[00:00:00] James Lawler: You are listening to Climate Now. I’m James Lawler, and today I have the pleasure of speaking with Dr. Asmeret Asefaw Berhe who is professor of biogeochemistry and Falasco Chair in Earth Sciences, Life & Environmental Sciences Department at the University of California Merced.
[00:00:20] James Lawler: Dr. Berhe is a soil biogeochemist who studies the impact of erosion, fires, and drought on the carbon storage potential of soils. She is a globally recognized advocate for the role that soils can play in removing and storing carbon from the atmosphere. In 2021, Dr. Berhe was nominated by President Biden to direct the Office of Science in the United States Department of Energy.
[00:00:43] James Lawler: Dr. Berhe, thank you so much for making time to join us today.
[00:00:46] Dr. Asmeret Asefaw Berhe: Well, thank you for having me.
[00:00:48] James Lawler: The goal of our conversation today is to understand how soils can be used as a carbon dioxide removal strategy, but before we get into those details, maybe you could tell us a little bit about how you became a soil biogeochemist in the first place.
[00:01:02] James Lawler: What inspired you to become a soil scientist?
[00:01:06] Dr. Asmeret Asefaw Berhe: I grew up in East Africa, in Eritrea. In the capital city of Eritrea, in Asmara, is where I was born and raised. When I went to college as an undergraduate in the University of Asmara, I heard about this department of soil science. And until then, I never knew that there was a science of soils. Like most people, I think, especially most 18, 19 year-olds. Once I discovered that I could study the sciences that I was fascinated with already within this amazing world of soils, I just never stopped. I’ve been learning about soil since then.
[00:01:42] James Lawler: What was it that intrigued you so much as a teenager in the science of soils? What was it about that, that really grabbed your attention, would you say?
[00:01:52] Dr. Asmeret Asefaw Berhe: I think it has to be discussions about the ecological functions of the soil. See, most of us recognize that our food largely comes from soil, plants that grow in soil or animals that feed in soil, but what we don’t necessarily know about is that soil is also responsible for recycling our waste, regulating our climate, provision of water and clean water, and it also serves as a building and engineering medium for so much, and is home to the most abundant and diverse forms of life than we know of anywhere in the Earth system. I think learning about the ecological functions of soils more than anything else was just eye-opening.
[00:02:36] James Lawler: Was there a connection between your interest in the ecological function of soil and the region where you grew up?
[00:02:43] Dr. Asmeret Asefaw Berhe: Yeah, actually there is an important connection because East Africa happens to be one of the earlier known settlements that we know of anywhere in the world, and so the soils in the region are amongst the longest cultivated soils.
[00:02:58] Dr. Asmeret Asefaw Berhe: They have some of the longest cultivation and human use history. By virtue of that, and in some of the climate complications, the soils also happen to be among the most degraded in the world right now. So there’s a huge emphasis placed on trying to understand soil degradation and also soil and water conservation practices to protect the soil resource, obviously, from further degradation and rehabilitate as much of it as possible.
[00:03:29] James Lawler: So, give us a description of your research. What do you and your research group focused on?
[00:03:36] Dr. Asmeret Asefaw Berhe: My group at the University of California Merced is a soil biogeochemistry research group, and largely our work revolves around trying to understand the role that soils play in maintenance of the earth’s climate.
[00:03:49] Dr. Asmeret Asefaw Berhe: To a lesser extent, we also spend a lot of time asking questions about human-soil relationships. So, the political ecology context of what it means when humans are interacting with soils and the subsequent degradation, and even questions about ownership and other issues that are associated with it.
[00:04:09] Dr. Asmeret Asefaw Berhe: Within the first major area that I mentioned in soil biogeochemistry, our work tries to improve our understanding of how and why organic matter that would otherwise decompose rather quickly remains in soil for long periods of time. So, think about the fact that the organic compounds and the carbon within that are in soil.
[00:04:32] Dr. Asmeret Asefaw Berhe: Are these rather fast cycling, if you will, thermodynamically unstable group of reduced organic compounds. If you left a residue to decompose on top of the soil surface under most environmental conditions, it would be decompose within a year or so, but if it actually enters in soil, that carbon can be retained in the soil for thousands of years, sometimes longer.
