AoR 142: Understanding Grazing Effects on Soil Carbon, the sequel with Dr. Paige Stanley

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>> Welcome to the Art of Range, a podcast focused on rangelands and the people who manage them. I'm your host, Tip Hudson, range and livestock specialist with Washington State University Extension. The goal of this podcast is education and conservation through conversation. Find us online at www.artofrange.com.

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Welcome back to the Art of Range. My guest today is a return customer, Paige Stanley. We did a fairly recent episode on a paper that she wrote with a number of other authors in the journal Global Change Biology titled Ruminating on Soil Carbon: Applying Current Understanding to Inform Grazing Management. We're coming back to this because there's a lot of talk about grazing and carbon sequestration potential, the stability of carbon storage, and the potential for grazing to increase carbon capture rates. One of the central features of the paper is the conceptual framework that they developed describing carbon soil organic matter creation and stabilization through five ecophysiology elements or drivers of soil carbon. The paper is a synthesis of how different grazing patterns affect those drivers because this is not simplistic. So I've invited Paige back to talk about that in a little bit more detail. We introduced the importance of soil carbon and how grazing generally affects soil carbon globally and why it matters, but didn't quite have the time to get into the details of how specific grazing patterns affect all of these different drivers and how that translates through a few different pathways into soil carbon increase or decrease. We'll jump into it. Paige, welcome back.

>> Thanks so much, Tip. I really appreciate the opportunity to be back on here.

>> I would recommend that people go back and listen to that other episode to get a little bit more context if they have not heard it previously. And if they have heard it previously, they might be looking forward to what we're going to talk about today. But just for the sake of it, in case people don't go back and listen, provide a little context for where that paper came from and what you were hoping to communicate with it.

>> Yeah, sure thing. So, a couple of years back, myself and some of my collaborators that wound up co authoring this paper with me, we were all at the Society for Range Management Conference. That year, I believe it was in Boise. And that was the first year at SRM where soil carbon was kind of a central focus of the meeting. And throughout that week, there were several soil carbon sessions. And there was this one panel in particular that stood out to me and I think spurred the creation of this whole idea and the resulting paper, and that was a carbon markets focused panel. And I think myself and other scientists in the room that work on soil carbon were just    we came out of that panel thinking, oh my goodness. That felt like a lot of misinformation and possibly was causing even greater misunderstanding in the realm of grazing management and soil carbon and what we know and what we don't. And that's been exacerbated by the recent, I think    I would say emergence of rangeland soil carbon markets. And so, after the meeting, myself and my collaborators were talking and we were like, what's going on here? And what can we do to possibly synthesize the things that we do and don't know and hopefully drive better research and sort out some of this misunderstanding that's happening. And I think    we were all of the understanding that some of the mechanistic literature is a little sparse. And so, we were thinking about ways that we could synthesize all of these things. And we came up with this idea for this conceptual framework, which is a review paper of sorts. And the point was to say, you know, we've got enough pieces of this puzzle to be able to piece it together and get a better sense of what's going on. And so, that's what we did. Over the course of the next year, we got together and we created this conceptual framework where we combined kind of the impacts of grazing on ecophysiology elements, which are what we call EEs in the paper. So these are like ecosystem and plant community level impacts that we know have been measured as a result of various grazing strategies. And then the second level to that is, okay. Well, we know most of the impacts on soil carbon from grazing happen indirectly through these ecophysiology elements. And we know enough about that to be able to add that as another tier to this conceptual framework. And then, one additional deeper level is that, you know, the ways in which we've come to understand soil organic matter dynamics has shifted quite a bit in the past decade. So now we largely understand soil carbon dynamics in terms of these soil organic matter fractions. And so, that's even one step deeper into these mechanistic pathways. And so, what we did is combined our understanding and kind of a literature synthesis on how the impacts of grazing flows through each of these different things. So grazing impacts these ecophysiology elements. These shifts in ecophysiology elements is what is actually changing soil carbon dynamics. And then what is the changes in these different soil organic matter fractions that we've come to realize as important. And then the last aspect that I would say is an important contribution that we made is, if you were to go to the literature and type in something like impacts of grazing on soil carbon, what would come up is all of these papers on, for example, stocking rate or maybe grazing intensity. And those papers turn out to be really difficult to disentangle the actual causal mechanisms because grazing management is such an umbrella term. And so, we dug a bit deeper into what we wound up calling grazing pattern as a function of a couple of different grazing management levers, which tend to be what is reported amongst ranchers when you were to ask them. So it's digging deeper into the grazing management levers and then all of those downstream ecophysiology, soil carbon and soil organic matter fraction components is what we managed to put together.

>> Yes. And one of the things that struck me in re reading the paper is that we    by we, I mean, I think all of us, those of us who are sort of in this middle space between practitioners and scientists, as well as practitioners like ranchers and scientists    we underestimate the extent to which factors that we don't control are also major drivers of soil carbon. Because sometimes there's this idea that if we just apply more management effort, anything is possible. And I think I've said it on here a few times, but in a conversation with Jeff Herrick one time, he said, there are some fairly hard limits to what's possible and they're at least constrained by depth to bedrock, precipitation, and soil texture. And yes, while particularly in a more mesic ecoregion, we can manage the plant soil interface in a way that increases soil organic matter and maybe make some modest changes to soil texture, you know, the relative components of sand and silt and clay are not going to change a lot. And if you only have, as we do in a lot of places in Eastern Washington, other parts of the West, only a couple of inches until you get to solid basalt that goes down thousands of feet, that's not going to produce a plant community that grows 10,000 pounds of forage per acre per year. So there are some limits there. And this was brought out in the paper that we're trying to push these levers, but the levers can only go so far. And then you're up against a rock, sometimes literally. Well I'm going to mention now that we'll have    this is an open access paper. I think I said that right. This is an open access paper. And we will have a link in the show notes, both on the episode page and also directly from the podcast episode description in your podcast app, that'll go directly to the paper. And we're going to be talking about a graphic that I mentioned a minute ago that's on page three of the paper. And it might be useful to be looking at it, assuming listeners are not driving at the time that they hear this, to take a look at that graphic or after the fact to better understand it. Well let's talk through these drivers because    if you have a different opinion about where to start, feel free to do that. But I feel like it would be most useful to understand how these individual ecophysiology elements affect soil carbon and then step back to think about how grazing affects each of those and therefore affects soil carbon. So, yeah, let's jump in with    maybe describe how your team conceptualized these ecophysiology elements because that particular word probably doesn't have meaning for some people.

