Measuring and building soil carbon
University of Minnesota Nutrient Management Podcast Episode: “Measuring and building soil carbon: Challenges and solutions for farmers and researchers”
July 2023
Written transcripts are generated using a combination of speech recognition software and human transcribers, and may contain errors. Please check the corresponding audio before referencing content in print.
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Jack Wilcox:
Welcome back to the University of Minnesota Extension's Nutrient Management Podcast. I'm your host, Jack Wilcox Communications Generalist here at U of M Extension. In this episode, we're talking about soil carbon measurements. We have three panelists here with us today. Can you each give us a quick introduction?
Anna Cates:
Hi, I am Anna Cates. I'm the Extension Soil Health Specialist for the University of Minnesota.
Jessica Gutknecht:
Hi, I'm Jess Gutknecht. I'm a faculty in the Department of Soil, Water and Climate at the University of Minnesota.
Gregg Sanford:
And I'm Gregg Sanford. I'm a Research Scientist in the Department of Plant and Agra Ecosystem Sciences at the University of Wisconsin, Madison.
Jack Wilcox:
Why is carbon in the soil being considered as a credit in the global carbon cycle?
Jessica Gutknecht:
The idea behind considering carbon as a credit is the idea that if we can kind of grow more plants or kind of shift the carbon balance, that we're pulling in more carbon than we're releasing, which is what a lot of farmers are doing with regenerative and continuous living cover practices. That we can pull a little bit of the extra C02 from the atmosphere that we have kind of abundantly emitted as a society. And so the credit part of this is that we should be paying farmers for their good work to do this.
Gregg Sanford:
I think a key piece of Jessica's comment is that all of the carbon in the plants around us, crops, trees, grasses, et cetera, is built through photosynthesis. So it starts off as carbon dioxide in the atmosphere. As those plants build their above ground and below ground structures, their roots and their leaves and their stems, et cetera, that carbon is transformed into a physical carbon that we can then move around through agricultural management. And the idea would be that Google or General Mills or another company might have areas in their supply chain where they can't necessarily go carbon-neutral, but maybe they could pay a farmer who is able to take that carbon out of the atmosphere and lock it into their soils over some long period of time. And that carbon that the farmer is locking into their soils can be traded on a market financially. The farmer's essentially being paid for the service they're doing for another company.
Jessica Gutknecht:
And it's interesting that you brought up the example of a food company because you could argue that that company, if they're kind of targeting the credits toward products that they're sourcing, they can kind of count that toward lowering their company's emissions. And I think different companies tackle this in different ways. They look at it as an indirect offset, or they're looking at it as part of their own sourcing of materials or food or goods.
Gregg Sanford:
Sure and actually Organic Valley, I know uses the language of in setting a lot rather than trading credits, they're cooperative, they're farmers. The practices that they're doing that have the potential to sequester carbon or build carbon in the soils can then be incent into the milk product or cheese product and lower the carbon footprint of that product.
Jack Wilcox:
What practices change soil carbon?
Gregg Sanford:
Anytime we monkey, if you will, with the soil system, we have the potential to shift or change carbon. There's a lot of what we call natural climate solutions that we think could improve our carbon resources in the soil. And they all tend to kind of coalesce around reducing the amount of tillage or soil disturbance, increasing the amount of living cover, whether that's through cover cropping or growing perennials, reintegration of crop and livestock systems. So we have a lot of systems in our part of the world where crops are grown in the absence of livestock.
And so there's several kinds of nutrient management pieces to that, that actually impact the greenhouse gas footprint and the soil carbon scheme. And then there's a lot of discussion also about diversification, just that different plants fill different ecological niches. And if we can have niche complementarity and from an agricultural system, one way to think about that is this common coupling of grain crops with forage crops or grain crops with legumes. That grain crops produce a lot of biomass. They produce a lot of energy. Legumes produce a lot of proteins, so they give things back to the farmers. But from a biogeochemical standpoint, things like corn can produce a lot of above ground carbon, but they need nitrogen. Whereas things like alfalfa can produce nitrogen through fixation and then put a lot of their carbon below ground. So there's ways that plants working together can also improve soil carbon resources theoretically anyways.
