animals. And most of these animals are no longer with us, especially the largest ones. Of course it’s called the mammoth steppe because of mammoths but there were other large amazing creatures such as the wooly rhino. And there were a multitude of other mammals which as I said some of them are still with us, they haven’t gone extinct, for example the muskoxen, the reindeer and the horse. 

M: Not only that but we also had predators lurking around. And a fully functioning ecosystem that had not been defaunated

S: Meaning that the all of the parts of the ecosystem were still there, it hadn’t lost key parts of the food chain.

M: Large carnivores for example, or key functional groups within the herbivores, there was quite a high diversity.

M: But the density, the overall numbers and the overall biomass that could be sustained, was high. On the order of magnitude of the densities of African game reserves that we still have today. Many times, it has been compared to a Serengeti, but in the north. 

R: Wow, so the Arctic used to be host to all these different animals, big herbivores, big carnivores, it was this huge ecosystem that’s so different to how we think about the Arctic today.

S: A serengeti in the North, you never think of the Arctic as being like an African game reserve.

R: Yeah

S: Yeah, I just think this is such a wild idea, and really beautiful to imagine

R: Absolutely

M: It is definitely a mind-blowing idea. We are not used to thinking of northern lands and cold lands as extremely productive. One thing that can hamper this productivity are the low temperatures. Low temperatures are a great way to slow down certain processes. 

There are certain key processes that might be occurring pretty fast if you have a bunch of very large animals, even if it’s cold, just because the size of these animals allows the process to be transferred into the animal. 

Jeppe: Because basically, herbivore guts are big bio-reactors, 

[SFX/MUSIC: bioreactors]

S: This is Jeppe Kristensen again, postdoctoral researcher at the University of Oxford

J: they’re hot and they’re moist, and they’re great environments for decomposition, as are our guts.

M: They can speed up processes even in these cold areas that mean that the same area with no such animals might be extremely slow in its way of recycling nutrients and dispersing them

R: Wow so in these cold environments microbes, it’s harder for them to do their job in the general environment because it’s so cold, so instead they’re doing their job and breaking things down and helping  nutrient cycles within animals because the gut is warm enough, so what does that have to do with the rest of the ecosystem?

S: Fundamentally this means that the ecosystem can be quite productive. Speeding up nutrient cycles means pulling out the nutrients from whatever you’re eating and turning them back into a usable form, so that life can again incorporate that matter. So in this case this nutrient cycling is happening inside the guts of herbivores, and when they pee or poo, they deliver lots of lovely nutrients to the soil and to plants. So, turning our attention now to the vegetation that the mammoths and wooly rhinos would walk through, and fertilise, I asked Jeppe and Marc what kind of plants we would see looking out from that hilltop, and this turns out to be important for carbon storage. 

 Jeppe: We would see grass, herbaceous communities, rather than these dwarf shrubs. 

[SFX: sounds of dry grasses. MUSIC]

M: You would also be looking at a drier landscape than the current tundra, less wetlands. The herbs and the forbs with their deep roots and high evapotranspiration would make the landscape drier. 

S: What are forbs?

Forbs are herbaceous plants, flowering plants that you would see in an open meadow. 

[SFX: background rumble of fire in an open landscape]

S: Grazing pressure from the herbivores, and fire, helped to maintain these relatively open landscapes.

M: Overall, less shrubs than today, much more grass, much more open landscapes, longer horizons.

J: The shrubs, though beautiful, are not effective carbon pumps. Grasses are. They allocate much more of the carbon that they fix into the ground. They do not need to maintain woody structures, so they invest more in the ground. 

R: So when plants grow, they take in carbon from the atmosphere, and they put it into their structures, but they also can invest it in the ground. And what he’s saying is that grasses invest lots of carbon into the ground, more than plants with more complex structures, but what does it mean to invest carbon in the ground, what is that and why would a plant do that?

S: Yeah it’s a good question, and it has a really interesting answer… Jeppe will explain how plants can use this carbon as a kind of currency…

[SFX: cash register noise]

S: To exchange with microbes in the ground

Jeppe: It may be most beneficial for the plant to invest you into the microbes surrounding the roots: the rhyzosphere(?). 

S: [for emphasis]: The rhyzosphere

[SFX: soil/root sounds]

J: The soil immediately surrounding the roots

J: What the plants get out of that is nutrients in exchange to carbon. However the microbes in the soil depend on the plants to deliver carbon to them. But they can be much better than the plants in terms of getting nutrients from the soil. So there are a lot of these synergistic relationships. Where the plants give the microbes something in return for something else. A carbon for nutrient trading scheme. Everyone gets better off that way.

MUSIC

R: So these plants and microbes are doing deals, everybody’s getting richer off this.

