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New Discovery on the Mechanics of Keeping Carbon in the Soil and What It Means For Your Pastures

By   /  October 21, 2019  /  4 Comments

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Imagine you’re a carbon molecule floating in the atmosphere and your mission is to get from there into the soil and stay there for decades.

Your first step – slip into a plant through an open stoma.

Stomata are microscopic openings on the surfaces of plant leaves that allow for the easy passage of water vapor, carbon dioxide and oxygen. They are crucial to the function of leaves as photosynthesis requires plenty of carbon dioxide as well as the release of waste oxygen and excess water.

 

Inside the plant you go through your first transformation: photosynthesis. You’re combined with water (H20) and photons from sunlight to become glucose (C6H12O6). You’re now part of the body of the plant. From here, there are multiple routes to your destination, some that take much longer than others. You could become part of the body of a cow, or part of her manure. You might be part of a plant that gets trampled onto the soil, or you might be part of the roots that get sloughed off periodically underground.

Which ever route you take, you eventually end up in the soil as organic matter – a tasty meal for soil microbes. As they eat, they respire carbon back into the atmosphere as CO2. That means that if you’re going to accomplish your mission of staying in the soil, you have to avoid these hungry microbes.

How do you get away and become sequestered?

That’s the puzzle that scientists have been working on, and they’ve recently discovered how carbon molecules escape: through very tiny pore spaces in the soil.

A team of researchers led by Alexandra Kravchenko found that the pores in the range of 30-150 µm (about the size of 1 to 3 human hairs) can trap carbon molecules, making them inaccessible to the microorganisms that might otherwise consume them and send. Of course, the more of these tiny spaces there are, the more carbon is effectively sequestered in the soil. Knowing how to create those environments will help us sequester more carbon, improving soil fertility, improving forage production and wildlife habitat, and increasing resilience to droughts and floods.

To help us with this, over a nine-year period, Alexandra Kravchenko and her team studied five cropping systems: continuous corn, corn with cover crops, a switchgrass monoculture, a poplar system with trees and undergrowth, and native succession. In the end, only the two systems with high plant diversity, poplar and native succession, resulted in higher levels of total carbon.

“What we found in native prairie, probably because of all the interactions between the roots of diverse species, is that the entire soil matrix is covered with a network of pores. Thus, the distance between the locations where the carbon input occurs, and the mineral surfaces on which it can be protected is very short,” says Kravchenko. Having these readily available escape routes means that more carbon is sequestered for the long-term.

Kravchenko writes that the 30-150 µm pores are associated with the most active microorganisms that can respond rapidly to increased carbon inputs. When these pores are spread throughout the soil, as they were in the more diverse systems the team studied, the volume of the soil matrix receiving and protecting the products of microbial decomposition is greater as well, and the more soil carbon is accrued. So, while the switchgrass monoculture had the largest root mass and did create the small poor spaces necessary, there was an absence of the necessary volume of pore spaces. Once the layer next to the pore was saturated, most of the carbon was oxidized into CO2 and returned to the atmosphere.

Figure 5 from the paper: Microbial footprint defines the soil volume available for C protection. Schematic representation of the effect that the abundance of 30–150 μm pores has on the size of the spatial footprint of microorganisms residing in such pores in perennial switchgrass monoculture and biodiverse native vegetation systems

 

What this tells us is that simply increasing biomass, in the form of above ground residue or below ground roots, does not necessarily help us accumulate more carbon in the soil. We now know that, not only does the plant community help determine the soil microbial community, but by adding to and changing soil pore space, they help define where microorganisms can live and how well they can function. The larger the “footprint” of the microbial community the better it is for keeping carbon in the soil.

What can you do with this?

The lesson once again is that diversity is important. If you’re looking across your pasture and see one species, think about how you might add more. Some folks have found that all it takes is better grazing management to create an environment that helps a greater variety of plants to thrive and grow. If you’re considering seeding, talk to your supplier or with Natural Resources Conservation Service, Conservation District, or Extension staff in your area about what kind of mixes will work best for you. If you’re managing row crops, use a variety of cover crops. Avoid monocultures whenever possible.