[00:04:57] Dr. Asmeret Asefaw Berhe: So, that’s the kind of mechanisms that we’re interested in studying.
[00:05:01] Dr. Asmeret Asefaw Berhe: Maybe this would be a good opportunity to get us and all of our listeners on the same page. What actually is soil. Can you give us a basic definition of soil as well as a sense of how much carbon can be stored in soil?
[00:05:14] Dr. Asmeret Asefaw Berhe: Soil is that loose material and consolidated loose material that’s a mixture of minerals and organic matter. And technically speaking, soil is anything that is sitting on top of an unaltered rock that we refer to as parent material, and that is what defines the depth of soil. Globally speaking, I think it’s fair to say that the depth of soil can be approximated by about six feet of new soil material, but there’s a lot of variability based on a number of ecological climate-related variables.
[00:05:51] Dr. Asmeret Asefaw Berhe: There’s a large variability in depth of soil, some soils are just 10 centimeters deep, shallow, others can be tens of meters deep, 10 meters and beyond. It’s in fact a very interesting reason why we can’t approximate carbon storage very well is because we have a hard time estimating the distribution of soil depth.
[00:06:12] Dr. Asmeret Asefaw Berhe: But, a lot of the accounting that’s out there in terms of soil carbon storage, right now includes at least the top two meters of soil, if not the top three. So, that’s the level of estimate that’s out there. When you consider those depths, then, there’s roughly close to about 3,000 billion metric tons of carbon, 3,000 gigatons of carbon stored in soil.
[00:06:41] Dr. Asmeret Asefaw Berhe: That’s compared to about 650 gigatons that’s stored in vegetation, and another close to 800 gigatons that’s stored in the atmosphere. I think it helps to remind folks here then that soil stores more carbon than the atmosphere and the vegetation combined and then twice over.
[00:07:04] Dr. Asmeret Asefaw Berhe: Wow.
[00:07:04] James Lawler: That is a significant amount of carbon, and while we’re still talking about soil science basics, I’m wondering, could you also paint a picture for us of how soil fits into the carbon cycle? So, how has that giant reservoir of carbon forming and how is it interacting with the rest of the environment?
[00:07:20] Dr. Asmeret Asefaw Berhe: So, the carbon cycle, the biogeochemical cycle of carbon that soil is an important part of basically operates in this manner.
[00:07:30] Dr. Asmeret Asefaw Berhe: There’s a lot of CO2 in the atmosphere, and when plants photosynthesize, they take out CO2 from the atmosphere and use it to build their biomass, or the green matter. When those plants die or the animals that fed on those plants die, their remains return into the soil. That’s the main entryway for carbon to the soil.
[00:07:52] Dr. Asmeret Asefaw Berhe: So, once the carbon is in soil, though, it doesn’t just sit there. In fact, there’s an active community of microorganisms that decompose that residue of animals and plants and causes the release of greenhouse gases in the form of dominantly CO2, but also methane and nitrous oxide into the atmosphere.
[00:08:14] Dr. Asmeret Asefaw Berhe: I think at this point, it’s important to mention that decomposition is an extremely important ecological process. It’s the process by which all the nutrients that plants took up from the soil when they were growing is actually released back into the soil so that it can be used by the next generation of plants and organisms that live in the soil.
[00:08:36] Dr. Asmeret Asefaw Berhe: Photosynthesis is the primary pathway that carbon comes into the soil, and a large part of that carbon that comes into soil gets decomposed by activity of microbes and returned back into the atmosphere again as greenhouse gases, but in the process, some of that carbon might actually not decompose rather fast and it could be stashed in what I like to refer as the soil carbon bank.
[00:09:02] Dr. Asmeret Asefaw Berhe: When I’m talking about this, I like to use a bank analogy to understand carbon storage in soil. It’s very useful to not just rely on the rate of input of carbon or the rate of output, but rather the balance of the two, because soil carbon storage works basically like a bank. You and I have a bank account where in the beginning of the month when we get paid our salaries, there’s a lot of carbon, if you will, with photosynthesis that enters the soil, but once we pay our bills, there’s very little that’s going to remain, and the question is, if we can maximize how much is saved as the end of the time after we pay our bills, we can build a rich bank account, right? And in the same manner, if we can slow down the decomposition of even a small amount of the carbon that enters to soil over time, then we’re able to build a rich bank of carbon in soil.
