Soil Carbon [Carbon Pools, Carbon Farming and Quality Organic Matter]

Carbon is probably one of the most well-known elements today. There is a global emphasis on carbon dioxide emissions and carbon capture. In terms of agriculture, much of this focus has centered on capturing carbon by increasing soil organic matter. Organic matter encompasses all organic materials present in soil (living microorganisms and undecayed residues).

Roughly, 55-60% of soil organic matter is made up of carbon. Around 80% of that carbon is from dead, nutrient dense microbial bodies. However, we are now finding out that not all carbon in organic matter is the same. Broadly, the different carbon pools vary by shelf life.

The larger particles of organic matter (greater than 53 micrometers) are typically plant-derived. These particles make up the “active” soil carbon pool. The tinier pieces (less than 53 micrometers), roughly clay- and silt-sized, are allotted to the “stable” pool. They are protected from further breakdown by minerals.

This is why all soil carbon isn’t the same. The chemical properties vary depending on the source (plants or microbes), which influences how it behaves in the soil. Standard organic matter tests only look at total amounts, with no information provided on chemical or physical composition.

Why is looking at the composition of soil carbon important?

Ultimately, these different pools relate directly to organic matter quality. The “stable” pool is made up of microbial biomass being the nutrient dense portion that supports plant growth. Quantifying stable carbon pools can give you an indication of your long-term carbon stocks, carbon sequestration, and how much of the nutrient dense carbon you’re building up with time.

“ACTIVE” SOIL CARBON POOLS

If you’ve ever looked at testing your soil’s active carbon pool, you probably noticed that different tests may refer to it as assessing the active carbon pool. Which ones are the right pools to look at? Well, it depends on what you really want to find out. The main active pools we generally look at are:

1. Particulate organic matter (POM): the greater than 53 micrometer portion.

POM, or POM-C as it’s sometimes referred to, is the largest active carbon pool and contains (POXC, WEOC, and CO2). POM can make up around 20 to 25% of soil organic carbon and roughly 11 to 15% of soil organic matter (Table 1).

Because POM is active, it has a shorter shelf life than the “stable” pool, from around 10 to 100 years. This is very short on a geologic timescale. This is because it’s mostly made up of plant matter that soil microbes use for energy. So, it gets continually broken down and converted into more stable complexes.

2. Permanganate oxidizable carbon (POXC): the amount of carbon oxidized or eaten away by permanganate solution in roughly 10 minutes.

POXC is more active by comparison since it’s determined from what can be lightly oxidized, and WEOC is even more active since it’s simply pulled out with water. Mineralizable C or CO2 respiration is a pool but one that draws on either of these very active fractions, and one that is determined by microbes. According to a study by Awale et al. (2017), POXC, WEOC, and CO2 make up around 17% and lower of the POM fraction. Keep in mind that the CO2 used for this study was a 30-day incubation instead of 24-hours.

3. Water extractable organic carbon (WEOC): the carbon pulled out with water.

4. Mineralizable C, or CO2 respiration: gaseous carbon (CO2) respired by microbes in 24 hours.

Table 1. From Awale et al. (2017). Percentages of each active pool compared to total soil organic carbon (left table), and then compared to the POM fraction (right).

This is an important distinction because, broadly, the active pool is a food source for soil microbes but is also very susceptible to changes in management strategies, like tillage. Statistical comparisons between the tillage practices and the carbon tests showed that POXC and WEOC were the most useful indicators of changes from one management strategy to the other.

LONG-TERM “STABLE” SOIL CARBON

As mentioned previously, stable carbon is called “stable” because it’s effectively protected by minerals in the soil. It’s so effective, in fact, that this carbon can stay in the soil from 50 to 1,000 years. As a result, this stable carbon is also called “mineral associated organic matter”, or MAOM.

If the POM fraction is anything above 53 micrometers. Then, the MAOM fraction is everything less than 53 micrometers (silt and clay sized). MAOM is primarily composed of dead microbial components and smaller secondary compounds. These secondary compounds form as they breakdown larger pieces of carbon from the POM fraction. This is why MAOM has a much lower C:N ratio, much higher carbon density, and much higher nutrient content compared to POM.

Take aways:

• The active pools of soil organic carbon can be determined by physical (POM), chemical (POXC, WEOC), and microbial (CO2 respiration) means.
• If you’re going from till to no- or minimal-till management of your soil, these pools are a good indicator of how you’re transitioning from one system to the other.
• MAOM is microbially-derived while POM is plant-derived but are part of a system; POM is the energy source for microbes which increases MAOM, which in turn supports the production of plants and their inputs.
• Increasing your stable carbon (MAOM) promotes carbon-dense, long-term carbon stocks as well as enhancing your organic matter nutrient content and overall quality.

 INCREASING LONG-TERM STABLE SOIL CARBON

So, whether you’re looking to build up your carbon-dense, long-term carbon stocks or enrich your soil with high quality, nutrient-dense organic matter, the main approach you will need is promoting soil health. Microbes both prosper in and facilitate the development of a healthy soil. In turn, healthy soil increases organic matter quality, carbon density, and carbon stability.

However, microbes need some support to make this happen. Think of healthy soil as a brand new, well-oiled, and fueled woodchipper. As long as the chipper doesn’t overheat, its components don’t become clogged or rusted, the fuel doesn’t completely burn up. Or, if a large piece of carbon dense material doesn’t jam it altogether, it can keep tearing apart most plant matter thrown into it.

Similarly, if the soil has optimal water, nutrients, warmth, texture, pore space, and carbon, microbes can effectively break apart almost all residues they encounter. Those residues go into making more microbes, producing more nutrients for plants and their residues, and decreasing the soil density making it optimal for more of the same to occur.

But, if faced with drought or heat, or too much carbon rich material comes in, such as wood or grasses, the microbes cannot function, and everything stops.

The main things to keep in mind when implementing soil health practices to support microbial function are:

1. Water: Water facilitates many chemical and biological reactions in soil, especially for the latter since it’s the medium microbes use to get around.
2. Plants and residues/biomass: cover crop diversity, including perennial grains, produces large rooting networks, which releases root exudates. Exudates are like sugar for microbes, and their activity is greatly enhanced.
3. Managed rotational grazing (if applicable)
4. Soil texture and minerals (sandy versus clay)
5. Soil pH: too acidic or too alkali and both the microbial and plant production are highly impaired.

Figure 1. Diversity above-ground promotes diversity-below ground. Exudates from roots are the main drivers to coerce microbes into populating the root zone. The larger the root network, the more the exudates are produced.   

Conclusions

In summary, soil health isn’t so much facilitating a process as it is supporting the function of a system that feeds back into itself. Testing for either active or stable components is extremely important in order to determine your progress. But, remember that no one component can tell you everything. Everything in the system is a gear in a larger machine.

However, supporting this system pays off for everyone. Not only will the nutrient density of your organic matter increase and fertilizer inputs decrease, but water holding capacity, porosity, and bulk density are all greatly improved. On top of that, long-term carbon is continually tucked away. This could result in a financial return if you’re actively participating in the carbon market.

The best part of knowing what to test for and how to interpret the results is that a grower has multiple ways to benefit from promoting soil health, from limiting your inputs to enhancing quality carbon stocks, which will ultimately increase your bottom line.