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Does Grazing Sequester Carbon? Part 2 – Some Background

By   /  October 9, 2017  /  4 Comments

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We’re working on a series of articles that explore what the research tells us about grazing and soil carbon sequestration. It’s a complex topic, and as with all things science, there is a lot of background information that may not be covered in the papers themselves. So, before we take a deep dive into the literature, we thought it would be helpful to give you some of the background. (Here’s part 1 in the series.)

Why is carbon important? 

Carbon is one of the primary building blocks of life on earth. It is found in everything, and can be a solid, a gas or a liquid depending on which other elements it combines with and where it’s located. In the atmosphere, we can find it combined with oxygen as carbon dioxide (CO2) or with hydrogen to create methane (CH4). In the soil it’s the bodies of soil microbes and plant litter.

What is “Carbon Sequestration?” 

For this series we’re using Rattan Lal’s definition:

“Carbon sequestration implies transferring atmospheric CO2 into long-lived pools and storing it securely so it is not immediately re-emitted.”

Not all soil organic carbon meets this definition.

Most soil organic carbon is very active. Found in plant tissues and the bodies of microorganisms in the soil, most of it cycles back to the atmosphere after only a few weeks or months.

Thanks to photosynthesis, soil organic carbon increases dramatically in the spring as plants green up, and increases even more when there’s good spring moisture. When plants go dormant in the winter, CO2 is respired back to the atmosphere. Seen from space, the change of seasons almost looks like the earth is taking a deep breath in spring, and then exhaling again as fall and winter come on.

Below ground, there’s a whole world of micro-organisms. They eat carbon, supply nutrients to the plants and to each other, and respire CO2 back into the atmosphere as a waste product. In the process of all the soil microbes eating plant tissues and each other, some of the soil carbon becomes trapped inside microscopic particles of minerals. Think of CO2 as the tasty center inside a chocolate truffle. Trapped particles can stay in the soil for years or even decades before returning to the atmosphere.  It’s this trapped carbon that meets the definition of carbon sequestration.

Can sequestered carbon get released into the atmosphere?

Carbon that is sequestered in coal or oil is released back to the atmosphere when we burn them for energy. Likewise, carbon that is trapped in microscopic particles in the soil can be released into the atmosphere when the soil is disturbed  – by plowing, for example – or by management practices that leave ground bare or vulnerable to erosion. More recently there are indications that certain practices may create a microbe feeding frenzy leading to microbes breaking into the particles where the carbon is stored. That’s a story for later in this series.

Is Soil Organic Matter the same as Soil Organic Carbon? 

No. Soil Organic Carbon (SOC) makes up about 50 to 58% of Soil Organic Matter (SOM). Increasing Soil Organic Matter may or may not lead to longer-term carbon sequestration.

How much soil organic carbon can I have?

Thanks to the folks at soilquality.org.au for this graphic.

This depends on your climate, rainfall, the soil microbial community, management, and many other variables. Under favorable conditions, soils will sequester carbon until they reach their saturation (whatever that might be for that particular soil) and no more is sequestered. Think of it as a paper towel wiping up spilled water. There’s a point at which your towel becomes saturated and it will dribble water all over the floor as you run to the sink. To increase sequestration beyond this point, you’ll have to change management.

Where does nitrogen fit in?

If your plants don’t have enough nitrogen they can’t grow and they can’t sequester carbon. Nitrogen fixation by legumes, mineral fertilizers, manure, or compost are all avenues for getting nitrogen to plants. Each has strengths and weaknesses, adding to the complexity of our management decisions.

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We hope this initial background will help make sense of some of the research we’re going to be sharing describing the interactions of grazing and soil carbon. We’ll be adding additional background as we go along. If you have questions that you’d like us to address, do let us know in the comments below or by sending us an email.

Finally, what’s On Pasture’s position on all this?

When we started On Pasture, we promised to bring you the best science – science that tells us what’s happening in the world around us so that we can make good decisions about how we manage grazing. That’s what this exploration is about.

We recognize that there is an invitation to engage in the debate between those who think grazing is the cause of all problems, and those that believe that grass fed livestock represents a solution. That’s not what we’re doing here. We’re simply reviewing the research and sharing its results.

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About the author

Rachel and Kathy are co-editors of On Pasture. They often collaborate on articles so that you get the best they have to offer.

4 Comments

  1. Gene Schriefer says:

    Wisconsin has a 28 year running cropping system trial, one of the treatments is managed grazing with dairy heifers. Compared with every other system it the most favorable for building soil organic matter.

    https://wicst.wisc.edu/

    • Kathy Voth says:

      Of course. When comparing pasture to cropping systems, as these folks are doing, it is normal for the pasture treatment soils to be better than cropped soils.

  2. DWK says:

    I liked the time lapsed video. However, it didn’t look like there was much of an impact from global warming. Did I miss something?

    • Rachel Gilker says:

      Nope. You didn’t miss a thing. We’re not covering climate change here. We’re just talking about carbon.

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