The list of how humans are causing trouble in the soil is pretty mind-blowing, kind of overwhelming. It’s connected to all that is central to our modern human societies – industrial agriculture, synthetic chemistry, city sprawl, global mobility and so on and on. In this episode, Matthias Rillig, Anja Krieger and their guests Maddy Thakur and Asmeret Asefaw Berhe explore the human impacts on soil and their cascading effects.

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TRANSCRIPT

Theme Deep in the Soil by Sunfish Moon Light

Asmeret Asefaw Berhe
We’ve been extracting resources from soil, but we haven’t been tending to the soil in the way that it needs to be tend to, to actually support the production of food, feed, fiber that we need to extract from it.

Diana Wall
We are seeing more pollution in soils. We’re seeing more paving the soils. The effect of climate change on soils, if you think about fires, we’re looking at temperatures, we’re looking at flooding, we’re looking at erosion, dust. All of these are threats.

Richard Bardgett
It’s been estimated that something like a third of soils on Earth, are damaged as a result of intensive agriculture.

Anja Krieger
You’re listening to Life in the Soil, a podcast by the plant, fungal and soil ecology lab at the Freie Universität in Berlin, Germany. 

Richard Bardgett
Things like droughts are increasing in intensity, and frequency and heat waves, and floods, and all these extreme events can have dramatic impacts on the diversity of life within the soil.

Maddy Thakur
And it may take few weeks, it may take few months, it may take few years. And sometimes you even have cases that communities or organisms or biodiversity, they never recover back. 

Vocoder sings Life in the Soil

Matthias Rillig
This is reality and we need to face it. But it also means there is hope. We can switch some of these factors off.

Anja Krieger
Hey there! My name is Anja Krieger, and I welcome you to this series on soil ecology. In this fifth episode of our series, we’re going to explore the impact of the human species on the soil. As you may know, healthy soils are absolutely crucial for life here on Earth. The green beauty you see above ground, the charismatic animals we all love, our own provision with food and fibres, and the stability of our climate – all of that depends on healthy soils and their biodiversity. But the soils are at risk, all around the world. Human activities lead to unprecedented changes on a global scale. Soil biologist who’s my podcast co-producer, is one of the people investigating how this human-made global change affects the soil, and the life in it. 

Anja
Matthias, how are we humans impacting the soil?

Matthias
Well, in many, many different ways. This is actually a realization that, for me, is relatively new when you think about it systematically. So we are impacting soils in terms of chemical ways, in terms of physical ways, and in terms of biological ways. Each of these big categories have many, many different divergent examples. So for example, in chemical ways that includes atmospheric deposition of nitrogen, which is leading to an enrichment of ecosystems with nitrogen. But you have also environmental pollution, with a wide range of substances actually an mostly unknown range of substances.

Anja
U-hu. Where do these come from, these unknown substances?

Matthias
Well, from a wide range of applications. Microplastics, for example, come from our everyday life, from industry and all kinds of origins and all of the additives, for example, that are in plastic compounds that end up in the soil in the end will contribute to this level of pollution. But there’s also you know, many, many household chemicals and many industrially produced chemicals for various industrial applications, I should have said, they also end up in the environment and in the soil, and they contribute to quite a range of pollutants. And then, of course, there’s all the chemicals that we on purpose spray, for example, on our agricultural fields, like various pesticides, including fungicides or nematicides, herbicides. Those are all chemicals that we release into the environment on purpose for a specific goal. 

Anja
What can you tell me about the other impacts you mentioned, besides these chemical ones?