Anna Cates:
I was going to point out that a lot of the practices you bring up are mimicking the practices that built soil carbon in the first place, which is mixed perennial polycultures, like grassland, forest soils also built a lot of carbon over time, although less than grassland soils. And so some of the most promising things for changing the carbon content of soil is actually to move back to those systems or to restore wetlands. They actually have a really high potential carbon increase relative to changes in annual cropping systems. Because unfortunately, we didn't get carbon in the soils in the first place by rotating corn and soybeans and feeding it to animals. So when we mimic those practices that brought carbon into the soils in the first place, we tend to see more success in carbon increases but it's always slow.
Gregg Sanford:
To get to Anna's point, we often talk about ecological intensification or this idea of bringing those ecological or historical landscapes or biomes, the functionality of those, back into our agricultural systems as much as possible. Trying to mimic the prairies if we can, or in some ways mimicking even forests with agroforestry or silvopasture, those types of things.
Jessica Gutknecht:
And when we're thinking about how we do this in the future, some of the key features of those native prairies where there was continuous cover, they were perennial, a lot of them were deeply rooted. And so when we look at perennial species now, diverse prairies or switchgrass or some other kind of longstanding perennials that are now on landscapes agriculturally. The depth of rooting and some of those things are starting to influence carbon.
Jack Wilcox:
What makes soil carbon complicated to measure?
Jessica Gutknecht:
I just spoke to one of those things, all of these different factors. Is it the amount of the plant growing? Is it the depth of roots? Is it the diversity of species? Greg actually alluded to this earlier too when we were first describing carbon as a credit. It starts with photosynthesis and carbon moving into a plant and how does it move through the plant and into the soil system? Every species might have slightly different ways of doing this. Is that plant respiring, re-breathing, the CO2 back into the atmosphere quickly through its roots? Some species do that. Some species are good at fueling microbes that'll recycle and build the soil carbon and make it more permanent. Some soils and some landscape positions are better at holding carbon than others. So there are just so many facets to how plants grow and how they cycle carbon and the soil structure and soil biology that's processing the carbon that we want to keep in the soil.
These are such diverse, intricate systems that getting a handle on it is really tricky. And it even goes beyond that. There are some threads of evidence that these decades of disturbing soil through our current agricultural practices actually might make it harder to start storing carbon again. Have we altered these soils to the extent that they don't even store carbon to the same degree? Another kind of existential threat to understanding this is that there's some evidence that the best we can do is tread water because global warming might mean that we're kind of naturally losing carbon. I saw a big synthesis paper on this a couple of years ago that our whole system is changing in the context of trying to understand how carbon is stored.
Gregg Sanford:
Just basic complexity from an analytical standpoint to assessing carbon. So kind of the gold standard method that we have right now for estimating soil organic carbon is dry combustion of soil samples. And that dry combustion estimates total carbon from a soil. And so one of the first things that we have to deal with is are we sure that all the carbon in the soil is organic carbon? That is, it came from plants and photosynthesis originally. Or do we have some carbonates in the soil? There's lots of soils that have carbonates in them, which are basically carbon in rock form that came from ancient seabeds or wherever the soils' parent material comes from. So first we have to sort that out. So that requires typically analysis of soils with and without an acid treatment to get whether or not it's organic carbon or total carbon.
And that total carbon has inorganics in it. So we have to pull those inorganics out. And then depending on the historical management of those soils or the kind of pre-agricultural biomes, we may have a lot of background carbon to deal with. So Anna talked about wetlands. Huge swaths of the Midwest used to be prairie potholes or wetlands, and they've been drained. Huge chunks of the Midwest used to be just tall grass prairie. Many of the soils around where I'm at in southern Wisconsin, where you're at in Minnesota. And so there's a lot of native carbon. And what that means is that if you couple that big pool of native carbon and then the variability that's inherent in the soils, you end up with a really huge amount of noise in the data. And so that big amount of noise makes tiny changes that we're making year to year.