>> Yeah, absolutely. So, I think one of the real challenges that we had as we started on this paper is a framework, right? So, if you begin to think about how grazing impacts anything in the environment, you tend to go down a bunch of different rabbit holes seemingly all at once. And so, figuring out a way in which we could kind of organize each of these pathways and understand how they're toggling on each other was a challenge that probably we spent the most time on in developing the paper. It wasn't even the writing or the literature searching. It was like thinking about how to organize our thoughts on this. And the more I was reading, the more I was like kind of in my brain chunking up these pathways into these buckets. And so, what we came up with are these five ecophysiology elements, and these are kind of our way of organizing the ways in which grazing can impact individual plants in the short term. But more than that, what those kind of collection of short term impacts in individual plants adds up to in the long term. And so, that begins with ground and canopy cover, which is like how much leaf area do you actually have? And how is grazing influencing that? The second is productivity or how much net primary productivity. How much biomass is actually accumulating. So if you're to think of soil carbon accumulation as something of a mass balance equation. So you can think of it similarly to calories in, calories out for weight gain or weight loss. So the productivity is your main lever there, like how much total carbon is going into the soil. The third is input allocation, which I think is maybe a tougher one to understand. So plants accumulate mass in their bodies in what we call this root to shoot ratio. So how much biomass are they adding above ground versus below ground? And that differential accumulation actually has real downstream impacts on how much soil carbon is accrued and where it's being accrued. And so, grazing or defoliation in general can influence where and how much plants are adding to these different places in their bodies. And the fourth is input quality. So that also has something to do with input allocation. But stoichiometrically, there are differences in the quality of a plant input and how it forms soil carbon or doesn't. And so, if you were to boil it down to like the carbon to nitrogen ratio of plant inputs or microbial inputs, grazing will have an effect on that because it influences energy allocation, for example. Then that is another kind of ecophysiology element that can influence downstream carbon and nitrogen dynamics. And then lastly is plant diversity. And this one actually was a bit tougher because the impacts of grazing on diversity    Actually, you can think of diversity in terms of the prior four ecophysiology elements, but there does seem to be something specifically to impacts on plant diversity, whether that's diversity per se or functional diversity that we can't quite slot into those four other ecophysiology elements. And so, those five are what we came up with in terms of the    I'm sure there are others that we didn't think of, but these are the primary ways that grazing influences the ecophysiology around it that will then have downstream impacts in the pathways of soil carbon formation.

>> Yeah, I like those. And I like the way the graphic in the paper puts it together. You read the text and you can read through how each of these different ecophysiology elements relates to soil carbon creation and storage. But then in the graphic in the paper, you're showing how undergrazing, optimal grazing, and overgrazing affect each of those drivers. And there's some interesting comparisons there. And part of what's, I think, useful is that you can see how, for example, with undergrazing may result in higher canopy cover but lower productivity because of the decrease in photosynthetically active leaf area. So maybe let's go through each of the elements and describe in a little bit more detail how they specifically affect soil carbon fractions and then how that is influenced by different grazing regimes, so to speak.

>> Yeah. So if you were to ignore the five ecophysiology elements just for a moment and think about how soil carbon gain and loss happens in any soil, there are three, I think, primary pathways to soil carbon formation. One is increasing the amount of just carbon that's making its way into the soil. So increasing carbon fixation, which will have downstream impacts on just increasing soil carbon input. So if you think back to the calorie thing, that would be essentially like eating more. The second pathway is just reducing the amount of soil carbon that's being lost from any given soil. So that could be either by reducing decomposition or preventing erosion. And so it's less of an increase per se but more of a reducing the amount of loss. And then the third way, which is probably the most difficult to conceptualize, but we call it increasing the efficiency of below ground transformations. And so this is where that kind of mass balance equation starts to fail a little bit, because there are things that happen below the soil surface. Despite having biomass removed on top of the soil from defoliation, there are ways in which grazing can improve the internal functionality of the soil, particularly by way of microbes to actually increase soil carbon formation. And so it's not necessarily a function of additional carbon or reducing loss of carbon, but it's kind of making it more functional in a way that actually does have measurable increases in soil carbon. And so each of the five ecophysiology elements, so the canopy cover, productivity, input allocation, input quality, and diversity, will have impacts on soil carbon in one of those three pathways. And so throughout the paper, what we tried to do is connect these ideas of grazing, and then impact on any one or combination or interaction of those ecophysiology elements, and then which pathway or pathways are those outcomes on ecophysiology elements having on downstream soil carbon. And so that was the way in which we kind of conceptualized that framework and came up with that figure. And so what we tried to do in the figure in particular    and this is honestly thanks to Erica Patterson, who is a PhD student in the Soil and Crop Science Department at CSU. She's a fantastic graphic illustrator. And so she kind of took this brain baby and turned it into this really beautiful figure that I think really helps illustrate these concepts. But what we tried to do is stack them all on top of one another. And so you get this really nice visual where on top it's grazing pattern and then ecosystem structure, so what the outcome of those ecophysiology elements might look like. And then these kind of sliding bars of ecophysiology elements, and then downstream impacts on soil carbon and allocation into these different organic matter pools.

>> Yeah, I will echo that sentiment. The graphic is excellent. I tend to be a word based person and not a picture person. But with these really complex ideas, you can sort of get lost in the words. And a really good figure is not something I think that is particularly common in scientific literature.