Matthias
I mean this is a huge field. And there’s all these physical factors, of course, most well known is global warming. But there’s also increased disturbance, there’s sealing soils off, there is, you know, depending on how we view it, fragmentation. You know, UV radiation, you name it. I mean, the list is really very long of all these different factors. And they’re all totally different, where they come from, precisely how they’re related to other factors and how they impact soils, what are the entry points in soil, so also in just the way they act and the way the effects also differ extremely. And then we have biological entities. There’s invasive plants, of course, that also change the soil. This is known. There is very little known about other invasive microbes, because we don’t know who’s been there. There’s a bit of work on invasive fungi, but only those that form the macroscopically visible above-ground fruiting bodies, but the ones that are just mycelia in the soil, we know very little about them. And those are biological agents of change that we also contribute.

Music – Sketch 3 by Klangpflaster

Anja
The list of how humans are causing trouble in the soil is pretty mind-blowing – kind of overwhelming. It’s basically connected to all that is central to our modern human societies – industrial agriculture, synthetic chemistry, city sprawl, global mobility and so on and on. One of the biggest factors of global change is caused by the increase of greenhouse gases in our atmosphere. And while we often think of the air when we speak of climate change, the process is intricately connected to the life in the soil.

Maddy Thakur
We try to understand how soil organisms first respond to temperature extremes and extreme drought. But there, the story is not complete.

Anja
At the university of Bern in Switzerland, Maddy Thakur explores the impacts of climate change on soil organisms. 

Maddy Thakur
…the response is one thing. Recovery is something which is happening after the extreme event is over. And it may take few weeks, it may take few months, it may take few years. And sometimes you even have cases that communities or organisms or biodiversity, they never recover back. 

Anja
With warmer temperatures, extreme weather events are becoming more likely, or more severe. Like heat waves and droughts. In their lab, Maddy and his colleagues have looked at protists, which are another group of small soil organisms. Many protists feed on bacteria, which the scientists also added to the experiment. Next, they ramped up the temperature by ten degrees Celsius – so, the scenario of a future heat wave. After three weeks, they turned off the heat.

Maddy Thakur
With this setup, we could look into two things. One thing how bacteria and protists will respond when we have plus 10 degrees Celsius. But it also will tell us that once we get back to 20 degrees Celsius, which is you know, from week 3, how will they recover, you know? So we looked into both the response and the recovery. And what we found, very surprising, was bacteria were never able to recover back. A heat wave of seven days had such an extreme effect on that community. On the other hand, protists, which were the predators, they didn’t care much about extreme heat, they sort of remained the same. And then we were a bit puzzled, because we were thinking that predators will also respond to climate extremes. But in this case, what we saw that these predators, they were able to adjust their physiology to that heat event, and what they did, they became inactive. 

Anja
So the predators, the protists, just slowed down during the heat event. They also started to shrink their body size, so they got smaller. But the bacteria did not recover from the heat event. They weren’t able to adjust their lifestyle to the new conditions like the protists. But protists need bacteria to feed on. So if one soil organism cannot cope – what happens to the others?

Maddy Thakur
If you respond in a certain way, and particularly if you recover in a particular way, how would that have an effect on the entire community? How would that have an effect on the trophic interaction? And how would that have an effect, also on ecosystem processes, right? Like, I mean, soil, as you know, Anja, it’s super important for decomposition, right? 

Anja
And decomposing dead organic matter is not the only important function soil organisms provide. They also feed on each other, forming a complex food web of several trophic levels. Just by making a living, they run part of the carbon and nitrogen cycles, they break down pollutants, they protect soil against erosion and help it store water. So what would happen if one heat wave struck after the other, and some of the bacteria wouldn’t be able to cope? As we discussed in episode 3, they are at the basis of the soil food web. 

Maddy Thakur
So, when a system which is highly connected, if you disturb that system, you start to lose more interactions, and the response is much faster. In the brown part where there are a lot of weaker interactions, when you disturb that system, there is often a lag, it takes some time when you will see the response. 