[00:10:02] Dr. Asmeret Asefaw Berhe: In fact, all the carbon that’s in soil right now was built like that over long periods of time, you know, kind of base slow rates of accrual, but slow rates over long periods of time was able to build all of that carbon that is in soil right now.
[00:10:18] James Lawler: Now, you mentioned one mechanism by which carbon re-enters the soil and then is metabolized by microorganisms, one byproduct being CO2, which is emitted into the atmosphere, but by what mechanism does carbon become permanently sequestered in the soil? Is it also through microbial decomposition, or is it through some other processes that the carbon remains? And when it remains, in what form does it remain in the soil?
[00:10:48] James Lawler: As a soil biogeochemist, I prefer not to use the word permanent. When it comes to discussing organic matter, mainly because these are, as I mentioned earlier, thermodynamically unstable, organic compounds. So, they don’t necessarily stick around in soil forever. Rather, what we talk about is the balance again, at what rate can the decomposition in soil be slowed enough so that there is an accrual over time. So, a higher rate of input than output. That’s the focus of what I discuss. So, from a perspective of why carbon persistence oil for long periods of time, that largely has to do with the environmental conditions where that carbon finds itself.
[00:11:38] James Lawler: For example, the organic carbon can be trapped inside aggregates or clots that are made by a combination of minerals and organic matter. And once it’s physically trapped in these aggregate spaces, then this rate of decomposition is slower because if the microbes are going to decompose it, they’re going to have to break down those aggregates first and that’s energetically very costly. And even more of the carbon that persists in soil for long periods of time is stored because it forms a chemical association with surfaces of minerals. And here, it helps to remember that soil is made up of not just the organic matter that’s cycling, but largely actually is made out of mineral bits and pieces of rocks that are weathered and altered over long periods of time.
[00:12:28] James Lawler: And those minerals tend to be reactive to a different degree, depending on which minerals we’re talking about. But in particular, when would these organic compounds that have charged surfaces come in contact with the mineral surfaces, which are also charged, they can form chemical bonds and those chemical bonds are actually largely the reason why most carbon persists in soil for long periods of time.
[00:12:53] Dr. Asmeret Asefaw Berhe: Just like that physical association and side aggregates that I described, when carbon is bound to the surfaces of minerals through these chemical bonds, it’s also hard for microbes to decompose. So, if microbes have fresh source of residue to decompose, then they leave that alone for way longer, and so it accumulates over time, basically.
[00:13:14] James Lawler: Excellent. Thank you. So, by what mechanisms have you or others learned that the release of carbon from the soil can be slowed down? Since that seems to be the key question: how do we actually slow the process by which carbon is re-emitted into the atmosphere?
[00:13:32] Dr. Asmeret Asefaw Berhe: Yeah, so when you think about the actual processes involved in land management, that could slow down the rate of decomposition, it helps to think about what conditions, the environmental settings, that microbes cannot operate optimally at. And by this, I’m referring to the fact that it’s the decomposition that’s facilitated by microbes that is responsible for loss of the carbon from soil or release of greenhouse gases. And so if we want to slow down the decomposition, then we have to figure out what are the conditions under which the microbes operate optimally, and how can we change those?
[00:14:08] Dr. Asmeret Asefaw Berhe: For that, thankfully, we know a lot about this, right? For example, tillage physically mixes soil. It breaks down aggregates and arid soil. It creates optimal conditions for microbes to actually efficiently break down the carbon that’s in soil. So, if we can slow down tillage, if that’s appropriate for the soil, you could slow down the rate of decomposition.
[00:14:34] Dr. Asmeret Asefaw Berhe: If you could provide mechanisms for organic matter to be trapped, either inside the aggregate spaces physically, or bound to the surfaces of reactive minerals, that would also create conditions that would slow down decomposition, right? So there’s a whole host of mechanisms, including reducing the application of excess amounts of agricultural chemicals.
