The Exchanges

Q then now we're interested in perhaps trying to mess with it again, only this time to try to maximize the amount of carbon that soil takes up and keep it in there as long as possible for the purpose of mitigating climate change. So before we talk about the things we might want to do to mess with it now, how did we mess with it when we started agriculture?

A The answer to that is complicated, but I, I think there are a couple main ways that agriculture has altered soil and its role in the carbon cycle. So, plants are fixing carbon, and, and they're adding it to the soil through a number of ways, through their, their leaves, and to a large extent via their roots, and also through carbon compounds that leak out of the roots or are exuded from the roots. We call them exudates, and so they're adding carbon to the soil, and that carbon builds up because microbes can consume it, they can eat it, but they can't keep up with the rate at which plants are adding that carbon under all circumstances for a number of reasons, and so the, the sort of natural baseline condition is that there's an ecosystem on the surface, there are plants, they're fixing carbon, they're introducing it, there are microbes that are constantly breaking it down and releasing it back to the atmosphere, But those two processes ultimately come into some kind of balance that allows carbon to build up to a certain level in the soil. And, you know, how much builds up depends on a whole bunch of factors, but at some level, it depends on how much plant growth is happening in a given place. So when, when humans come along and start developing agriculture, they're really finding another use for that plant growth at some level. And so agriculture involves harvesting biomass, rep…

AI assessment note: “replacing forests with cropland, potentially, converting ecosystems that were composed of deeply rooted plants”

Q And get your perspective on where they fit in the mix, how much certainty we have around them, where the big questions lie, and so on. There's now a bunch of, you know, conversation around enhanced weathering, which is sort of independent from Soil carbon itself, right? We're not really trying to create new soil when we are doing enhanced weathering, right?

A Yeah, I mean, enhanced weathering, that's, that's a whole nother can of worms, um, at some level. I mean, I, I, um, is that also a soil pun? Maybe. Um, so, and what I've talked about so far is, um, organic carbon in soil, but soil has this huge role in the inorganic half of the carbon cycle as well, Um, uh, regardless of whether that inorganic carbon is stored in the soil or not, and, uh, that's because the weathering reactions that happen in soil, and these are basically transformations of minerals that formed at high temperatures and pressures, um, deep below the Earth's surface, which are not thermodynamically stable in soil or in, in, in ecosystems at the Earth's surface, and as they're transformed, um, the reactions basically consume acidity, um, And release, um, divalent and monovalent cations, um, so like calcium, magnesium, sodium, potassium, um, for instance. And that reaction on balance ends up Basically pulling CO₂ out of the atmosphere, and, um, and that CO₂ then ends up as bicarbonate in water in the soil. It could end up in a stream and then flushed out to the ocean, where it's essentially sequestered in the ocean, or it could end up staying in the soil as a carbonate mineral, right? So that, that process is going on all the time, and it's hugely important in regulating Earth's climate at really long timescales, and might be influenced by things like mountain buil…

AI assessment note: “soil has this huge role in the inorganic half of the carbon cycle as well”

Q in the soil or keep the carbon in the soil longer. I'll maybe bucket a few of them together and then just get your take on what we, what the state of the science is on And what we know about how effective these might be, um, but there are things like cover cropping and agroforestry, no-till farming. How should we be thinking about all those things as of today?

A Oh, where to start here? Um, I mean, there are a whole range of agricultural practices that were originally developed, uh, for, uh, Perfectly good agronomic reasons that aren't actually related to sequestering atmospheric CO₂, and that would include cover cropping, um, which is partly about erosion management and nitrogen management in, in agricultural soils, and maintaining soil fertility, um, because organic matter is good for soil fertility. Or, um, no-till, for instance, which was developed as an erosion control measure, right? Or for that matter, agroforestry, which, which also has benefits in terms of soil health and, and, you know, erosion control, et cetera, right? So, so these practices were, were explored, um, or, um, promoted, uh, because they have real benefits in, in the right context for agriculture. And then what's happened over the last couple decades is that there's been increased interest in using these practices to, um, Also to fight climate change. There are difficulties though. All of these, um, agricultural practices, they yield increases in the amount of organic carbon in soil. And organic carbon is not necessarily a long-lived sink for atmospheric CO₂, and at some level, I think that's the central thing to keep in mind, is that microbes like to break down organic matter. It's how they make a living, um, and so any one of these practices might succeed in …

AI assessment note: “ultimately it shouldn't be treated as equivalent to a, a more durable or permanent”

Q All right. So I guess final question, um, a lot of all of this thinking and accounting around soil carbon relies upon modeling ultimately because we're not measuring everything all the time and certainly can't measure the future. So what type of modeling are we doing? How sophisticated is it? Is it well set or is it problematic? Give me a sense of the world of soil carbon modeling.

