Earth & Sky

The following person was interviewed for today’s program. Our thanks to:

Syndonia Bret-Harte
Research Associate Professor
Institute of Arctic Biology
University of Alaska, Fairbanks

Interview with Syndonia Bret-Harte:

ES: Thanks for talking with me today. Let’s start with some background into the Arctic tundra experiment that the Institute’s been conducting.

SBH: Let me give you first just a general background of why we might care about what’s going on in the tundra. And, one of the reasons we might care is because we expect from the global circulation models that climate change is going to be most pronounced initially near the poles as compared to the tropics. So, since we think that the burning of fossil fuels is increasing the atmospheric concentration of carbon dioxide and that’s acting as a greenhouse effect – your listeners are probably all familiar with that – we expect that climate will be warming because of these human activities, and we expect that it will warm most intensely near the poles. So, looking at the tundra, because it’s a northern ecosystem, we might expect that we’d see changes there first before we’d see them in places like Iowa, and in fact, over the last 30 years, scientists have seen a lot of changes happening in the Arctic. For example, the sea ice is thinning dramatically and it’s also decreasing in area. And both winter and summer temperatures are warmer, but especially in the winter, the temperatures are warmer in the temperature record, and the permafrost is warming up and it hasn’t all started to melt yet, but it’s getting closer to that threshold.

And one of the other reasons why we should care about the tundra is that actually, in the northern parts of the world, which includes the tundra and boreal forest, have a lot of soil carbon. And the reason why they have a lot of soil carbon is because when plants and animals die, they get decomposed and they break down and eventually the nutrients that are tied up in them are released, and the carbon that’s tied up in them is released in the form of CO2, and it goes back to the atmosphere. But because the tundra soils are really cold and wet, in the boreal forest the soils are also cold and wet, a lot of that carbon hasn’t decomposed. So in fact the Arctic and Boreal regions of the world have about 40% of the world’s soil carbon, even though they’re only about 1/6 of the world’s land mass. So there’s a huge amount of carbon stored in the soils there, and if that carbon was to become decomposed, that would be a tremendous positive feedback in terms of increasing the concentration of CO2 in the atmosphere. There’s enough carbon in the soils up there that it would just about double the concentration of CO2 that’s currently in the atmosphere, so it’s actually a much larger amount than is being released by fossil fuel burning. So what is going to happen to that carbon is a sort of intense research question, and that sort of sets the stage for the work that we were doing.

Now, because decomposition is slow, because the soils are cold, and so things happen very slowly, there are also all those nutrients that plants need to grow are all tied up along with that undecomposed soil and organic matter. So it’s all in the soil but it’s not accessible to plants. And we would expect that as the Earth warms and the soil warms up, decomposition would speed up, and more nutrients would become available. And so, having more nutrients available should make the plants able to grow better. And that’s one prediction of the models that are predicting what would happen with the tundra, is that more nutrients will be released as the soils warm up, and the plants will grow better, and as the plants grow better, of course, they’re going to take carbon dioxide out of the air through the process of photosynthesis and turn it into their own biomass. So, having that positive effect of taking CO2 out of the air and forming plant biomass won’t mean that will act as sort of a brake on the release of carbon dioxide from the decomposition that’s happening in the soils. So, people have been very excited about the idea that plants might grow better, and that would act as sort of a brake on the increasing global warming as you decompose all of this soil organic matter. So, because we think then that nutrients are going to become available as decomposition proceeds, one way that you can kind of simulate that without actually warming things is to put on some nutrients.