Anja
Let me explain this: Maddy says that there’s a difference between the food webs aboveground and those belowground in the soil. He calls the aboveground food web the green web, so where all the plants are. There, animals often specialize on a certain plant or other organism as food source. So, when their main food source disappears, they might starve or need to move. But in the soil – the brown food web as Maddy calls it – there are plenty of alternatives. Even if the bacteria don’t survive the heat wave, the protists might find other food. So, this doesn’t have instant effect, but it will also cause subtle changes in the food web, and those might be felt later on. Ultimately, these shifts might also affect what happens in the green part, aboveground…

Maddy Thakur
These two systems always are exchanging mass and energy, right? Because when plants die, or when organisms aboveground die, they go into the soil and below-ground organisms convert them into inorganic matter, and inorganic matter is then taken up by plants. So that’s sort of the balance that is really required for healthy ecosystems. 

Anja
Climate change might shift this balance and change the communities of life in the soil. How exactly, we don’t know yet. The scientists will need to conduct long-term experiments, because they won’t see those lagging impacts in just a year or two. 

Maddy Thakur
And we, in our research group, we are building up a experimental system, where we will really look into how such green-brown imbalance due to climate extremes could then affect various ecosystem processes. 

Sound 26 Minutes of Rain (a Berlin Thunderstorm with City Sounds) by Anja Krieger

Anja
Because humans emit greenhouse gases into the atmosphere, temperatures and sea-levels rise, and extreme weather events become more likely or severe – like hurricanes, droughts or heavy rain. 

Sound of thunder

Sound Digital Wildfire by autumncheney on Freesound under a CC-0 license

And there’s another toll we’ve been seeing in the past years. Huge wildfires now regularly devastate regions of the world, like in Australia, South America, Africa or the West Coast of the United States. It’s not that wildfires are anything new to these regions, but there has been a big change in their power and frequency.

Asmeret Asefaw Berhe
The nature of the fire matters a lot when we’re talking about fertility and soil health in these fire-adapted or post-fire ecosystems.

Anja
Asmeret Asefaw Berhe works at the University of California at Merced. She studies the impacts of fires on the soil biogeochemistry.

Asmeret Asefaw Berhe
Historically, native and indigenous communities had practiced fire, both to clear agricultural areas as well as to thin forests. Because low-intensity fires tend to create conditions where most of the vegetation, the overstory, is retained, but the understory gets burned and the burning of that understory opens, you know, the area to sunlight to penetrate for other plants to grow and successions of plant communities to happen. 

Anja
So fire can actually boost the life in an area. At low intensity, the understory vegetation burns down to char. And that has beneficial effects because it helps the soil hold water and nutrients. 

Asmeret Asefaw Berhe
But what we are having right now is really dominated by these large intensity severe fires. So stand-replacing fires, as they are referred to, that are so destructive that they don’t just take away the understory vegetation, they take everything on the path.

Music – Melancholy Mole by Klangpflaster

Anja
When forests burn down, much of the carbon that was stored in stems, branches, needles and leaves gets lost in the form of CO2. Roots can become charred, and soil organic matter can be burnt off in high-intensity fires – so carbon is lost from soil as well. If that happens too frequently, ecosystems are not able to recover. And by releasing carbon, they add CO2 to the atmosphere.

Asmeret Asefaw Berhe
So we’re in a very losing proposition, if you will, where the soil, the amount of carbon stored in soil is getting reduced, and the amount of carbon in the atmosphere is getting increased. And obviously, as the amount of the concentration of carbon dioxide in the atmosphere increases, we cause more warming of the Earth’s atmosphere. 

Anja
And of course, wildfires are not the only way that the carbon bank of soils is depleted. In the Arctic, some soils were frozen for millenia, and with them huge amounts of organic matter. Plants and animals and organisms that died long ago, their carbon stored in the ice-cold soil. With the rise in temperatures around the Arctic circle, the permafrost gets warmer and thaws. Microbial communities are now able to decompose this ancient organic matter, and release huge amounts of greenhouse gases in the process. Similarly, carbon is lost from soils all around the world when temperatures rise.

