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Understanding Soil Carbon Dynamics

By Caitlin Youngquist, Ph.D. 

Carbon is an often overlooked, but very important com­ponent of the soil. We know how to manage nitrogen, phos­phorus and potassium for maximum production, and that micronutrients play a critical role in crop yield and disease resistance. Deficiencies can usually be corrected relatively quickly through the addition of soil or foliar fertilizer ap­plications. While soil nutrient status can change quickly, changes in soil carbon status are generally much slower and effects are less obvious in the short term.

Soil is a living system and has both inherent and dynam­ic properties — land managers work within the constraints of the inherent properties to change the dynamic proper­ties. Changes in type and amount of soil carbon is one of our biggest opportunities for soil improvement.

Soil health can be defined as the capacity of a soil to function in the areas of biological productivity (i.e. plant growth and decomposition), environmental quality (i.e. water filtration and erosion resistance) and plant and ani­mal health. It is also one of the best indicators of long-term sustainability in land management. The primary unifying factor in all of these areas is soil carbon, the major compo­nent of soil organic matter. It is what gives healthy soil its dark brown color and rich, earthy smell.

Soil organic matter encompasses all organic components of the soil system. This includes living and dead plant and animal tissue, as well as excretions and soil microbes. Soil organic matter is typically a small percentage of the soil but has a very important role to play in soil health, disease suppression, drought resistance, water quality and quantity and long-term agricultural viability.

Example of soil carbon difference between two samples
These two soils came from neighboring fields separated only by a fence. This Wyoming ranch recently converted half of the irrigated hay ground into pastures and implemented a rotational grazing system. The soil on the right is from the field that remained in hay production. The soil on the left is from the field that was converted to well managed pasture. Note the change in color (soil carbon) and rooting depth after only one year.

The terms soil organic matter and soil carbon are often used interchangeably, and while one is a component of the other, they are not the same thing. Carbon is the primary component of SOM, accounting for approximately half of the molecular weight. Nitrogen, phosphorus, calcium, magnesium, iron, zinc and other plant nutrients make up the rest. While plants do not take up any significant amount of carbon from the soil (instead they get it from the air), or­ganic matter is the food and energy source for soil bacteria, fungi, worms and the rest of the soil food web.

When it comes to managing for soil health, it is actually the organic soil carbon that is of interest. Soil organic car­bon was once a part of a living organism and will be again someday. In contrast, soil inorganic carbon includes things like charcoal and calcium carbonate (lime) and does not provide the same benefits to soil health.

Soil microorganisms (nematodes, bacteria, fungi, etc.) rely on organic matter as a food and energy source. These microbes break down complex carbon-based molecules in crop residues and manure like cellulose, lignin, fat and protein into smaller components. As a result, nutrients are made available to plants, and carbon dioxide is released as a byproduct.

The bacteria responsible for the most rapid organic mat­ter decomposition are aerobic (require oxygen). Tillage in­troduces oxygen into the soil, stimulating microbial activity. This burst of microbial activity leads to increased rates of organic matter metabolism in the soil and subsequent loss of soil carbon as carbon dioxide. This is why tillage is a pri­mary factor in loss of soil carbon and declining soil health.

Plants cannot use the nitrogen or many of the other nu­trients in organic matter until the microbes break it down. The process of releasing nitrogen from organic matter is “mineralization.”

soil layers
Grass and legume roots sequester carbon and help increase soil organic matter levels. Note the dark color that surrounds the large alfalfa root. This is caused by root secretions (polysaccharides) and resulting microbial activity.

Bacteria in the soil are also responsible for the conver­sion of ammonia to nitrate, the preferred form of nitrogen by most plants. Both processes require oxygen and warm temperatures. This is why plant-available nitrogen may be limited in saturated or cold soils.

Active, Slow & Passive Pools of Soil Carbon

Looking a little closer at carbon in the soil, there are several different pools that serve different purposes. The active pool (also called labile carbon) is composed primar­ily of living organisms, crop residues and manures. It turns over in seasons to years as soil microbes break it down and convert it into more stable forms. This pool plays an important role in structural stability (resisting erosion) and as a food source for soil microbes. Because it is made up of primarily “fresh” materials, nutrient release from this pool is relatively rapid. Levels of active soil carbon change rela­tively quickly with tillage practices and cropping systems.

The passive pool of soil carbon turns over in hundreds to thousands of years. It is very stable and physically pro­tected from the activity of soil microbes because it is bound up in organic-clay complexes. This pool of organic carbon is the major contributor to cation exchange capacity (the ability of the soil to hold nutrients), and water-holding capacity. It is very slow to change and primarily lost through wind and water erosion of topsoil. Humus is part of this pool, which has been shown to promote root development and plant growth.

The slow pool of soil carbon is an intermediate pool that turns over in decades. This pool also provides food for soil microbes and is especially valuable for its slow release of nitro­gen and micronutrients. It provides some benefits of both the active and passive pools as well. Changes in till­age and cropping systems will also impact this pool but effects may take longer to manifest than in the rapid pool.

Think of the active active, slow and passive pools of soil carbon as a checking account, savings account and retirement plan. You can add to these “accounts” with cover crops, manure and compost, and by in­cluding soil-building crops in your rotation. You can minimize losses by reducing tillage, leaving crop residues in the field and protecting the soil from erosion.

So, what does this mean for land managers and stewards? There are many soil functions that are directly or indirectly affected by soil carbon.

  • Soil microbial activity — plant nutrient availability, degradation of pollutants and disease suppression.
  • Soil structure — water infiltra­tion, rooting depth, resistance to ero­sion and compaction, and oxygen availability for roots and microbes.
  • Water-holding capacity — drought resistance and water storage.
  • Crop quality and yield — dis­ease resistance, seed germination, root development, and plant growth.

Changes in soil carbon can be measured in the lab or in the field. The simplest method requires only a shovel while more advanced methods involve laboratory analysis. By dig­ging a small hole and taking note of the color, smell and structure of the soil you can tell a lot about soil carbon status.

A soil with more carbon will be darker in color, have a stronger earthy smell (humus) and better tilth. You may also notice more earthworms and deeper roots. Compare soil from a cultivated field to soil from a pas­ture, fencerow, or garden. Observing changes in these three basic character­istics (color, smell and structure) over time can tell you a lot about the effects of your current management on soil health and carbon status.

Laboratory soil tests will typically include soil organic matter (as a per­cent of soil by weight) along with N, P, K and micronutrients. Watching how this number changes over time can be very informative, especially if you are making any changes to crop­ping or tillage systems.

There are also several lab and field tests available for soil microbial bio­mass and activity. As they say, “if you can’t measure it, you can’t manage it.” As you manage soil N, P and K for maximum crop production, consider ways to manage C too. The long-term benefits will be well worth the invest­ment.

By Caitlin Youngquist, Ph.D. This article appeared in the December 2017 issue of Acres U.S.A. magazine.

Caitlin Youngquist is a University of Wyoming Extension Educator in northwest Wyoming. While she tries to answer any question that comes through the door, her area of expertise is soil man­agement and composting. You can find some of her other articles in her blog: drcaitlin.us, or email her at cyoungqu@uwyo.edu.