This experiment that Michelle Mack and Terry Chapin and I engaged in, among other people, at Toolik Lake is a very long-term experiment where people have been fertilizing the tundra for about 20 years. It’s about the longest term fertilization experiment that I know of, certainly in the tundra. And over the course of that time, that was 20 years of fertilization, we have seen a huge increase in plant biomass. In particular, the shrubs that are normally a fairly modest part of the ecosystem have become very tall and bushy, and they’ve grown a lot more, and of course they’ve made all of this wood, which is not very easily decomposed, and people think, oh good, meaning that they’re going to be trapping a lot of carbon dioxide in those stems. And we’re seeing perhaps this braking effect on further warming. What Michelle and I did in this experiment that was just published in Nature, was we looked at both the above ground portion, where the plants were growing, and the below ground portion. We did a carbon and nitrogen budget for the entire ecosystem. And when we did that we discovered something really surprising, which was that even though the plants were growing a lot better, and definitely fixing more carbon above ground, below ground, a huge amount of carbon was being lost. And, in fact so much carbon was being lost from the deeper layers of the soil that it totally overwhelmed the amount that the plants were taking up. And this was really surprising to us for a couple of reasons. The first is that you wouldn’t expect, based on the current scientific paradigm about what controls decomposition, you wouldn’t expect nitrogen fertilization by itself to increase decomposition rates. You might expect nitrogen availability to increase as decomposition increases because of warmer temperature, but you wouldn’t expect fertilization by itself to cause a big increase in decomposition. And, in fact, that’s what we were seeing. And that implies that as the soils warm up, and more nitrogen gets released, that that nitrogen then acts as a positive feedback on further decomposition, and speed up decomposition even more than what people were originally thinking based on temperature alone. And so what this experiment suggests is that there is the potential for a lot more carbon to be lost from the soils in the north much more quickly than people had previously thought. And that’s why it’s a big deal. Both for the implications for the future, and also because it suggests that the current models that we have that are predicting what’s going to happen to the balance in decomposition in plant growth in ecosystems, tundra ecosystems in particular, those models aren’t as good as we thought. In other words, we don’t understand the process of decomposition as well as we thought we did if we could have this really surprising result, and it’s not captured in our models.

ES: Could you “break this down” for me, what’s happening in the process of decomposition in these Arctic ecosystems?

SBH: The standard paradigm of how decomposition works is that the microbes that do the decomposition, which are mostly fungi and bacteria living in the soil, that they are primarily limited by having a carbon source. So they break down organic carbon to get energy for themselves, and they also use the nitrogen because they have a certain amount of nitrogen that is needed to make their own biomass. But the paradigm is that they’re almost never limited by nitrogen availability, or if they are, they’ll just mineralize some more, they’ll break down the carbon a little bit more to get the nitrogen and build up their biomass. So, you don’t normally expect that you would be able to increase their activity just by adding nitrogen to them. You would expect instead that they mostly would be limited by the quality of the carbon substrate.

And the quality issue comes, in a system like the tropics, where decomposition is pretty quick, when leaves or tree branches fall to the ground, they’ll decompose within a year or so, and what happens is that all of the stuff that’s really easy to decompose ones first, and there’s one suite of bacteria in fungi that work on those. And the stuff that has more lignin in it, more wood, takes a little longer, and there’s fungi that are specially adapted to be able to make use of that substrate. And, in the Arctic, in the Boreal forest, because the soil is so cold and wet, and it’s the physical conditions that are limiting the microbes, a lot of that carbon that’s down there is really high-quality, and so if the microbes, as the soil warms up, and the microbes are not limited by temperature anymore, and they have a chance to go to work on that, they will have lots of good quality carbon, and they’ll be able to break it down pretty quickly. And you wouldn’t expect them in this situation, where they’re limited by temperature and physical factors, to be limited by nitrogen availability. And so, it’s very surprising to us that just by adding nitrogen alone, and not increasing the temperature, that you could suddenly see this big increase in decomposition that happened in this experiment, and such a large increase that actually, at the lower layers of the soil, the ones that are farthest down not near the surface, the coldest layers, there was almost 50% of the carbon was lost over this 20 year period, which is quite a large amount of carbon.

ES: How much was that?

SBH: Well, it was about 2 km/m^2, which is quite a lot. One way that you could think about this that the tundra soils are kind of like a peat bog – they’re not quite as extreme as a peat bog – but there’s a whole lot of organic material on top, which is black and squishy, and looks like peat, which it is, basically – old, undecomposed plant material. And then even farther down into the soil, when you get into the mineral, there’s still a fairly high carbon content. So the carbon content of the organic and the mineral layers in the tundra is much higher than it is in an agricultural field or in a tropical forest, where stuff doesn’t hang around as long because it decomposes more quickly. So these soils have been accumulating carbon. We know from dating the peat that it’s been accumulating on the order of 8,000 to 9,000 years. So, over that time period, plants have been growing and dying, and animals too, potentially, but mostly plants, and sinking into the soil and becoming one with soil, and maybe they have a little decomposition that goes on, but not very much. Enough that there’s carbon in there that’s at least 8,000 years old, which you would never find that in a tropical system because it would pretty much all have been decomposed by that time. So, again not only is it significant because we see this large loss of soil carbon, with fertilization, which we weren’t expecting, but also, you’re sitting on this huge store of carbon, very old carbon, which has been accumulating for the past 8,000 – 9,000 years, and what this suggests is that the process of global warming if it increases nitrogen availability, it’s going to get all that stuff mobilized into the atmosphere in a very short period of time, compared to how long it took to accumulate.