[SFX: Deal-making microbes]

S: they’re collaborating. We often have this view of nature of being really competitive and a kind of dog-eat-dog world, but it’s nice to know there are some friendly exchanges going on, even underground.

PAUSE

S: So, in our mammoth steppe biome, we have more herbaceous plants like grasses and forbs, which are the flowering plants, and they are effective carbon pumps, and we know we also have big herbivores munching away on them.

J: and the grasses that are really well adapted to herbivores being part of the system, they even have compensatory responses to herbivory, they actually exshoot additional carbon in the ground 

[SFX: herbivores chomping]

to compete better for nutrients when under grazing pressure. These systems, nutrients are in free flux with herbivores, so you need to be a good competitor for sporadic, immediate nutrient availability. They pee and poo, so you need to be ready. Some of these grasses are really well adapted for that.

R: So when the herbivores come and eat these plants he’s saying that if they’ve taken a bite out of some of them, in order to grow faster they take more nutrients from the soil by chucking more carbon in and trading that? And what was the thing about them pooing and peeing but only randomly… is it that the nutrients are delivered back into the soil through this, but that happens in like discrete places, and therefore the nutrients aren’t very well spread out, so when you can get them, you try as hard as you can sort of thing? 

S: Yes that’s exactly right–they get these big nutrient dumps thanks to the wandering beasts. They’ve essentially developed to co-habit really effectively with grazing herbivores. 

R: I wonder if there’s a link in that when the herbivore comes and eats them, they will poo near them…

S: yeah they’ve just got to stay alive long enough…

BREAK

S: Ok, so we’ve seen how the grasslands of the mammoth steppe ecosystem, with a wide variety of grasses and herbaceous plants, and these hungry mammoths and whooly rhinos–these great bioreactors–roaming around, munching grass, dispersing nutrients, how this was actually a very productive ecosystem, with a lot of carbon going into the soil. But, there’s more to this story… these grassy ecosystem may also have helped the carbon become stored in a more stable form. Because it turns out–not all carbon in the ground is the same.

To place this in context, it’s worth us recapping at this stage Roberta what we learned in episode one about how organic matter can stay in the ground if decomposition is slowed down–do you remember the three ways to do this? 

R: Yes the first mechanism was when the microbes didn’t have enough oxygen, so anoxic environments so that’s places like wetlands.

S: And that’s like putting a lid on your food or shrink-wrapping it.

[SFX]

R: The second mechanism is when things are acidic, and the worms and the millipedes that break down that carbon into smaller chunks, they really don’t like those acidic conditions, so that helps to stop the carbon decomposing, so that would be coniferous forests, right? With lots of pine needles, or maybe forests with beech trees in them?

S: Yeah well remembered.

R: And the final one was temperature, and that’s the key for permafrost right? When you lower the temperature, you slow down those microbes too.

S: Precisely, and that’s like freezing your food. 

[SFX of closing a freezer door]

S: But in all of these examples, the carbon is being preserved in its original, organic form, this is known as particulate organic matter 

J: Particulate organic matter which is basically this undecomposed plant material

S: This is what we compared to spaghetti bolognese in episode one, this complex tangle of long organic molecules in a tasty sauce of smaller compounds. And this particulate organic matter stays in the soil if microbes aren’t able to break it down

J: That’s right, and then you’d be conserved in that Bolognese state. 

PACE

S: But, let’s say the microbes are able to decompose this carbon

J: if you go through multiple decomposition rounds, 

[SFX: boxing bell. MUSIC]

you’re incorporated into one micro-organism…. 

S: And then that bacterium dies, and is eaten by another

[SFX: bacteria eating]

J: And that can happen several times. And each time it happens the organic molecules becomes a little simpler each time, 

S: some carbon is lost to the atmosphere as CO2, the rest is kept in this simpler form

J: so imagine that the spaghetti becomes shorter each time…

S: so we’ve taken the scissors to our spaghetti and are left with shorter, simpler molecules 

J: And when we get simple molecules as a product of decomposition, these simple molecules can adsorb to the surface of mineral grains in the soil. 

S: and this is known as mineral-associated organic matter

J: …small chains of organic matter stuck to mineral surfaces in the soil. This is an effective way of storing carbon for millennia. 

PAUSE

J: It depends on the mineralogy, but there is some capacity in all soils for that.

J: If you include the subsoil, this can be much larger than the POM. 

J: This is particularly something that’s important in grasslands. You might think there’s not a lot of carbon in that soil, so you think let’s plant some trees. But if you actually measure the carbon in that soil, down to 3 m, you’ll find that there’s actually huge amounts of carbon stored in those soils. 

R: So so far we’ve been talking about POM, but now we have this new way to store organic matter in the ground… which is to stick it on the side of minerals… so how does it stop it being decomposed by microbes?