>> Yeah.

>> And I think this did a really amazing job of pulling that together in a way that communicates well. And it does take some time. You've got to sit and look at it and think about it when you're encountering this for the first time. But it has to be that way because this is not simplistic, and there's a lot going on there. So there's a lot going on in the figure, but it's worth taking the time to think through it and then to follow up. I think the paper is also laid out well enough that you see a bar that's in a particular location on one of the ecophysiology elements, and you can find the description of why that is that way in the paper, along with quite a bit of scientific literature that's backing up that concept.

>> Yeah, and I think the kind of real benefit of the figure is that    throughout the paper, we go in depth into these kind of mechanistic pathways one by one. And that's how it's written out because that's the easiest way to understand them is individually. What we tried to do in the paper, though, is to demonstrate kind of three different ecosystem structures that might arise from different grazing management strategies and then what the kind of combination and interaction of all of those ecophysiology elements could be to arise in the form of that structure. And so it's a little bit of taking all the things that you might read about in the paper individually and thinking about what that might look like as an accumulated ecosystem as a result of grazing. And so, toggling on any one of these ecophysiology elements might give rise to a different kind of ecosystem structure and soil carbon stock, for example. And I think that is a real benefit of the figure.

>> One of the surprises, I think, for people reading the paper will be the effects of undergrazing.    because for a long time, we've had the idea, because of some history of overgrazing in many places and other more complex structural changes to landscapes and soils that resulted in significant soil carbon loss, undergrazing for a time can bring back the plants that are necessary to stabilize soil and build floodplains back up and sort of recover. But then that sort of reaches a plateau where you're not necessarily experiencing any further ecological benefits from having no grazing at all. And of course, most arid lands ecologists would say, or rangeland ecologists, not just arid lands, would say that these grassland and shrub ecosystems are disturbance driven, meaning that they're most healthy when they experience some level of plant harvest. So I'm sort of rethinking our flow for the discussion while we're talking here. Why don't we start with the overgrazing column and describe what the direct effects of overgrazing are on ecosystem structure, plant community structure, and then how that affects these different drivers.

>> Sure. So in the top of the figure, for those that are listening and can kind of look at this while we walk through it, we tend to think of overgrazing as a kind of over harvesting, over defoliation, and that could be grazing too often or taking off too much. Whatever the circumstances are that arise in this pattern that we tend to think of as overgrazing can have downstream impacts on soil carbon through all of these ecophysiology elements. So I think at its most basic level, if you are grazing, if you're taking off too much of a plant, essentially what's happening is you're reducing its capacity to photosynthesize by reducing its leaf area index. And so there's a very fine balance in any given plant, depending on its structure and its age and its kind of energy reserves, where it can regrow if it has the correct amount of energy reserves. It can regrow after defoliation. But if you take off too much of that leaf area, it won't have any way to photosynthesize and regrow, especially if it doesn't have those energy reserves. And so at the individual plant level, you can think of that happening over and over and over again, and then those plants will wind up terminating themselves because they've got no way to exist anymore. And the accumulation of that will mean plant death kind of on the whole, and then you'll start to see things like bare ground or maybe some shifts in plant community composition that are less desirable. And so when you've got bare ground, you've got all of the kind of ecophysiology elements will start to arrest. And so you've got less canopy cover, less ability to photosynthesize, which will turn into less productivity, so less total amount of carbon entering the soil. The input allocation almost becomes a moot point because you have fewer plants in general. And what that will also tend to do is reduce plant diversity because you're going to favor an ecosystem in which the plant community composition is very tolerant to grazing. And then you can imagine other outcomes like    say, you're years down the road in an ecosystem that's been overgrazed. Then you might have large swaths of area that are bare ground and other plants that are very grazing tolerant. And then the amount of kind of very lignified, kind of tougher carbon that's entering the soil from the plants that are still existing will be less likely to form that kind of mineral associated, organic matter that we tend to think of as very stable. And so it's a combination of having fewer plants and less total soil carbon accumulation is what we tend to think of as like an overgrazed system. And that can happen through a bunch of different ways of grazing. And I think what the readers might gather from this paper is we try not to offer credence or support for any particularly grazing management strategy by name. Because these kind of grazing patterns can arise through any kind of grazing, but it's really about the management levers and how they're being kind of toggled together that will give rise to these different kind of outcomes in which we, in the paper, call over, optimal, and under.

>> I think in the paper, one of the words you used for overgrazing was chronic defoliation where there's just sort of persistent pressure on the plants where they never get a chance to fully express the leaf area index. And the result is not enough soil coverage, low rooting depth, a whole host of things. And you say that bare soil    in an overgrazing situation, bare soil loses soil organic matter via erosion, reduced aggregate stability, exposure to increased temperature extremes, and shifts in microbial communities and activities that increase decomposition of the native soil organic matter.

>> Yeah. So I think if you were to, in its most simplest form, take the temperature at the soil surface of bare ground versus soil that's covered by plants. You'll see a giant difference, especially on a hot day or a cold day where bare ground, the temperature swings are much more significant. And so then that kind of becomes an inhospitable environment for the organisms that we want to be working for us in terms of soil carbon accumulation    tend to kind of teeter off because they can no longer survive in such a hostile environment. And so that's one simple, very simple way to think about the ways in which soil carbon might be lost    I mean, soil erosion, which is just the bare soil becoming more susceptible to loss by wind or water is one way. But the other is really like oxidation of carbon that was once in soil because the things that were keeping it there are no longer present.