So, right now, in the current climate, the soils are really cold and wet, which means that things don’t decompose quickly. And, that means that all of the dead plant bodies that are in there still have the nitrogen that is in the form of their proteins, and in their DNA molecules, and, in other components in the living plants. Even though they are dead, those nitrogen molecules are still tied up in organic materials like proteins. And they haven’t been transformed into inorganic forms, which are easily taken up by plants and other microbes. So the process of decomposition does just that. It breaks down all of those organic carbon and nitrogen compounds and makes them into the simplest forms, which is inorganic nitrogen like ammonium, or nitrate, and also the endpoint for carbon is turning it back from sugar molecules and whatnot into CO2. And so, because decomposition in this planet has receded so slowly, all of those nitrogen molecules are tied up in organic matter – they’re not available to the plants. So there’s very low nitrogen availability in the soil. So we think that as decomposition proceeds, just because the temperature warms up, that makes decomposition go faster, that’s going to release nitrogen just from the process of decomposition and make that available to the plants, and then also – our work suggests – make that available to the microbes that are doing the decomposition, and that will potentially speed up decomposition even more than just the temperature effect alone.

ES:

SBH: So there’s a lot of carbon that’s in the soil now. It’s actually been accumulating in the Arctic and Boreal forests for about 8,000 years. And where it all came from originally is that it was all plant bodies that the plants grew up, when they grew they took the CO2 out of the atmosphere, and they turned it into sugars and their own body parts, lignin and whatnot. And then they died, and they fell over onto the ground, into the ground, and then, because decomposition is so slow, in these cold, wet soils, they’ve just been sort of preserved there, as it were, and they’ve been decomposing very slowly. So there’s a lot of carbon, and there’s also a lot of nitrogen that’s tied up in that organic matter, which is these dead plant bodies. And, so, that’s why there’s so much soil carbon there. Because right now the soils are so cold and wet, decomposition goes very slowly. And so, the northern part of the world has managed to accumulate 40% of the world’s soil carbon. And so as climate warms, the soils should get warmer, because the air temperature gets warmer, the soil should get warmer too. And depending on what happens with the precipitation patterns, there should be less precipitation, and even if there is the same precipitation, or a little bit more, still, as the soil warms up there’s probably going to be more evaporation in the summer, so the soil’s going to get drier. Either thing, getting warmer or drier is going to tend to make it easier for microbes to decompose, because those microbes are limited by cold temperatures and wet conditions. They have to do decomposition under aerobic conditions where there’s oxygen available. And so, the though is that as the soils warm, and get drier, the microbes that are already there in the soils are going to start working on all of this old, dead organic matter.
And they’re going to start breaking it down, and turning it into CO2 and into inorganic forms of nitrogen, which will then be available for both the microbes themselves, to increase their numbers and their body size, and also for the plants to take it up and be able to grow better. And so, people have always thought that yes, decomposition will increase under warmer temperatures, and that that will make more nitrogen available, and that the plants will get some of that nitrogen, probably not all of it, but they’ll get some of it, and that they’ll be able to grow bigger. And in fact, that’s what we see when we add nitrogen. Plants do grow bigger, but then we also have this surprising result that in addition to growing bigger, apparently having more nitrogen available in the soil by itself, even without any warmer temperatures, also speeds up the process of decomposition. So if you look at it from the microbe’s point of view, as the soils warm, the temperature goes up. Not only is it nice and warm, so that they feel happy and they want to decompose, but also all of this nitrogen is going to start to become available, and if they’re nitrogen limited, they’re like, “wow, we can really take this nitrogen and decompose even more.” So there are two positive feedbacks now on the decomposition, not just one, the warmer temperature and the increased nitrogen availability, and then that suggests that the amount of the decomposition that’s going to happen is going to be much larger than we previously thought. So there’ll be sort of a positive feedback, to have more decomposition then, even though the plants will also benefit, and they’ll grow bigger, and they’ll fix some carbon. That fixing of carbon that they do is going to be small compared to the amount of carbon that’s released by the soils by decomposition.