S: The molecules are physically and chemically adsorbed–or stuck–to the mineral grains, and protective aggregates can form around them. This basically means it’s much harder for the microbes to access and attack this organic matter, so it decomposes much more slowly than particulate organic matter. 

S: It’s like… what was a tasty snack is now…

R: Stuck on the side of a bit of rock

S: Yeah, and then you can’t eat it, I guess

R: Makes sense

S: Now, Jeppe told me that there are a few important things that help the carbon turn into this mineral associated-form. One is the availability of iron in the mineral soil, in its oxidised form. This is reactive and the organic matter tends to bond with it. The cold and dry conditions helped to keep the iron in this oxidised form, and the dustiness of the period helped supply iron and bury the carbon
Say here’s Marc

[SFX of dusty and cold environment]

M: A key process that ensured that this was a big pool of carbon, was the fact that it was quickly frozen and buried by this aggrading sediment that came from the dustiness of the Earth in glacial times.

And the second thing is that the microbes appear to have been particularly efficient at using the carbon for growth, rather than for respiration. I think one thing that’s really interesting about this method of storing carbon is that it actually involves decomposition

R: Yeah so you’ve had to chop the carbon up many many times to get it small enough so that it’s able to be stuck on the side of a mineral grain. 

S: Yeah, and we know that that results in carbon being released. But because these microbes were efficient: they had what’s called a high microbial carbon use efficiency, as they chopped up the spaghetti, they didn’t actually emit that much out to the atmosphere. 

Why were the microbes particularly efficient with their carbon? Well Jeppe says that the carbon use efficiency goes up when organic matter is available in certain forms that are easy for the microbes to handle. The efficiency is particularly high when you have root exudates and animal excrements in the soil–both are characteristic of grasslands. 

A final thing is that animals helped mix the soil and bury the carbon downwards

J: Also grasslands are rich in bioturbation agents 

[SFX of digging animals in the soil]

S: i.e. animals that move stuff around in the soil

J: earthworms, prairie dogs, moles, which can bury the carbon and expose it to the interaction with the mineral particles. 

J: We know from deposits on the Colymer River, that there are mammoth tusks and ground squirrels in the same layer. And there’s no ground squirrels in the Arctic today. So we know that during the last ice age, mammoths and ground squirrels were contemporaries. 

S: All of these factors led to the mammoth steppe accumulating a lot of mineral-associated organic matter

J: And the reason why I think grasslands deserve more priority in our carbon accounting is that mineral-associated organic matter is more persistent to perturbations. And we know these will become more prevalent with climate change. There’s no one who wants to log mineral soil, so it’s just there. Once it’s there it is an extremely persistent way of storing carbon.

R: So if there’s a forest fire and it burns down all your grassland and it burns down all your grasslands and all the grass catches fire, then that carbon in the ground is still there and it’s not released by that, even though the grass is gone. 

S: Yeah, we just can’t see the carbon, that’s why it’s hard to get our heads around this. When we look at a forest, we see all that carbon literally in those trees, whereas the vast majority of the carbon is actually under the surface, and in grasslands, that ratio of above ground to below ground carbon is particularly in favour of the below-ground.

R: So in grasslands there’s loads of carbon hidden below the surface.

BREAK

So Roberta, we’ve stepped back in time and peeked into an Arctic of sweeping grasslands and abundant herbivores. And we’ve seen how the dynamics of this ecosystem helped put lots of carbon into the ground. Where it has been mostly happily frozen since. But if we return for a second to the modern Arctic, we not only of course see that the ground is now thawing, but we are also greeted by very different sights, smells and tastes: woodier and wetter ecosystems with shrubs, with mosses, berry bearing plants, and lichens, and far fewer herbivores. 

R: So what changed?

OUTRO

S: That is a great question, and we will tackle it, but it’s going to be in the next episode. We will try to understand what has changed and what’s caused this ecosystem shift, and we’ll also learn about how some scientists are trying to turn back the clock, and recreate this ice age ecosystem in a place called Pleistocene Park. Part of the reason that they’ve been doing this, is that there are compelling signs that shifting to the Pleistocene-type ecosystem can help protect the permafrost. So stay tuned. 

R: Hope you enjoyed this episode, please do subscribe, rate and share.

S: And visit our website polar.ox.ac.uk to learn about the Polar Forum, our wonderful members and their fascinating research–which of course is not just limited to this topic! And you can also find us on Twitter: our handle is @OxPolar.

CREDITS

Polar Pod comes to you from the Oxford University Polar Forum. It’s co-hosted by Sam Cornish and me, Roberta Wilkinson, reporting, production and original music by Sam Cornish, and sound design by Jihad Zgheib.