Soil and The Carbon Cycle

Module Progress:

As you’ve seen, soil has cultural significance, can be classified in multiple ways and has both physical and chemical properties. Soil also has relationships with our broader atmospheric and natural cycles. This is particularly interesting and important to explore when we look at soil’s interaction with atmospheric carbon and the process of decomposition. In this module, you will begin to understand soil’s broader role in global ecosystems, as well as begin to think about how soil management choices in agriculture can impact the carbon cycle.

Film 1: The Carbon Cycle

The illustrations below show the carbon cycle. Through the carbon cycle, carbon moves from its abiotic form as carbon dioxide in the atmospheric reservoir to a biotic form in plant or animal biomass as complex carbohydrates. Carbon spends time in living or dead organic matter, or in humus in the soil, but it eventually returns back to the atmosphere as carbon dioxide again. Soil plays a key role in the carbon cycle- more carbon is stored in soil than in earth’s vegetation and atmosphere combined. In soils, you can find carbon in both organic carbon compounds and inorganic carbon compounds. In most soils, carbon exists predominately in the form of soil organic carbon (SOC).



Check out the image below of a soil profile, in which you can clearly see carbon-rich humus in the dark uppermost level of soil.


The Decomposition Process

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The carbon cycle is dependent on decomposition. In unmanaged ecosystems, dead plant and animal material is broken down into minerals and humus, which adds to the soil structure and is absorbed by plants. Carbon from the atmosphere enters the plant as CO₂ through photosynthesis. When the plant dies, decomposers break down the plant. When the plant dies, the debris is broken down by organisms in the soil. Some are broken down quickly, others are broken down slowly over time, and contribute to soil structure as they slowly decompose. Organisms utilize the carbon, and release CO₂ back into the atmosphere as a result of their decomposition actions.

Fresh residues consist of recently deceased micro-organisms, insects and earthworms, old plant roots, crop residues, and recently added manures. Crop residues contain mainly complex carbon compounds originating from cell walls (cellulose, hemicellulose, etc.). Chains of carbon, with each carbon atom linked to other carbons, form the “backbone” of organic molecules. These carbon chains, with varying amounts of attached oxygen, H, N, P and S, are the basis for both simple sugars and amino acids and more complicated molecules of long carbon chains or rings. Depending on their chemical structure, decomposition is rapid (sugars, starches and proteins), slow (cellulose, fats, waxes and resins) or very slow (lignin).

Soil organic carbon (SOC) is the main constituent of soil organic matter (SOM). SOM is formed by the biological, chemical and physical decay of organic materials on the soil surface and below the ground. Basically, soil organic matter (SOM) is composed of anything that once lived, including:

  • organic bits and pieces of plant and animal remains in various stages of decomposition, sloughed off cells and tissues of soil organisms, and substances from plant roots and soil microbes.
  • living soil microbes: bacteria, fungi, archaea, nematodes and protozoa, as well as plant roots. If we weighed all of the organisms found in soil, soil microbes would comprise about 90-95% of that weight.
  • humus: a chemically stable type of organic matter composed of large, complex organic carbon compounds, minerals, and soil particles. Humus is resistant to further decomposition unless disturbed by a change in environmental conditions. If undisturbed, humus can store soil carbon for hundreds to thousands of years. This makes humus a very important carbon sink.
  • charcoal: incompletely burned plant material. Charcoal can remain in the soil for decades to centuries without decomposing.

Essential to the carbon cycle is decomposers, who process carbon into forms that are available for plants to utilize for growth, who are also involved with converting other essential nutrients into forms available for plants. As you can see from these graphs, carbon cycles have been changed by human processes, such as combustion.

Chart 1: Fluctuations in microbial biomass at different stages of crop development in conventional agriculture compared with systems with residue retention and high organic matter input

From Balota, 1996.

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Soil organic matter has been directly and positively related to soil fertility and agricultural productivity potential. Benefits of increasing or maintaining a high level of Soil Organic Matter/Carbon Sequestration include:

  • Reduced bulk density
  • Increased aggregate stability
  • Resistance to soil compaction
  • Enhanced fertility
  • Reduced nutrient leaching
  • Resistance to soil erosion
  • Increased biological activity
  • Reduction of greenhouse gases by soil C sequestration

Film 2: What is Carbon Sequestration?

Carbon and Global Climate Change

The global carbon cycle is in balance in natural ecosystems, but as a result of human activities, CO₂ is out of balance. Fossil fuels burning, in our homes, cars or in factories, releases CO₂ and other greenhouse gases into the atmosphere in amount that cannot be taken up by plants. Therefore, we have more CO₂ in the atmosphere than we have had in thousands of years.

Film 3: GAO: Description of the Global Carbon Cycle Changes Over Time