We planted a cover crop. We didn't till. We spread some manure. It makes it really, really tricky to pick out a signal, if you will, or some sort of change amid that swirling noise in the data. Those are two things. But then, what Jessica was saying, a lot of our agricultural management begins to change the soil properties. And one big property that it changes is bulk density or how compact the soil is. Certain systems can make soils less compact, but other systems can make them more compact. And so if we're tracking carbon over time, not just it got inorganic carbon in it, and how big is the background pool? But then it is, have our systems changed the density of the soil? And if they have, we have to correct for that.
So there are several pieces that have to come together to get an accurate estimate of carbon and that's at one time point. To actually know whether or not anything that we're doing is building carbon. We have to then track that carbon over several years. Usually five would be a minimum depending on where you're starting but decadal measurements are typically kind of what we would really be pushing for.
Anna Cates:
I'll just add one summary comment to that, which is that when we talk about carbon in the soil, we tend to think of it as a mass, a static body of adder. But actually it's always transforming. It's changing from residue to microbial bodies to gaseous form to other forms of organic material. So that just makes it inherently complicated. We're taking a snapshot of something in the process of transformation.
Jessica Gutknecht:
And I think going back to some of the goals and the context that we're discussing in terms of credits and how do we understand carbon in order to give credits? That temporal aspect that Anna just spoke to the time course and even when we put carbon in the soil, is it even staying in the soil and how long is it staying in the soil for, is a whole other kind of series of questions.
Gregg Sanford:
That's why the term sequestration is so problematic because sequestration by definition suggests that you're taking something and locking it away. A monk in his cell or whatever it is, something's never coming out, and that simply isn't the way the soils work. They're constantly changing their dynamic. And then you have this whole overarching changing climate, which took away any potential equilibrium. So you've got a changing climate coupled with a dynamic soil system. Things get complicated really quickly.
Jessica Gutknecht:
I think the closest we get is in trees. A tree trunk is locking in carbon, but even that you could burn it or it could decompose if that tree dies.
Gregg Sanford:
There's actually a discussion of sequestering carbon by returning to using wood as a building material in cities, basically growing the carbon and then sticking it in buildings.
Jack Wilcox:
What do you think is the ideal or is there an ideal carbon measurement scheme? What would it look like?
Gregg Sanford:
I don't know if there is an ideal. I think that a good carbon measurement scheme is going to need to hybridize empirical measurements of carbon with landscape models. And just to clarify that, the carbon markets that are out there now are kind of a mixture of those. Some rely heavily on modeling tools. You basically plug in what it is you're doing or how you're changing what you're doing and it churns out some sort of carbon credit. There are other crediting groups that are using that modeling, but ground truthing it with an empirical actual estimate of carbon, some change in carbon. And I think that's the direction that things should be going. From the standpoint of how one would most accurately measure carbon, there's some key things that have to happen. We've got to have empirical baseline data for any type of measurement that we're taking, and that needs to include organic carbon and it needs to include the mass of soil. And that soil carbon needs to be tracked over time because everything that we've talked about now this afternoon has gotten to that point that things are dynamic and changing. We do need to make those corrections for changes in soil mass, but we also need to be sampling deep. And this gets a little bit to what Jessica is saying or has said, that most of the protocols stop at 30 centimeters. And the reason for that is because most of the action, if you will, is happening in those top 30 centimeters. That's where most of our cropping systems' rooting depths are. Most of the root biomass in that top 30 centimeters are tillage, and doesn't go deeper than that. And so that's the area of the soil that we can really manipulate. But the climate is changing and things are happening below that 30 centimeters. And there's a lot of data that has shown that if we ignore those deeper depths, we're not getting a full picture.