To learn even more about what Kravchenko and her team learned, download and read her journal article published in Nature Communications.

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About the author

Publisher, Editor and Author

Kathy worked with the Bureau of Land Management for 12 years before founding Livestock for Landscapes in 2004. Her twelve years at the agency allowed her to pursue her goal of helping communities find ways to live profitably AND sustainably in their environment. She has been researching and working with livestock as a land management tool for over a decade. When she's not helping farmers, ranchers and land managers on-site, she writes articles, and books, and edits videos to help others turn their livestock into landscape managers.

4 Comments

  1. Bruce Howlett says:

    A recent article in Science found that for boreal forests in Sweden, much of the carbon recently sequestered in the soil had passed through mycorrhizal fungi. Thus, although some of the soil micro-organisms (especially bacteria) release carbon, other micro-organisms convert it to stable forms that persist in the soil. I would guess that even in grasslands these processes can sequester much more carbon and more stable carbon than can be trapped in pore spaces.

    • Kathy Voth says:

      There may have been some miscommunication here, maybe from my translation. Dr. Kravchenko isn’t saying that the pore spaces are where the carbon is stored. Those small spaces are simply the escape routes for carbon that then becomes stored long-term in the soil matrix. In the past, we weren’t sure how this was happening, and this research has provided insight into the mechanism that makes it possible. Her research goes well with the article you read. Sorry for any confusion I may have caused. But you can read the full paper which is linked in the article.

  2. Andrew McGuire says:

    It is interesting research, but I don’t think the results can be confidently attributed to the diversity of plants. The researchers only compared a few monocultures and not all the plants in the diverse systems were grown out as monocultures. Without growing all the species represented in the mixtures as monocultures and then determining their effect on soil pore sizes and C-storage, it is not possible to say that diversity is the cause of this effect. It could be that one or more species in the diverse mixes are providing the effects. The individual traits of individual species may be more important than the effect of diversity per se.

    • Kathy Voth says:

      Hi Andrew,
      I checked in with Dr. Kravchenko and here is what she shared:

      “Technically, he is correct. If it were just this one study where monoculture switchgrass is compared with a diverse plant community it would not be suitable to attribute the outcome of this study to just the effect of the plant diversity. But there is a wealth of literature that demonstrated that greater plant diversity is beneficial for soil carbon gains. So, in our case, greater biodiversity is safe to suggest as a possible explanation of the obtained results. We are not saying that we have discovered the benefits of plant diversity for soil carbon gains here! Those benefits are already very well known. What we found is one of the mechanisms by which these benefits are likely created.

      “Our study does bring up this important follow-up question – are there specific plant species that are particularly good for creating the soil pore architecture beneficial for promoting soil carbon gains or are all plant species good for it as long as they are growing in a diverse community? This was my first question when I first contemplated our findings and I am glad to see that this is the question appearing in minds of other people who read our work, like Mr. McGuire. Because to find the answer will require a lot of work of many research groups.

      “To answer this question, the suggestion he has about growing different species in monoculture and in mixtures makes a lot of sense – we can see what each species is doing and compare its performance when grown alone with being grown together with other plants. There is a lot of evidence that in many respects the plants can behave differently when grown alone as compared with when they have to compete with other species for resources. We just don’t know how it works for roots and subsequent carbon gains yet. Actually, this is what me and my group have been doing for the past few months – starting greenhouse studies with switchgrass and other plants from our native prairie restoration to see their individual and joint performances.

      “But even though such approach makes a lot of sense research-wise (that is as a tool to understand the underlying mechanisms), from a practical perspective it would not make sense to have a pasture or a vegetation restoration effort built only on monocultures of some individual species – diverse plant communities have so many benefits!”

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