ES: Could you describe the research area and the work you did?

SBH: So these experimental plots are pretty big plots – they’re five by twenty meters, so they’re pretty large, and when we sample them, for both plant biomass and the soil components. We obviously can’t sample that whole area that would just be too much. So we do a random sampling procedure, where we took five individual samples, and they were little quadrates where they were 20*20 cm squared, and within that quadrate we took all of the above ground plant material and all the litter, and we cut out a chunk of the soil, so we looked at all of the soil in that area. And we looked at cores that went down deeper, into the very deep soil. So we took out some of the organic layer of soil with that square, and we had cores down below that to get into the deeper organic material and the very old soil. And that way, by knowing the exact dimensions of the piece that we cut out, we can say that there’s this much carbon in the litter, 20*20 cm, so that we can calculate how much it would be per meter squared, and that way we have it on a sort of ecosystem basis. And so, we did that sub sampling five times on each of these plots. And there were four individual replicates of each of these plots, so there was a sort of sub replication with the plots, and there were four examples of these plots. And what we did to actually do the carbon budget – in the field, we cut these things out and brought them into the lab. We had quite a lot of people, probably at least 10-15 people who sat down together and separated the plant biomass into all of it’s different species, and what are the different components, and what are the new leaves and the old leaves, the new stems and the old stems, the below ground stems, the roots. And then the different layers of the soil were also separated by depths of 0-5 cm, 5-10 organic, and then mineral soil that’s deeper than that. And after we got all of these components dried and weighed, so that we knew how much of them there were, little sub samples of each of those classes were taken, and they were analyzed for their carbon and nitrogen content using a machine that’s a carbon-hydrogen and nitrogen analyzer that takes, and you grind them into this fine powder and put them in very small containers, and then basically they’re heated up to a very high temperature, and they kind of vaporize. And then with a machine you can measure, because all of them have different weights, how much is carbon and how much is nitrogen. And on the basis of those numbers, and then knowing how much there was to start with, we were then able to calculate how the fertilizer plots compared to the unfertilized plots. The one thing about this experiment that would have been nice to do that no one had done, and we couldn’t do, is to look at the same plots over time. So, look at it 20 years ago and look at it now. And we couldn’t do that. We actually have records of the plants going back twenty years, but no one had looked at the soils in depth before. So just compared the fertilized with the control, but the fertilized had been fertilized for 20 years, and so we are assuming that the fertilized would have started off like the control, which is a common scientific assumption, which is very valid because this is very homogeneous vegetation, and homogeneous in the soils too, it’s not different across this particular area that we were looking at. And so, when we did that, we could see that not only were the plants much larger in the fertilized plots, but that the depth of the organic material was shallower, because some of it had either disappeared, or simply not been added to over that 20-year period. One of the things that happens with fertilization is that you get a shift in species composition. In the normal tundra, there are a lot of mosses, and they disappear in the fertilized plot. And mosses add a lot of material to the organic layers, so those mosses being gone were not adding organic material to that layer in the fertilized plot. And then in addition to that we can see in the lower layers of the soil and in the mineral soil, the carbon content was only about 15% of what it had been in the control plot. So that carbon, which was down so deep, was presumably pretty old carbon, so this study also suggests that, you know it’s not just the new carbon that’s had its quality changed by the species composition changing and be different, but also the old carbon that was there before as a result of adding this fertilizer to these plots.

ES: What are some of the surprises that came out of the Toolik experiment?