Asmeret Asefaw Berhe
When there is fast rate of CO2 into the atmosphere, then you cause even more warming that causes even more decomposition, so more loss of carbon from soil. So this process basically enters into something like a vicious cycle, if you will, that keeps reinforcing itself. 

Anja
So more carbon lost from soils will mean more warming, more extreme droughts and heat-waves, and more frequent and intense wildfires. How do we get out of this cycle?

Asmeret Asefaw Berhe
If we can figure out management practices that would allow us to take carbon from the atmosphere and store it in soil in pools that can last for hundreds and thousands of years, then we would, you know, potentially be adopting a more sustainable climate future that makes makes sure that the soil remains healthy, and the atmospheric burden of carbon is also regulated. 

Music – FungusDance by Klangpflaster

Anja
Matthias, I would think that there has always been global change over the millennia, since soil existed, and that it has never been the same. So why would the current global change happening with the impact of humans be any different?

Matthias
Yeah, no, I mean, that is a good question. I mean, there’s always been a change in the climate system, for example. And of course, soils will have had to respond to that. What we are talking about here really is anthropogenic global change, not the geological time-scale global change. And one of the distinguishing features is, I think, just the sheer multiplicity of factors that are at play now. There’s so many more factors at play now, with all the chemical pollutants and microplastic. There was no microplastic before 1950. You know, I mean, it’s just a huge number of factors that are now all coming together, not just climate change. Climate change is in itself already complex in its effects and its manifestations. But it is way more now that we are talking about in terms of global changes, anthropogenic global change. And you know, one of the big features is just the rates of change. You know, it does matter how fast things change, I mean, on a geological timescale, there would be like a spike. 

Anja
Uhum. Is there one factor that you find most worrying?

Matthias
No. I find most worrying the entirety of the factors with which we’re affecting our planet. This has been sort of also driving my thinking in my research in the last few years and is what we also focus on in the lab right now. It’s just the sheer number of factors that are impinging on our systems, especially the soil. That is what I find very important. And I have a bit of a problem with these arguments out there that, you know, is microplastic getting too much attention? And should we not rather focus more on warming, which is clearly more significant and should be more worrisome. I don’t like these discussions where you pit one factor against another in terms of, you know, getting attention in the public, I think the important thing to realize is they are all manifestations of the same thing. This is what we as humans do, they’re different manifestations. And therefore, I think it’s unproductive to pit one factor against another and say, like, Oh, this one is much more severe. And so I would say that it’s just the collection of factors that is really worrying, just the whole set, the joint impacts of all these various factors is what worries me most.

Music – Regenwurmwalzer by Klangpflaster

Anja
Imagine someone loses their job, splits with their partner and gets thrown out of the house all at once – does it really matter then which was most important? That’s the analogy I sometimes use to imagine the multiple factors of global change. And Matthias even has some data to back that up. Together with his colleagues from the lab, he randomly combined more and more factors of global change. Then he checked their impacts on soil processes and the biodiversity of fungi. And the result was: The more factors, the more negative impact. It didn’t matter which ones – just how many.

Matthias
And we had all, we had no effect, positive and negative effect of the single factors. It didn’t matter! In the end, when you had more and more and more of these factors impinging on these soils, we saw a linear decrease in diversity. So we lost species that went extinct, we saw a decrease in functions, a decrease in soil aggregation, decrease in respiration, a decrease in decomposition rates. And that was just incredibly amazing to me, because it meant that the more of these factors we had represented in the experiment, the more negative the effects became. And that’s on the one hand, it’s sobering because, you know, we don’t know about these effects. And we had some, we had some surprises, for example, soil became suddenly water repellent when we had more than five factors. I…we still don’t know why. And so there’s just these surprises lurking behind the high number of factors that we simply have not been examining experimentally so far.

Anja
I mean, we humans tend to…just psychologically, it’s easier to focus on one thing and say, you know, there’s a hierarchy of importance, this factor is worse than that one, and we have to focus on that. But with that information, how could we start thinking about global change and also the way we have to go into the future – if we consider those results?

