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Soil Science- From the Textbook to the Practical. Lesson 4 Carbon in the Soil

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This article comes from the NOFA/Massachusetts 2019 April Issue Newsletter

By Noah Courser-Kellerman and Julie Rawson

Noah: I think it’s worth framing this conversation with the fact that all life, in the soil or otherwise, is carbon based. Often, especially among those who care about the environment, carbon is only talked about in regards to greenhouse gasses and climate change. It’s worth appreciating that carbon is the element that defines life on this planet, and that its role in how our climate works is only one part of a much broader biogeochemical cycle.

Julie: What is the importance of soil organic matter (SOM) to agricultural soils - water infiltration, aggregate stability, nutrient retention, etc. 

Noah: Just to clarify, the word “organic” as it is used in the term “Soil organic matter” refers to its composition primarily of carbon, oxygen and hydrogen. It doesn’t mean “Organic” as in USDA Certified Organic. I’ve always thought it was funny that the people who develop pesticides are organic chemists.

I have always thought of SOM as the living part of the soil – the meat of the soil. People have long recognized- long before soil science existed- that soil that is dark and spongy tends to grow better plants. That component of the soil is what originally was derived from living things- this is the organic component of soil. The term Soil Organic Matter is used to describe everything that is carbon based (but not living plant roots) in the soil. SOM is roughly 50% carbon by weight.

SOM is the only practical way to build CEC in a soil that is low in clay.

Soil with higher SOM tends to be more aggregated and crumblyWater retention is improved in a soil with higher organic matter.  For every additional % of SOM in the top 6” you can hold 20,000 gallons of water per acre. This figures out to a little less than a ½ gallon per square foot. If you are managing your soil as a biological system adequate water is critical for the microbes to feed your plants. I often think that when people use exclusively drip irrigation it often manifests as nutrient deficiency. If only the soil around the drip tape is wet, the plants will get enough water, but unless nutrients are delivered by fertigation, plants will get hungry because the soil around them is too dry to function. All this is to say that SOM helps even out water content fluctuations in the soil, which can result in more even supply of nutrients to the plant from the soil.

Beyond water retention, a soil with higher SOM tends to be more aggregated and crumbly and have more pore spaces and drains away excess water. It will infiltrate better after a big storm. NRCS has been doing demonstrations where they measure infiltration rates. Soils can infiltrate as poorly as 1/2 inch per hour or as well as up to 8”/hour. Some infiltration concerns are directly related to tillage and compaction, but this goes hand in hand with increased organic matter.

Sufficient or high organic matter makes the farmer’s life easier and provides for more profitable crops even with a wide variation of weather events.

Noah: One concept that is useful is the idea of “carbon pools” in the soil. A “pool” is category of a type of substance and there is flow between the different categories.  A useful way of thinking about carbon in the soil are pools of living, dead, and the very dead carbon. Soil is teaming with living things but only 2-4% of the carbon in the soil is incorporated in living organisms. This includes the bacteria, fungi, nematodes, protozoans, earthworms and other animals. Then a large pool, depending on the soil, are dead but not very decomposed – plant matter, dead roots, carbon based chemicals that are floating around, cell broth when a molecule breaks apart.

Julie: Why does that happen?

Noah: A nematode, for instance, dies and it bursts open and there is nematode juice floating around. All of the compounds that the nematode had in its body are now available for other animals, bacteria or fungi to eat. There are always variable percentages of the recently dead. If you plowed down a bunch of rye or a frost happened on sorghum Sudan grass, for example.  The size and turnover rate of this pool changes throughout the season. Another term for the “dead” carbon pool is “Labile carbon”, or “Active Carbon”. It is rapidly turned over C which is oxidized to CO2 or made into more stable forms or incorporated into growing plants. It might only be in this pool for a few hours or days before being incorporated into living tissue, oxidized to CO2 or changed into a more decay resistant form.

Active carbon is really important to the soil for several reasons. As it breaks down, it provides CO2 to the plants growing above. It feeds earthworms that make burrows that get air down deeper in the soil, and it releases nutrients like nitrogen that were previously locked up in the tissues of the living soil organisms.

The “Very dead” is the SOM that sticks around for years and years. Some has been measured to be thousands of years old. This is the mysterious portion. The central question around stable soil carbon is how is it resistant to microbes? How is this organic matter not quickly oxidized by soil organisms like the labile, fast turnover carbon pool?