SBH: Well, as I say I guess the most surprising thing was this change in the composition, but there were a couple of other things. So, having much less carbon than was there before. But there were some other things that were different. One thing that was really different was the distribution of where their roots were. And this probably also relates to the carbon loss. So, in the control tundra, there are four different types of vegetation. One type, the grammanoids, they’re sedges, and they look a lot like grasses, although they’re not. They’re related though, and a lot of them have very deep roots – they go all the way down to the thaw line. And so, in these Northern systems, that are underlying by permafrost, which is very deep. At Prudhoe Bay it’s almost 1800 meters deep, so it’s very deep. And basically, if you think about this, this giant mass of frozen soil, and then on the top of it there’s a little, shallow layer that thaws out very year in the summer. And so, that little shallow layer – that’s where all the action happens. That’s where all of the plant roots are, that’s where all of the microbes are doing their decomposition, and everything below, because it’s frozen, nothing is really happening there. And so one thing that we can see is that when we fertilize these plots, and we got all of these shrubs doing really well and growing up, and out competing all of the other plants like the grammanoids. The shrubs have much shallower roots, so the roots were much closer to the surface. And, in the control plots, there were these gramminoids that have deep roots, and the roots would go all the way down to the thaw line. So there was a difference in the distribution of root material, with the roots being a lot shallower, the fertilized plots and in the control plots. And it’s quite probable also that some of that carbon that was being lost was in fact ancient plant roots that were maybe even from the graminoids, because they were so deep. We didn’t actually look in our experiment to see what diversity of below ground organisms. That was different than the control and the fertilized plots, but there’s another scientist that works in our area, John Moore, at the University of Colorado, and he has seen that in the normal tundra situation, in the control, the current climate tundra, the contribution of fungi and the bacteria is weighted toward more contribution by fungi to decomposition. And that’s actually what you would expect for a situation where the soils are cold and wet, because the fungi tend to tolerate that better than the decomposing bacteria do. And that also is another reason why decomposition is slow, because fungi are just kind of slower at decomposition. Bacteria are, they don’t multiply as fast, and they tend to in general, break down more specialized compounds, woody compounds and whatnot, that require enzymes that take more energy to make. So, pathways of decomposition that are dominated by fungi tend to be a bit slower than ones that are dominated by bacteria. And one thing that John Moore noticed in these same types of experiments, these long-term fertilization experiments, was that the decomposer pathway shifts from one being dominated by fungi to one being dominated by bacteria. So there’s lot fewer fungi and a lot more bacteria. And the decomposition goes faster, in part because of the nature of the organisms that are doing it. We would expect that to be the case in our experiment as well.

ES: Would you like to share some thoughts about the meaning of the tundra experiment?

SBH: Well, I think that what it means is first of all, what happens in northern regions is going to have a strong impact on what happens in the rest of the globe. And so if we proceed on the present course that the planet is on, not curbing fossil fuel emissions and allowing warming to proceed quickly, as it’s doing now, that there could be – after you cross a certain threshold, where you start getting all of this decomposition happening – there could be a tremendous positive feedback to further warming, from the northern regions of the world, which potentially could cause the release of a lot of carbon dioxide that you wouldn’t be able to control anymore, just by driving less SUVs or whatever. So I think that there’s probably, I mean it’s a continuous process, but I think that there’s probably a threshold that you’ll cross over to where you’re not going to be able to stop this sort of run away warming anymore. At least, that’s what, as a scientist, I worry about. So, I guess from my point of view, the lesson from the policy makers would be is, don’t delay in trying to deal with this problem. Don’t think that just because nothing bad has happened right now, that “oh, a little warming in Alaska, who care about that, it isn’t having any impact on me and my life in Texas.” But in fact, that’s a dangerous thing to assume, because if warming continues, and you get to the point where you start mobilizing all of this northern carbon, you may have very strong and severe effects that you can’t really do much about any more. So, personally, I think that political decisions are not necessarily made on the basis of scientific information. But it’s not like you necessarily need to have a rocket scientist tell you what to do on the basis of fancy computer models or anything. You can sort of see from the information that’s already been gathered that the smartest way to deal with the potential for global warming would be to keep it from getting from out of hand. In other words, don’t push the system so hard that you pass the point of no return. And so I would say that this work is just another in a line of evidence that suggests that, in fact the consequences of warming could be large, and if I was a policy maker, I would think seriously about trying to address the problem and reduce greenhouse gas emissions now, rather than waiting to deal with it 50 or 100 years from now when it basically may be too late.

Author’s notes:

From: Carbon Dioxide Storage in Soil (Enviroliteracy.org)

According to NASA scientists David Herring and Robert Kennenberg, “Scientists estimate that while the boreal forest occupies about 21 percent of the Earth’s forested land surface and contains 13 percent of all the carbon stored in ‘biomass’ (or living matter), it holds about 43 percent of all of the world’s carbon that is stored in soil.