Matthias
Yeah, I mean, this is a very important point, I think. We focus on a few main factors, usually, you know, elevated CO2, warming, nitrogen deposition, and invasive species, maybe drought – those are some of the main factors that a lot of people work on. And it’s presumed that they are the dominant factors. And maybe they are from the single effect size. But it is not clear how they combine and how other factors that are out there, would influence their effects. So yeah, I mean, one of the problems in communicating certainly is, well, you know, it may not be enough to just focus on those main factors. And that’s just… how do you communicate that? I don’t know. I mean, it’s like, it’s inconvenient. You know, I mean, you have to focus on all these factors. Who can do that? I mean, it’s just, this experiment was super difficult to set up. I mean, they are very difficult experiments to do, to deal with all these factors, you need different application methods, different exposure systems, it’s hard. But I think this is the way we need to go, we really need to do this hard stuff. And look at the many factors and their joint impacts, and I think we can no longer afford to just conveniently study the factors that we have pre-determined to be important. I think this is maybe missing some very important effects, and is therefore potentially dangerous. But of course, there is also a positive message in there, you know, that this progressive worsening of effects with more and more factors, it also means any factor you can eliminate, is gonna be good! You’re back on that trajectory towards lessening impact. So it’s not only depressing, you know, I don’t want to do that – I’m actually a fairly optimistic person. So, doesn’t mean that you don’t want to know the truth of real impacts, you know, even though it may be not so nice. But we’ve just repeated that, and we got the same results. I mean, it’s…I think this is reality, and we need to face it. But it also means there is hope – we can switch some of these factors off! You know, people now talk about pesticide-free agriculture, I think it was unthinkable many years back. And also you have to realize many of these factors are connected with each other. They’re not totally independent, you know, so I mean, by eliminating certain practices, we can actually move back on that line of number of factors quite a bit. And I think this is a very positive message.

Theme – Deep in the Soil by Sunfish Moon Light

Anja
So, there is hope: All that we do to take the stress off the soil, to replenish and protect it will matter. This was episode 5 of Life in the Soil. Thanks to our guests Asmeret Asefaw Berhe and Maddy Thakur for sharing their insights! Our podcast is a co-production between Matthias Rillig’s laboratory in Berlin, and me, podcaster Anja Krieger. We owe huge thanks to our story consultants from the lab: Stefan Hempel, Gaowen Yang, and Milos Bielcik. The music was composed by Julius Stucke, also known as Klangpflaster, and the theme is by Sunfish Moon Light. Cover design by Maren von Stockhausen. Special thanks to Joscha Grunewald for helping us improve the sound. We’re really grateful to our funders at the BiodivERsA research network for enabling us to make this special podcast. And thanks to you, listener, for lending us your ear! In the next episode, we’re going to explore paths to a sustainable future and ways to protect the precious life in the soil. So, hope to see you next time!

Vocoder sings “The Life in the Soil”­­­, end of audio.

FULL CREDITS

Produced by: Anja Krieger and the Rillig Lab
Funded by: Digging Deeper / BiodivERsA
Guest experts in this episode: Asmeret Asefaw Berhe, Maddy Thakur
Additional Voices in intro: Diana Wall, Richard Bardgett
Story consultants: Stefan Hempel, Gaowen Yang, Milos Bielcik
Thanks to Joscha Grunewald for helping us improve the sound!

Cover art: Maren von Stockhausen
Theme song: Sunfish Moon Light / Future Ecologies
Music: Julius Stucke / Klangpflaster

The Digging Deeper project was funded through the 2015-2016 BiodivERsA COFUND call for research proposals, with the national funders Swiss National Science Foundation, Deutsche Forschungsgemeinschaft, Swedish Research Council Formas, Ministerio de Economía y Competitividad and Agence Nationale de la Recherche.