>> Mm hmm. So then we often react to long term overgrazing situations by undergrazing or no grazing, at least for a period of time, in order to just recover plant material, to increase plant individual density, increase the diversity of plants that are present. And it's been observed lots of different places that if you give the plant community a bit of a break, you tend to get quite a few things back that maybe even sometimes we didn't know were still there. That happens in riparian areas; it happens in uplands. So we often react by undergrazing, but there's also, of course, some institutionalized undergrazing scenarios like the Conservation Reserve Program. Yeah, I guess this is my show. I can say whatever I want. But CRP was a good program to recover bare soil from places that got farmed that probably shouldn't have been farmed. And so it had significant benefits and still has some significant benefits, especially for wildlife habitat, but it is a good example of undergrazing and the effects of undergrazing. So let's walk through that one next because I think this will be, again, the specific effects of undergrazing on these different drivers was enlightening for me. And I felt like I knew something about it already, but this was really useful.

>> First of all, I'm very happy to hear that because I think there are two ways for this paper to go. One is for it to be well received as kind of like a general framework, and the other was for people to really dislike it because they disagreed with all the things that we said. So I'm really happy. But yeah, over    Sorry. Undergrazing, I think, is a tricky one because overgrazing is easy to conceptualize, right? Like if you're overusing, you're over, you know, defoliating, you're chronically defoliating, it's easy to imagine the ways in which that has negative downstream impacts on the ecosystem and soil carbon. With undergrazing, often I change the way I talk about this depending on who it is that I'm speaking to and what their kind of baseline, maybe biases or understanding of this kind of thing is. But there's a couple of different ways in which undergrazing can arise. In the paper, I think we talk about it in a couple of different ways, including over rest or under defoliation. And so you can think of it as like too few animals, so that can be kind of patchy over and undergrazing. Or you can think of it as over rest, not grazing nearly enough. Or in a situation like you were just mentioning with CRP, you might have, you know, conversion of a landscape into something that's meant to be like a grassland or a rangeland, but you've removed the defoliation factor that once created that ecosystem. And that's a whole other can of worms. But what I think undergrazing can and does do    Maybe I'll start with the patchiness. So for example, if you have a continuous stocking operation, a continuous set stocking operation where you've got maybe just a few animals on a really large piece of land, what will happen is they will selectively defoliate. And so the kind of less palatable species, they might eat not at all or less frequently. And so those plants will continue to grow without any defoliation and they will senesce, lignify, turn that kind of crusty brown. And those plants tend to contribute significantly less to soil carbon formation because they are contained, you know, chock full of really structural plant components that tend to be really tough to break down in the soil. And so that can happen just by patchy underuse. In a scenario where, for example, you might be over resting    so say you've got inadequate stocking density or stocking rate, depending on your system, but you're not defoliating often enough, that same thing will happen but to a higher proportion of plants in a pasture. And so maybe you've got senescence and lignification across the entire pasture, in which case you've kind of arrested that pasture's ability to be sequestering carbon because those plants are no longer photosynthesizing. And this happens naturally, seasonally. But it can also happen, I wouldn't necessarily say artificially, but we can drive an ecosystem to that just by under utilizing in terms of defoliation. And so that is another option. In more humid or maybe temperate ecosystems, there's even another kind of option that can happen with undergrazing, which is that if you're not removing some of that leaf area from plants in a pasture when they'll just continue to grow, especially, for example, in the US Southeast where you've got an extended grazing season, you can get what we call canopy closure. And so the plants will just grow and grow and grow until the kind of smaller plants that maybe have a slower growth rate are unable to receive sunlight and photosynthesize. And so you might get some really tall plants that have essentially shaded out the ability of other plants to grow, and that can have some other downstream effects like reducing the plant diversity, or it can even reduce productivity by way of shading out those plants. And so it might look really healthy because the plants are tall and maybe green if you're there in the right season, but in reality, it's just    those really fast growing plants have outcompeted other plants and you could have minimized that and actually increased productivity by even some minor defoliation. And so what we tend to think of as happening in all of those cases is that it can't    well, what I might add here is that rest is an important component of any grazing system. And so I don't want to, you know, say all of this and people take away that we shouldn't be resting or anything like that. Rest can and is very important. But in all of these cases where it's underutilized or under rested, you'll arrest your ability to accumulate soil carbon in any environment by way of all of these ecophysiology elements or an interaction among them. And so you'll wind up with an ecosystem that probably has more soil carbon and nitrogen than an overgrazed one, but it doesn't have nearly the amount that it could have if you were grazing optimally for that location.

>> Mm hmm. I like the term that you used for that with optimal grazing. We wrestle with how to describe these things, sustainable grazing, proper grazing. You know, we come up with all kinds of different terms, but I kind of like this way that it's sort of balanced between undergrazing and overgrazing, where in order to harvest anything, you've got to give it a chance to grow first. But then once it's grown, it actually benefits the plant community for it to be harvested in some way. And I have seen locations, plant communities, like you just described in a more mesic environment, you know, either a sub irrigated situation in a low precipitation environment or a higher precipitation environment where it just naturally grows a lot of stuff. You know, situations where the grass grows up, gets big, falls over, that happens for a few years, and then you really do have a large thatch layer that actually kills out the grass. And those are often places where weeds come in. So now you've reduced the active photosynthetic leaf area because there's so much dead plant tissue laying around that all of that's interrupted.

>> Absolutely. It's essentially like suffocating everything that could be coming up underneath it. And you see that all the time, even in less mesic areas. But for example, in the Central Coast of California, which is a kind of uniquely Mediterranean climate, I see that all the time    especially in areas that tend to be, you know, have historical overgrazing or, for example, in the case that you mentioned, CRP, or even sometimes tend to be fire prone. I think a knee jerk response we see from government institutions, or NGOs, or kind of public trust land ownerships is just to take grazing away and, you know, for all the reasons that we know to be problematic, right? Like grazing is bad, grazing is all these things, but in reality, just the removal of those animals completely has other downstream detrimental effects. And so that's why, I think, this idea of optimal grazing is an important one. And I think what we tried to do in the paper is to be a little bit agnostic to what optimal grazing plan should look like. What we didn't want to say is, oh, well, you know, deferred rest grazing is the way to go. The thing about optimal grazing is that it can look different in a bunch of different contexts. So it's really about the correct manipulation, adaptation, and application of those four grazing management levers in a way that's appropriate to any given ecosystem, which we know are mediated by things like soil texture and climate and precipitation, in a way that gives rise to benefits in each of those ecophysiology elements that have known pathways to soil carbon formation. And that can look different anywhere and even on a given place, it can look different throughout a year or throughout a season. And so we really wanted to kind of highlight that and I hope that came through in the paper.