[00:14:56] Dr. Asmeret Asefaw Berhe: For example, that would decrease the rate of greenhouse gas flexes from soil. Why does reduction in application of chemicals improve the carbon retention profile of soil? Remember that the carbon and nitrogen cycling in soil are coupled processes. So, when you have mineralization of organic matter is when both these processes that unleash CO2 and nitrous oxide are activated to a large degree, or their rate can be enhanced. Application of agricultural chemicals, in particular nitrogen-based fertilizers, actually create conditions in soil that would cause a large degree of nitrous oxide. In fact, if you look at where nitrous oxide is released globally, or even in the U.S. you would notice that agricultural areas stand out.
[00:15:49] James Lawler: Okay, so just to clarify, you describe mineralization as when CO2 or nitrous oxide can be released from organic matter. So, mineralization is not the same as the mineral association that you were speaking of earlier, where the carbon or the nitrogen bind with minerals that are in the soil?
[00:16:03] Dr. Asmeret Asefaw Berhe: No, mineral association and mineralization mean very different things. So, mineralization actually by definition just means conversion of organic compounds into inorganic forms.
[00:16:16] Dr. Asmeret Asefaw Berhe: In essence, the conversion of the reduced organic compounds, where most carbon is part of soil, to CO2—that conversion isn’t a process of mineralization. In the same manner in the nitrogen cycle, conversion of that organic nitrogen in the residue or organic matter pool to nitrate and ammonium and eventually N2O, that is also a process of mineralization.
[00:16:41] Dr. Asmeret Asefaw Berhe: But, it’s very different than the mineral association part, the processes that we recognize as complexation or cation bridging, but they’re really just binding. That’s just binding, not conversion or transformation at that composition.
[00:16:58] James Lawler: Fascinating. I’d love to ask you to clarify one point, because I think for some, it might be a little bit confusing or perhaps counterintuitive. You mentioned that if our objective is to slow the decomposition rate that is affected by the microbial communities in the soil, then one might ask, why not just try to kill the microbes in the soil? Then, you know, certainly the rate of decomposition would slow, but of course, we’ve also heard that microbes are very beneficial to soil health. So, I’m wondering if you can explain what’s wrong with that thinking.
[00:17:36] Dr. Asmeret Asefaw Berhe: Yeah. So, no, we don’t want to do that because, as I mentioned earlier, decomposition is an extremely essential ecological process, and even beyond the composition, microbes have a lot of other social roles that they play in soil.
[00:17:50] Dr. Asmeret Asefaw Berhe: So, even though we might want the rate at which they’re cycling the carbon to slow down a bit, we definitely do not want to stop it. We don’t want to sterilize soils. That’s definitely not something we want.
[00:18:02] Dr. Asmeret Asefaw Berhe: Okay. So, what is the potential for additional carbon sequestration by soils? You know, if we can put some numbers around that.
[00:18:14] Dr. Asmeret Asefaw Berhe: There’s a pretty important and significant potential for carbon sequestration out there, but I think it’s important to make sure we’re clear about the fact that the rate of sequestration depends on what type of ecosystem we’re talking about and what type of management practices are also implemented in soil. But, a good way to start thinking about this is, think about the fact that since we started agriculture. There’s an estimate out there that about 120 billion metric tons of carbon that was in the top two meters of soil was released to the atmosphere, with the fastest rate of release happening in the last 200 years since the industrial revolution.
[00:18:57] Dr. Asmeret Asefaw Berhe: Can we reverse that? Can we at least put back this amount of carbon that was released from soil into the atmosphere because of human land use in working land space? That’s one of the important guiding principles for carbon sequestration around the world at this point. But then to give you some numbers to constrain the rate of sequestrations.
[00:19:21] Dr. Asmeret Asefaw Berhe: So, for example, you can sequester carbon really fast in wetland soils that we broadly classify as histosols, meaning organic matter-rich soils in wetlands. And the rate of sequestration there could actually be really fast, on the order of about 50 megagrams of CO2 equivalent per hectare per year.