For most of the last century, the main theory around the stable organic matter in the soil has centered around the idea of Humus and Humic Substances. The concept is that the stable, dark colored, crumbly stuff that make soil good and rich is a substance called humus. Humus, it was posited, was the residue left over after decomposition- as plant material decays, it eventually reaches a stage where it can’t be broken down further, but still hasn’t been completely oxidized. Finished compost is a good analogy.

To study humus, a strong alkaline solution of lye was leached through it. This dissolved much of the humus from the soil. When this leachate was neutralized to a pH normally present in soil, some of the humic substances leached from the soil sample precipitated out. These precipitated humic substances are known as humic acids. The light brown stuff that remained dissolved in the solution is known as fulvic acid.

When analyzed, it was found that humic acids are HUGE molecules comprised of bits and pieces stuck together resembling other types of organic compounds. A large number of phenolic rings are present. This led to the theory that humic acids arise from the breakdown of lignin in plant tissues by fungi. Lignin is a polyphenolic compound that stiffens plant cell walls and, along with cellulose, makes up the bulk of wood.

So the theory emerged that stable organic matter is made up of humic substances derived from the incomplete decomposition of woody plant material. The many organic acid functional groups on these compounds contribute to the CEC of organic matter.

This theory explaining the origin and composition of stable organic matter is still found in textbooks and taught in classrooms. However, some parts of it do not add up. For one, it takes more energy to make a humic acid molecule from its constituents that are released. Microbes in the soil need to gain energy from a reaction in able to “afford” it thermodynamically. Another thing that doesn’t add up is that if stable SOM is derived from lignin containing plant material, a conventional no-till cornfield should be the epitome of carbon sequestration- lots of high lignin plant material added to the soil each year at harvest. The opposite, in fact is true.

A new theory has emerged in recent years, as our ability to analyze organic matter in the soil without harsh chemical extractions has improved. This newer theory posits that stable organic matter is not a kind of super-molecule that is resistant to decay because of its size or composition, but is in fact “normal” organic compounds like fats, proteins and carbohydrates that form tight associations “stuck” to clays and metal oxides in the soil. They are resistant to decay because they are physically protected. Interestingly, further analysis has shown that most of the stable carbon in the soil most recently came from bacteria and fungi- not plants. This points to the importance of soil microbes and fungi beyond their role as decomposers in the soil.

If we still need a big, heroic molecule, we might find it in Glomalin. This compound is a kind of soil glue that is critical for holding soil crumbs or aggregates together. It was only discovered in 1996, but is one of the most important organic compounds in the soil. It is called a glycoprotein, because it is made of carbohydrates and proteins twisted together. It is produced by mycorrhizal fungi in the order Glomales. Glomalin, like other stable organic compounds, can last for decades in the soil.

From a practical point of view, the emerging view that stable carbon in the soil comes not from huge inputs of dead plant material but by fostering microbes that form symbioses with living plant roots has some real implications. Reducing tillage, eliminating the use of toxic synthetic chemicals, and optimizing plant nutrition to promote root exudates to feed the good microbes are all good ways to build stable organic matter in the soil.

I think it’s really interesting how these competing theories mirror the evolution of soil science as a discipline. The Humic theory began when it was assumed that soil is a mass of broken up minerals- studied by geologists- in which plant foods and other compounds- studied by the chemists- are found, so that plants- studied by the botanists and agronomists could grow. Because the only way to study the chemical nature of a substance at the time was to chemically extract it, the problem of defining stable organic matter became a chemistry question. Because it was the soil chemists studying the problem, it didn’t occur to them (and they were lacking the technology) to study how the mineral and biological components of the soil might be part of the puzzle. If the only tool you have is a hammer- chemistry, in this case, every problem starts to look like a nail.

But soil- like all of nature is complex and a real study of it needs to be interdisciplinary. This, to me, is the most fascinating thing about soil science.

To learn more from Noah, check out his Soil Science for Gardeners event on April 13, 2019, and keep an eye on the NOFA/Mass Events page for on-farm soil health and soil carbon sequestration workshops in July and September with Gaining Ground Farm, Red Shirt Farm, and Many Hands Organic Farm".

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