>> I think so. I think it actually provides some basis for thinking through what does optimal grazing look like in a specific environment because optimal grazing will have these effects. And we sometimes even define sustainable grazing backwards in that way, not by defining the grazing pattern, but by saying, well, if it is sustainable grazing, then it will have these effects. And so we define it by the results. And you've said, here are the results. I also think it provides a useful middle ground socially by describing this optimal grazing because the rancher may, even if they feel like they understand these things pretty well, they still may lean toward overgrazing because in their mind, you know, maybe mild overgrazing has the effects that you're describing with optimal grazing. But then on the other side of the spectrum, people who like seeing lots of residual dry matter out there and assume that that means it's healthy can be pushed back toward the middle by recognizing that that does reduce, you know, the functional photosynthetic area, and that's actually limiting soil carbon formation and storage, and you can bring both to the middle.

>> Mm hmm. Yeah, absolutely. And now might be a good time to also mention that the point of this paper was to synthesize the ways in which grazing can lead to soil carbon formation. Now, there are other ecosystem goals that any given producer or land manager might have. So for example, if it's very important to you to maximize habitat for an endangered species, then maybe your optimal grazing looks very different than the optimal grazing scenario would be to accumulate soil carbon, for example. And that could be true anywhere. But I think generally speaking, the ways in which optimal grazing leads to soil carbon formation can be beneficial to the ecosystem at large. But I just did want to highlight that there could be other ecosystem goals that might drive management that's not optimal for soil carbon, and that is equally as valid.

>> Yeah, the summary statement in the paper that I highlighted, because I thought it was good, was that undergrazed ecosystems accumulate more biomass in the short term, but ultimately result in less productivity, less root biomass, less diversity, and lower quality soil inputs because those specific plant community conditions are limiting all of the three pathways of soil organic matter accumulation.

>> Yeah, absolutely.

>> And to your point, that's going to look different on    improved pasture in Alabama is going to be different than a mountain meadow at 11,000 feet in Colorado is going to be different than salt desert shrub in Nevada.

>> Yeah, absolutely. And I also think it's important to note that producers who are toying around with what optimal grazing management might look like in their context, there will always be some trial and error and some failure inherent in that learning and what it takes to be an optimal grazer. And I think what I don't want to do is to say that any overgrazing event will result in an ecosystem structure that looks like this. It's more about the repeated interaction of grazing with that environmental soil carbon that will arise in these kind of patterns that we lay out. And speaking regionally, there are places that are more susceptible to having long term negative effects of a single overgrazing event, especially in arid and semi arid regions. But there is trial and error embedded in each of these things. And really, it's about that long term application of an optimal grazing strategy that will have probably bouts of under and overgrazing that    I just don't want to kind of discourage anybody from trying and getting it wrong if in the end they get it right.

>> Yeah, that's good. Well, let's talk about optimal grazing. But maybe before we get there, at the risk of getting into too much technical language, I actually think that this is really useful. Your description in the paper of how net primary productivity is a function of the interception of photosynthetically active radiation and the conversion of that absorbed photosynthetically active radiation into new plant tissues is actually really useful because I think it gets at some of this difference between undergrazing and optimal grazing, which can be pretty subtle, but which may have pretty big differences for soil carbon stocks. And I think this is something that people can conceptualize, and it gets at, I think, one of the conflicts in, I guess, the conversation about what to do with undergrazed systems, namely that    you know, you have the rancher who says, well, those    have a bunch of senesce plants and dead centers and ineffective plant tissue and too much litter and the person who thinks that that landscape looks pretty good, and it does. Because you've got oftentimes tall plants, robust appearing individuals; it looks better than an overgrazed situation. And so if you don't have much trust that moving toward something that would look like optimal grazing is going to be a safe bet compared to undergrazing or no grazing, that description of dead centers, senescent plants, they're all decadent. It's not good. Sounds like it's not scientifically valid. And I think this idea of the ability of the plant community structure to intercept photosynthetically active radiation is a really good way of visualizing because you can sort of feel it. You can see it. And you can kind of get your brain around why some level of harvest followed by periods of regrowth is more optimal. So can you walk through that equation?

>> Yeah, absolutely. So this equation is basically the foundation of all five ecophysiology elements, maybe with the exception of the diversity one. So I think the idea here is that if you were to conceptualize what it takes to grow biomass in a plant    In a pasture or at the ranch scale, we often define that in terms of what we call NPP or net primary productivity, which is like total growth. And net primary productivity is a function of a couple of things. One is sunlight. So how much sunlight is there? And we define that in terms of photosynthetically active radiation. So sunlight is a basis of what plants need to conduct photosynthesis. So we call that PAR. It's a function of that times the conversion or how much of that PAR is actually absorbed. So it's the fraction of sunlight that's captured by the plant. And that is primarily a function of leaf area index. So if you think of sunlight as being the first barrier you need to meet    You need to have light to grow plants. You also need to have leaves on the plant for it to capture the light to photosynthesize. And the amount of sunlight that's captured often depends on the things that we were just talking about. So not only the presence of leaves, but also how much shading is there. So maybe if you've got a chronically undergrazed situation, then you might have very tall plants whose top leaves might be photosynthesizing just fine because they're at the top of the canopy; they've got all the sunlight in the world to capture. All of the leaves underneath that are going to suffer because they're being shaded out, as an example. And then the last piece of this equation is what we call radiation use efficiency. So it's how capable are those leaves at turning sunlight into biomass. And so this is where you start to differentiate in terms of plant litter and input quality. And so if you've got young, photosynthetically active leaves, so leaves that are green and are actively turning sunlight into biomass, that would be an example of a high radiation use efficiency, as opposed to a plant maybe that's on its way to senescing and to entering that kind of reproductive growth, then those leaves are much less photosynthetically active. And so while they might be available for capturing sunlight or the sunlight is touching them, they're not actively using that radiation to convert into biomass because they're using that energy for something else. And so that is kind of the three major components of NPP. And I think sets the stage for the baseline understanding of each of those ecophysiology elements.