And so the empirical assessment piece I think is critical and it needs to be done pretty comprehensively, but there's no way that you can do that, these deep sampling over decades on every field that every farmer wants to enroll in a carbon credit. And so those empirical assessments need to be coupled with robust models that can then take data that's come out of specific sites in a region and extrapolate that to broader landscapes. The idea being then that if you paid a farmer X amount of dollars per ton for a carbon credit and they spent five years in a changed practice, you would go back after that five years and ground truth either with that empirical sampling or with your model, which has been calibrated regionally.
Jessica Gutknecht:
And to riff off of this a little bit, I think you mostly spoke to what I had in mind for the best scheme. Almost to go back to basics and take these soil forming factors into account. So you talked about time, and so the five soil forming factors are topography, parent material, climate, time, and biology like the plant or microbial community. And so time, climate, understanding different either temporal variation in climate or different ecosystems, geographical areas. The parent material, this got inorganic, organic, or what's the rest of the mineralogy that's underlying the carbon that we want to store? What's the matrix and infrastructure?
I think topography is an interesting one too because in different landscape positions like at the top of a hill or a bottom of a hill or a wetland, we've talked about all of those things. But the way that soil moves or changes or forms, then therefore the carbon within that can change too. And of course we've spent half our time talking about the biology in the plants. So I think a structure that hits all the soil forming factors in a scheme would be more informative. And that's really just the empirical measurements. There's a whole world that we haven't talked about a whole lot of in carbon modeling.
Gregg Sanford:
A huge part of what you're saying is going to have to be built into those models. There's going to have to be this iterative, empirical assessment model validation simply because those things are so complex and there's emergent properties to them.
Jack Wilcox:
What research are you excited about to address some of these challenges?
Anna Cates:
Well, a lot of it has to take place over the long term. So I'm excited about where we do have long-term cropping systems trials in place in both of our states and across the nation. I feel like those can offer us a lot. I'm excited about some work that I'm doing up in northwest Minnesota looking at the effect of installing drainage on organic carbon because I think the changing hydrology of the Midwest, both with artificial drainage and with changing climactic patterns could have a big impact on soil carbon. So I'm also really excited about research that looks at the effect of any kind of climatic patterns on carbon changes in the short and long term. Preferably over the long term for all the reasons we've discussed so far.
Gregg Sanford:
I concur 100% with Anna as somebody who manages one of the long term experiments here in the southern Wisconsin, North central states area. I think a lot of people when they set out to build these cropping systems experiments didn't envision them necessarily lasting 30, 40, 50 years. And the older they get now, the more valuable they become because they provide us with these long-term data sets on what's actually happening in the soil, which are truly invaluable. And if those experiments are set up well, they provide the opportunity to do some manipulative work.
So there's some work that Anna and I are actually working on where we're doing some manipulation of some of these long-term systems to see what the key drivers of carbon change. In our experience, we've seen loss of total carbon over the last 30 years, even in forage systems. And I think something that's a little bit terrifying that gets to a comment Jessica made earlier is that a lot of that loss is happening in really deep soils. So soils that are kind of beyond the reach of most of our agricultural management. So it feels like there's a climate signal happening there. And so I'm excited about research to try and figure out just where that carbon is coming from and if there's anything that we can do to stabilize those deep stocks or potentially even rebuild them.
And then another project that Anna and I are working on, which is exciting, is this coupling of the long-term experiments in Wisconsin and Minnesota and Iowa in a kind of scientist farmer collaborative that we're calling SOCNET, where we've got long-term monitoring sites and on-farm experiments set up in the three states. To track in real time over the next decade, kind of what happens when farmers make some of these ecological intensification changes, reducing tillage, adding cover crops, manure. It's a big project regionally, and it's a slow project because it's going to take a decade, but I think it's pretty critical for the questions that lie ahead.
Jessica Gutknecht:
Another couple of areas that I've been working on that I think are interesting and important are actually keeping a foot in the ecology world is one idea. I am taking part in a large long-term climate change experiment in a northern Minnesota boreal peatland and carbon there also is the big theme. We've been talking a lot about draining wetlands or draining peatlands. If you look at global emissions maps, northern Minnesota, southern Canada where we now because of climate change have a long enough growing season to actually grow something. We're draining peatlands to grow crops, and that could be leading to huge emission losses.