[00:19:43] Dr. Asmeret Asefaw Berhe: It’s a really fast rate of sequestration. This is as opposed to something on the order of one megagram CO2 equivalent per hectare per year, that we can sequester using crop land management, for example, or grazing land management. So, there’s a huge potential here, but it helps to remember that the climate envelope, where you can find wetlands and the geomorphic constraints that are out there of where wetlands can exist makes it so that there is limited area where this practice can be adopted.
[00:20:18] Dr. Asmeret Asefaw Berhe: But even though the rate of sequestration is low for cropland management or grazing land management, there is vast areas around the world, two, three orders of magnitude, bigger areas, where we can practice grazing land or cropland management compared to restoring the histosols. Even though grazing land management and cropland management have a very low rate of carbon sequestration, by the virtue of the fact that these practices can be adopted over large global areas, they can still achieve roughly equivalent or an even slightly higher rate, of global sequestration. So, we’re talking about 1.5 or 1.6 gigatons of CO2 equivalent per year in grazing land management or crop land management compared to 1.3 gigatons by restoring histosols.
[00:21:14] Dr. Asmeret Asefaw Berhe: For the record, all of these numbers come up from a paper that was published in Nature a few years ago, that I could share with you all.
[00:21:22] Dr. Asmeret Asefaw Berhe: We have our own paper coming out in press right now where a lot of this is reviewed too, that I could share, if that helps.
[00:21:28] James Lawler: That would be terrific, and we will have links to those papers posted on our website, along with the transcript for this podcast, so that our interested listeners can go and read more.
[00:21:38] James Lawler: Now I’d like to shift our conversation to how we can assess this potential. So, what is the current state of the art when it comes to measuring the amount of carbon that is stored in soils. What are the current tools available and how scalable are these tools? How accurate are these tools as well?
[00:21:54] Dr. Asmeret Asefaw Berhe: Yeah, that’s a really important question. For several decades now we’ve been able to rather accurately measure how much carbon is stored in soil, the plot or sample scale, right? We have measurement techniques that are getting more advanced, and the more common one that we use in the lab is the dry combustion technique, where you just burn all the carbon that’s in soil and try to measure how much CO2 is released.
[00:22:21] Dr. Asmeret Asefaw Berhe: We can get really precise measurements, but where things get complicated is when we try to scale up those measurements to plot and especially regional and global scales. And that’s because the amount of carbon stored in soil is not just the concentration that you have to look at. For you to be able to accurately determine how much carbon is stored in soil, you need to know how deep the soil is. You also need to know how densely packed the soil is, and a term that we use in soil science is called bulk density. Basically, how much soil is within a given volume. And we also need to know how much rock is in there because technically by definition, anything that’s greater than two millimeters is not soil, it’s either rock or pebble, or something else. So, to make the actual accurate measurements, we need to know the depth of soil, we need to know the bulk density, and we need to know the rock content in addition to the concentration of carbon emissions. And, as you can imagine, trying to do that is time intensive.
[00:23:25] Dr. Asmeret Asefaw Berhe: It’s not necessarily difficult, we’ve done it for decades, but it’s not quick, and of course, with time comes to money in the consideration. That’s really where the discussions of how hard it is to determine the amount of carbon stored in soil come from.
[00:23:41] James Lawler: So, I know very little about what I’m about to ask, so this may be irrelevant, but I know that geologists working for oil companies have various methods to assess the underground conditions in any given location to sort of make judgements on where to build. Or, physical approaches, right? Yes, like geo-physical approaches to it. So, you know, density of this sub-surface levels.
[00:24:05] James Lawler: I wonder if you could talk a little bit about to what degree some of these techniques are applicable to assessing soil conditions, or it could be perhaps adapted to do such things, and I’m kind of imagining like robots running around with these tools and collecting data over a large area in order to solve that problem, but what is in fact the state of the art when it comes to doing these measurements?
[00:24:29] Dr. Asmeret Asefaw Berhe: Yeah. So, application of geophysical techniques to learn more about the subsurface is an active area of research. There’s a huge potential there. It is being used, and in fact, it was used in a couple of projects that I’ve been involved in, but keep in mind that the way the technology works, or at least its application to soils right now, it is also a time intensive and not cheap.