>> Mm hmm. Yeah. That reminds me of this leaf area index and photosynthetically active radiation    There are actually only a couple of narrow bands of the visible spectrum that are active, which is why when you see deeper plant canopies, either a forest or a grassland ecosystem, as you mentioned, those lower leaves oftentimes, as it gets bigger, will die off. And it's because they're not receiving any active wavelengths that are effective for photosynthesis. So like on trees, they self thin at the lower edge of the canopy where the lower limbs just die and fall off and the canopy moves up as the tree grows. That's because all of the active radiation is being captured by those first canopy layers and there's nothing left by the time it gets down below. It's also why the understory, it doesn't usually have very much grass if it's a closed canopy forest. But it's similar in a grassland where you have    you know, for hay growers, they call it brown leaf    when the canopy gets big enough that the lower leaves then turn brown and die, not because they're not being supported by the roots of the plant and actively bringing up nutrients and water from the soil; it's because they're not receiving any radiation wavelengths that are useful for photosynthesis, and so they die and fall off.

>> Yeah, absolutely. And so then you can hypothesize all the ways in which a closed canopy or reducing the amount of leaves that are receiving sunlight    so essentially reducing that F par part of the equation can have downstream impacts, not even for plant growth, but even plant death. So you might begin to see plants that are just underneath that canopy start to die off. And so then you've got less plant inputs, less root inputs, less soil carbon, even though you've got some really big, tall plants. It's turning them into the kinds of carbon that once they hit the soil surface are not very efficient at forming soil carbon. And also, you've got less carbon entering the soil in total because you're not allowing for growth of all of the kinds of plants that could exist in that system. And so you can think of defoliation or grazing, in this scenario, as kind of the great equalizer of photosynthetically active radiation. So it's allowing more opportunities for sunlight capture by plants, which then is kind of a cascading effect on plant growth, input allocation, kind of canopy LAI, and then downstream soil carbon.

>> Yeah, that's fascinating. So let's walk through the optimal grazing and how that is pushing each of these levers. And one of the things that I'm interested here is the ways in which grazing animals are converting some of that material that would otherwise lay on the soil surface. I feel like I've got too many ideas and questions piled up in my brain to get them all out. But one of the things that I'm thinking about is that in much of the West where we have more dry    it's a low humidity environment. You've got dry conditions to soil surface. You tend to have an accumulation of dead plant material that does not readily decompose. And so you end up with this litter layer that is beneficial in some ways because some litter is certainly helping to protect the soil, provide some insulation, you know, ameliorates temperature at the soil surface. But you get too much of that, and now you have all of this plant material that instead of being converted into something that's useful like soil organic matter, it's just lying on the surface. Is that an accurate description of what's happening there in terms of that dead plant material that seems to not decompose in an undergrazed scenario?

>> It certainly could be depending on your ecosystem. But I think even in a case where you've got senesced plants or a thatch layer, or just plants that are not growing anymore, so that could not even be an undergraze scenario, but just late in the grazing season    what will happen is animals will defoliate, and through the process of enteric fermentation, what comes out at the back end in the form of manure tends to be much higher quality input to the soil in terms of forming soil carbon than the kind of senesced, lignified plant litter itself. And so there's a lot of ways in which optimal grazing can kind of fast forward the process of accumulating soil carbon. But one thing that I'm very interested in, and actually there's not been a lot of studies done on, is this kind of transformation of very lignified plant litter material into a very high quality soil input in the form of manure. And I think that's one of those ways in which that final pathway of soil carbon formation, where we talk about improving kind of internal transformations, that is kind of a shining example of that because    it might seem counterintuitive that you're taking off some of the carbon that's sitting on the soil surface in terms of plant litter, and that seems like it would throw off your mass balance equation where    okay. If you've got less total carbon on the soil surface, then there's no way for it to increase soil carbon formation on the back end. But in fact, when it comes to soil carbon formation, the ease with which microbes can break it down and utilize it has a big role in how much is actually formed. And so you can think of it in terms of like carbon use efficiency. And the carbon use efficiency of manure as a soil input is significantly higher than a dead plant, for example. And so you might actually wind up increasing soil carbon accumulation and even into that mineral associated, organic matter form, which is, we tend to think of as more highly persistent because microbes are more able to use it, and at that point is when it becomes kind of sorbed onto mineral surfaces and stabilized in the soil.

>> Mm hmm. That below ground transformation is occurring at a higher rate or happening at all with manure.

>> Yeah, absolutely. And some of those mechanisms, we hypothesize here, but I might just add a little note to another research project that I have going on where I really want to be able to answer this question    because most of what we've come to understand about the dynamics of manure on soil carbon has happened on manure application on tamed pastures or on croplands, which just function differently in terms of their soil carbon and nitrogen dynamics. And so I've got a grant that I'm waiting to hear back on where we'll actually get at some of this in terms of manure deposition on grazing lands and its mechanisms to accumulate soil carbon.

>> Yeah. On range lands    I would play the devil's advocate for just a moment, what if    oftentimes it seems like the manure just sits on top of the ground? Is that still more useful than if it's dead plant material also sitting on top of the ground? How do we know?

>> Well, I can imagine maybe both.

>> Yeah.