And so projects like this SPRUCE experiment in a southern boreal peatland where we're watching just how is a baseline in a natural ecosystem, how is warming leading to carbon loss, I think will be informative. Another area that I've focused a lot on is the more mechanistic tracing of carbon. So if we spike in labeled carbon and trace it through the system, can we start to understand where it's going with different practices or different situations? Can we understand where that carbon is going and how it's being stored versus re-respired out to the atmosphere? The pie in the sky would be mechanistic studies that are kind of wisely coupled with long-term monitoring.
Gregg Sanford:
I think that the work that both of you are doing, whether it's the tile drainage of inundated soils or the conversion of peatlands, I mean there's a lot of similarities in that. And that's, I think, huge, super valuable simply because those soils, whether they're inundated agricultural soils, inundated prairie lands, or these peatlands have been huge reservoirs of carbon for so long. The idea that all of a sudden they're becoming aerobic or being cultivated is going to be presumably a major loss of carbon.
Jessica Gutknecht:
There's this big potential for state changes too. One of the things that's happening at this SPRUCE experiment is that as the water table goes down, some of the shrubs are winning and this sphagnum moss is losing. And so you have these whole ecosystem state changes. What are the implications of carbon I think are really important to understand.
Jack Wilcox:
Is Minnesota uniquely positioned in any way to contribute to solving these issues?
Anna Cates:
So the focus on building carbon has really built awareness and interest and energy for practices that are good for other reasons. So as we think about whether Minnesota farmers can build soil carbon, a lot of our research suggests that we could be pretty skeptical of how much carbon they can build in annual grow crop systems really with or without tillage or with different tillage practices. But if they're looking at reduced till systems with cover crops, which certain markets want to pay them for because of the proposed carbon benefits, we could see benefits for nutrient sequestration on the land and in the plants instead of flowing into waterways.
And we could see benefits in terms of reduced flooding and we could see benefits in terms of better water holding capacity on farms. When we think about measuring soil carbon and what Minnesota farmers stand to gain from it, I think in a way what they really stand to gain is more resources poured into technical assistance for practices that could be beneficial for other reasons. And if we see a little boost in soil carbon and they're able to get a $5 an acre gravy payment for building soil carbon on top of perhaps a soil and water district payment for water quality benefits, then those could be a nice win-win.
Jessica Gutknecht:
Another thing we have to gain, it might sound a touch more symbolic, but I think it's important is that we're all talking about this more. One of the climate scientists likes to say that the most important thing we can do for climate change is to talk about climate change. And I think this is really, especially a lot of the trickiness and complexity and struggle with talking about carbon is leading a lot of us to articulate these processes better. But it's also leading to how many more, open, not political, conversations have I had with farmers about climate change. It's totally shifted the conversation. That we're all kind of talking about a lot of these big issues in a lot more holistic and a lot better ways than we used to. Which is going to lead everyone to think about how are we treating the land and what does it mean to be sustainable in the future. So I think that's actually a really exciting piece of this.
Anna Cates:
I agree, Jess, that's a great point. That we're building awareness of these practices and what's happening in the soil generally.
Gregg Sanford:
I don't have any grand ideas other than if farmers are interested in participating in research. We're about to enroll in year two SOCNET in Minnesota, so if you or any farmers you know are interested, reach out to Anna.
Jessica Gutknecht:
Another shameless plug that I would make is that a really useful tool for farmers has just come out. The Minnesota Farmers Union has just put out a guide to carbon markets and they did a tremendous amount of due diligence. They took contracts and had their lawyers look at them. And I've seen it recently and it's fantastic.
Jack Wilcox:
All right, that about does it for this episode of the Nutrient Management Podcast. We'd like to thank the Agricultural Fertilizer Research and Education Council or AFREC for supporting the podcast. Thanks for listening.
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