[00:24:52] Dr. Asmeret Asefaw Berhe: But it is there. So instead of having to dig all over the catchment, trying to figure out how deep the soil is, you can now use these really incredible geophysical approaches to tell you a lot, not just the depth, but also a lot about the subsurface. So, the technology’s there. It’s not necessarily fast and it’s not automated, not yet at least. But it’s definitely an incredibly useful tool that’s increasingly becoming part of how we understand the soil carbon. Not just carbon, but overall soil processes and properties.
[00:25:27] James Lawler: And is there any signature from satellite imagery that would tell us anything about what’s underneath the surface over larger scales?
[00:25:35] Dr. Asmeret Asefaw Berhe: That’s a little bit tricky because most of the remote sensing approaches that are available, they could tell you a lot about the very top of the soil surface, but they can’t really tell you about what’s going on in deeper subsurface. If you really want to know, for example, how much carbon is stored in soil, you can learn some things from the topsoil, but you really would not get a good, accurate characterization unless you know more about the subsurface.
[00:26:02] Dr. Asmeret Asefaw Berhe: There’s a lot of modeling approaches, computational approaches, where we are trying to use data from the surface to predict how much soil is in the subsurface, or even how much carbon it can store. The reason why these things are tricky is because soils are very diverse. The depth of soil, the type of soil, the mineralogy, i.e. the potential to stabilize carbon, and hydrology and temperature dynamics that dictate how active microbes can be, all of these things vary as a function of climate or parent material that the soil is formed from, the relief of geomorphology of the landscape, the kind of biota that you have in there, and how long even these soils have been weathering, these are factors that we collectively refer to as state factors of soil formation.
[00:26:51] Dr. Asmeret Asefaw Berhe: So, these five things dictate so much of the properties of the soil, and that’s the reason why soils are so diverse, even with small geographic area, let alone large swaths of land.
[00:27:04] James Lawler: Very interesting. I’d love to come back to a question that you posited earlier, which was, can we reverse the amount of carbon released from the soil since the start of the industrial revolution? That’s sort of a motivating question for a lot of this work. What is your opinion? Can we, can we do that and how?
[00:27:24] Dr. Asmeret Asefaw Berhe: It’s technically not impossible to reverse all of that carbon that we lost, but it would take a lot of time. Remember, carbon sequestration is a rather slow process. So, to be able to achieve that much carbon accrual, excess carbon accumulating in soil, compared to how much is being lost, we need concerted and sustained efforts for decades, if not longer. So, it’s not impossible, but it does require a lot of effort and patience.
[00:27:58] Dr. Asmeret Asefaw Berhe: Amazing. Thank you. That was, that was really great. It’s very exciting to hear you talk about all of these things, so I really appreciate your time.
[00:28:06] Dr. Asmeret Asefaw Berhe: I know I have the tendency to get animated when talking about soils. It’s just fascinating.
[00:28:16] James Lawler: After our conversation, we were curious about how much patience we would need to put back all 120 gigatons of carbon released from soils through human activity. Using Dr. Berhe’s estimate of almost three gigatons of total annual carbon storage potential across wetlands, grazelands, and croplands, it would take a little more than four decades.
[00:28:36] James Lawler: So, those kinds of practices could really have an important role to play in this century in removing carbon from the atmosphere.
[00:28:43] James Lawler: That’s all for this episode of the podcast. Climate Now is made possible in part by our science partners, like the Livermore Lab Foundation. The Livermore Lab Foundation supports climate research and carbon cleanup initiatives at the Lawrence Livermore National Lab, which is a Department of Energy Applied Science and Research Facility.
[00:28:59] James Lawler: More information on the foundation’s climate work can be found at livermorelabfoundation.org. To listen to other interviews from Climate Now, to watch our videos, or sign up for our newsletter, visit climatenow.com. If you’d like to get in touch with us, you can email us at firstname.lastname@example.org.
[00:29:15] James Lawler: We hope you’ll join us for our next conversation.