A Yeah, I mean, it's a, um, it's, it's kind of a thorny topic at some level. Um, um, the, the history here is that people first started, um, Modeling soil carbon in a really quantitative way in the mid-twentieth century, and, um, and so this was in an era when people still thought of soil carbon as, um, largely being just dead plant material that, um, was hard to decompose. I'm simplifying a bit when I say that, but that was sort of the dominant paradigm at the time, right? And The way they represented that, um, is that they basically, they did it in a very conceptual way, which is that they, scientists had figured out that some soil carbon, um, is very bioavailable, and it cycles quickly, and some of it is less bioavailable, um, and so maybe it's more recalcitrant, uh, is the word that was used for it, um, and because of its chemical properties, um, Or perhaps for other reasons, um, and so it cycles more slowly, and so they, they, they conceptually, they divided the amount of carbon in the soil into these pools, and they assigned different decay rates to them, and basically represented the The return of carbon to the atmosphere from soil organic carbon pools, um, the way you might model radioactive decay as a first order exponential process. Uh, so, uh, basically the amount leaving any given carbon pool is a function of some intrinsic turnover rate that's modified by environment…

AI assessment note: “they conceptually, they divided the amount of carbon in the soil into these pools”

Q and where it is geographically and everything. Now I'm going to change one practice. I'm going to stop tilling or, you know, pick your, pick your practice. How much certainty can we have around The amount of carbon that is additional carbon that is sequestered, and what is your take on the sort of world of measurement around that, which is also has its own whole suite of different solutions?

A Yeah, I, you know, at some level, the elephant in the room is, is the additionality question. Uh, you know, we, the, we meaning the scientists, people like me, love to talk about measurement, and issues with measurement, and how to measure best, etc. But, but I think that all pales in comparison to the, the, the more difficult question of what would have happened on that piece of land otherwise, right? Because if somebody was going to be practicing no-till agriculture, regardless of whether you paid them for it, then, It's, you know, that it doesn't count as, as a, a, you know, carbon removal or something that could be traded as an offset or anything like that, right? And this issue plagues the forestry sector as well. It's just as bad in the soil world. Um, and I, I, I personally, I think that challenge swamps the measurement issues, but there are also, um, Challenges related to measurement, it's, it's, I mean, we know how to measure soil carbon and how to estimate it, um, but it, it, you know, if you want to know the answer in one particular field, it requires taking a lot of samples. At some level, you can get around that by scaling up projects, um, because, um, Basically, if you want to get the right answer on average over a large area, you can get away with, um, with sort of achieving that with fewer samples. So there are sort of economies of scale when it comes to the mea…

AI assessment note: “the elephant in the room is, is the additionality question”

Q weathering. We haven't talked about, and we, we talked about some of the more sort of like practice agricultural practice driven stuff. We haven't talked about some other areas where folks are doing work, things like biochar application to soils and, um, some more like novel inorganic carbon maximization stuff. But like, what, what gets you excited? What do you think is really cool that you're seeing science built around?

A Yeah, I mean, I, I think what I'm most excited about, um, isn't a particular practice or strategy or technology. It's more of a way of thinking about how soil carbon works, um, that could kind of bleed into all of these applications. Um, and, you know, I, I, I guess, you know, to back up a bit, um, the, the way we've Thought about soil carbon has changed a lot, um, soil organic carbon has changed a lot over the past several decades. Um, you know, it, it, it used to be the case that the sort of dominant theory was that, um, soil organic carbon persisted in soil because it had intrinsically resistant chemical properties that made it hard for microbes to digest.

AI assessment note: “what I'm most excited about, um, isn't a particular practice or strategy or technology.”