>> I think what    and if I'm translating this directly, it would be something like, when you see a manure patty that looks like a husk of itself, but it still kind of maintains all of its structure, but is not the color of manure anymore, but kind of like that light brown just frisbee. So what will happen in that case, especially in an arid or semi arid environment is all of the yummy, high nitrogen portion of that manure will leach out into the soil. And so that could be a significant contributor to what we call dissolved organic matter. And so in a rain event, it'll kind of push some of that really soluble carbon and nitrogen through the manure patty and down into the soil. And that actually turns out to be a really powerful mechanism of that mound formation, too. And so I can imagine, yes, that that actually still would be more beneficial than plant litter because   

>> The input quality is higher, no matter what.

>> The input quality is higher, yeah, absolutely. But at the same time, that depends a lot on your system. So if you don't have much mineral surface, for example, in your soil and you're highly sandy, then maybe that leachate of dissolved organic matter that's flowing from the manure patty into the ground, maybe there's nowhere for it to stick around, for example. So I can imagine a lot of context dependency in terms of that being beneficial.

>> And in the graphic, the input quality is one of the features, one of the ecophysiology elements that's the most different between optimal grazing and undergrazing. With undergrazing, the input quality remains very low because of all the things we've just discussed. You don't have much    there's not much recycling of the plant material. It grows and then dies and remains on the soil surface. And what remains on the soil surface is mostly structural carbohydrates and not cell contents that are more easily broken down. Where in optimal grazing, you have most of the plant material. One, you're growing a lot of plant material and then using a grazing pattern that is an optimal harvest cycle that is converting that into a higher input, a higher soil input that results in higher soil carbon. Am I summarizing that correctly?

>> Yeah. I think that's absolutely right. If you were to think of this kind of input quality to be something like eating a piece of pizza for weight gain versus kind of chomping through the toughest beef jerky you've ever eaten, that structural component of plants, so for example, that kind of very lignified, brown, cellulosic material, would be akin to that really tough beef jerky to microbes. It's very, very, very hard for them to use and break down.

>> Or iceberg lettuce that dried out.

>> That too. Yeah, that could work. Piece of cardboard. And that's why you tend to see once plants start to dry out, there's not a lot more decomposition that happens easily, especially in arid and semi arid systems. On the other hand, green plant material, or plants that are just alive and photosynthetically active in general tend to have younger leaves and plant components that are higher in nitrogen. So they've got a lot more soluble components to them, simple sugars, carbohydrates. They're contributing to rhizodeposition, so their roots are alive and exchanging things with the soil environment. All of those things are significantly more efficient at forming carbon. The other thing is, you know, all plants will eventually senesce. But one thing that I think there is, I think, some convincing evidence for is that if you were to use grazing or defoliation in an optimal manner, there is some evidence to suggest that you might also defer plant senescence, and so you might have greater input quality in general, but also you might have it for longer. And so maybe the roots stay alive for just a smidge longer, or you have those plants are so green and photosynthetically active for a smidge longer. And we also have higher nitrogen. And so it's not only more carbon accumulation for a longer period of time, but it's more efficient carbon accumulation because you have that higher nitrogen component as well.

>> And for a longer portion of the year.

>> Exactly.

>> In the graphic, and we'll probably need to wrap up here in a minute    We've nearly discussed it to satisfy my own curiosity anyway, and we're about to run out of time. But in the graphic on the very bottom, it will indicate that the soil carbon stocks, both particulate organic matter and the mineral associated organic matter are significantly larger in the optimal grazing scenario. And the difference is pretty    is quite significant. It's not a marginal difference. Yeah, describe that. I assume that's backed up by some measurements. I'm aware of a number of other efforts like the three M big research project that's trying to get at some of that as well right now. Everyone's trying to figure these things out. Can you describe the basis for that significant increase in total soil carbon stocks between optimal and undergrazing?

>> Sure. So if you were to think about those combinations of ecophysiology elements, how they affect those pathways of soil carbon accumulation, what we've synthesized and tried to illustrate here is that the opportunities for additive soil carbon in an optimal grazing scenario far outweigh the other grazing strategies. And so optimal grazing defoliation could open up some of that canopy and increase leaf area index, which will increase total productivity, which is increasing the total amount of carbon that's making its way into the soil alongside that nitrogen. So that's also, you know, increasing the below ground transformation. So all of these pathways start to overlap in ways that just increase the total amount of opportunity to add soil and hang on to it with optimal grazing.

>> All five levers are working in that direction.

>> Exactly. If you were to look in that bottom panel, you might notice some sizable differences in the pool sizes    so not only total carbon, so that's the total amount of carbon that you can measure in the soil, but also the allocation among these fractions that have very different implications for soil carbon, longevity and maybe ecosystem productivity. So for example, in an undergrazing scenario where you have a lot of biomass, but that biomass isn't particularly useful for that long term carbon formation, you might have a moderate total carbon stock, but maybe more of that organic carbon is sitting in that particulate organic matter pool. And that pool of soil carbon tends to kind of turnover on a more yearly to decadal timeframe. So it's more of like a functional pool that is ebbing and flowing in the soil, but it's not a persistent    not what we think of as a persistent form of organic matter. Whereas in an optimal scenario, you know, you might be encouraging below ground allocation, for example, in the way that you're grazing. So you're encouraging more root growth and also rhizodeposition. So roots tend to be very, very, very efficient at forming mineral associated organic matter that long term pool, but also as you go down throughout the soil profile    So for example, if you're encouraging root growth but also maybe you're shifting your plant community composition to include more perennials through your optimal grazing scenario, what will also happen is you're encouraging more root growth at depth, and at a certain depth, you just    your microbial community starts to decline just by way of nature. It's a little bit anoxic down there. It's tougher for microbes to survive. And so in the in the kind of surface layers of soil carbon, those roots might be very efficient at forming mound.

>> At depth, though, because you've got fewer microbes to transform those roots and rhizodeposits into mound, those roots will actually contribute to a significant portion of a low ground pond that is more stabilized just because it's deep. So you know, you'll have greater soil carbon in general, but you'll also have, I think, maybe a more desirable proportioning into these pond and mound pools with optimal grazing.

>> Yeah, that is a major difference, and then, I think, counterintuitive, because most people would tend to think that undergrazing, where you have lots of big plants would result in more below ground material. But it doesn't quite work that way because of this cycle of root growth, and also the microbes being able to do something with it.

>> Right, absolutely. And also just total amount of root biomass is, I think, where some of that starts to happen in undergrazing, whereas if you're if you're closing your canopy, you're shading out growth, and you've got fewer plants in general, and the really, really big ones might have a bunch of roots, but you've got less root biomass at the pasture level because you've got fewer total plants. And I think that's where the kind of root shift in optimal grazing starts to happen. And there was also some evidence to suggest that, you know, well managed grazing in general can increase rhizodeposition. So this is like the sloughing off of roots and/or the exchange of root exudates. That tends to be very difficult to measure. I think we highlight that a bit in the paper. So I will just add that we've got a lot more to learn in terms of root response to grazing, especially in response to these grazing levers. But generally speaking, I think we've captured the major components and the things that we can generally say about root response.

>> Mm mmm. I asked you by email about the effects of fire, and thinking through this again, just in this conversation is making me see why that's a little bit more complicated. In a long term undergrazed scenario, fire would likely dramatically increase the amount of photosynthetically active leaf tissue that's available to support plant growth, but you're also removing some of the above ground portion of carbon, not solar carbon, but the above ground plant biomass is being removed and smoked off. Is anything known about what happens to the roots? And I realize that probably depends a bit on the fire. Because, you know, you have some fire where it runs across quick, does not generate enough heat to kill perennial plants, and you'll often see perennial plant shoots coming back within a couple weeks of the fire. I mean, you have a really immediate response sometimes in situations where it feels like there's not even any soil moisture, and the plants come back anyway. But you also have sometimes the scenario where it kills a number of the plant individuals, both shrubs and perennial grasses, and that may have a long term negative effect as well. Any other thoughts on that?

>> Yeah, there's so many. Fire is so complicated, and I feel like I'm maybe not the most up to date on some of the recent stuff done with fire, but I can speak of it generally on the types of things that you might expect to see. But I will add that it depends a lot on the ecosystem, the precipitation regime, the severity of the fire, and whatever kind of seed bank is remnant in that soil. And so typically, range and grasslands that have evolved with some kind of fire, especially if you've got, like, for example, a plot of CRP ground that has been chronically undergrazed, and so you've just built up this giant accumulation of senesced plant material, something like a fire, could actually be really beneficial, because it's a really effective way of burning off all that stuff that's just going to shade out new plant growth. But in order for that to be really effective, depending on your ecosystem, you might need that to happen in tandem with, for example, some water. You need enough precipitation for those new seeds to bust open and think that it's worthwhile to contribute energy to new growth. But I think in the correct time and placement, fire can be a really important, maybe reset on an ecosystem that could help you speed along to an optimal grazing scenario. I will also add that one thing that's particularly interesting about grass and rangelands that have fire history is that fire can create what we call pyrogenic carbon, and so that's    especially particularly hot fires where you turn components that were once sitting above the soil surface into this pyrogenic compound, which is a really complex type of molecule that microbes cannot break down in the soil. And so it's essentially turning some of that carbon into like a biochar, which can be really fantastic for an ecosystem. It kind of locks down some of that carbon to stay around for a very long time. And so I think that that's a really cool thing that folks in my lab are beginning to measure. And we actually just are    We purchased this piece of equipment to help us measure pyrogenic carbon because it's not something you can measure on your kind of standard elemental analyzer, and so we're excited to build that capacity out.

>> That sounds good. Now I'm probably just thinking simplistically. Some of the recent discussion around grasslands versus forests for carbon storage had been that even though grasslands store less carbon per acre than a forest, the vast majority of forest carbon is above ground and therefore vulnerable to being removed by wildfire. And grasslands are not like that. Most of it's below ground, and that's the point.

>> Absolutely. And actually, just saw a really fantastic presentation on this, Dr. Mike Wilkins, who is at CSU, who is a soil microbiologist, and essentially he had results pointing to exactly that. And we've seen results like that crop up all over California, for example, which is an extremely fire susceptible kind of ecosystem. But you're exactly right. Forests tend to be really great at storing biomass carbon. So that's carbon that's stored in the bark and all of the things that make up a tree. In the case of a fire, if you have a fire come through a forest, depending on the severity of the fire, a lot of that carbon that was once kind of locked down into that biomass will be respired to the atmosphere just by way of having it burned. Soils are unique because    you're right per acre, or at least in some cases, per acre, there's less total carbon, but more of that carbon is held below ground. I think the last estimate I saw is 80% of the total carbon in a grass and rangeland ecosystem, on average, is underneath the soil surface, And that tends to be a lot more resilient to fire. You actually see very little soil carbon efflux in the case of a fire, again, depending on the severity. There are beautiful things about every ecosystem, and I'm not an anti forest person, but that is a really unique aspect that, I think, makes soil carbon particularly in grazing systems really worth considering because it tends to stick around for a long time and it's a pretty resilient thing.

>> I think that's a good last word. Paige, thanks again. Maybe the next time we talk you'll be    I can call you the queen of pyrogenic carbon.

>> No, no. that would be my boss, Francesca, not me. But I'm certainly interested in it, and it's one of those things that's a very visual indicator in soil, so it's fun to like    pull up a soil core out of the ground and you can see pyrogenic carbon in a way that other types of carbon aren't very visible. So I really like it, but it's necessarily my expertise but   

>> Well, I want to thank you again, and officially thank Erica, in public, for producing that graphic. I think this has been helpful to other people besides me. Thanks again.

>> Yeah. Thanks Tip.

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