University of Vermont Extension 
Department of Plant and Soil Science

Anytime News Article

Dr. Leonard Perry, Extension Professor
University of Vermont
Did you know that by promoting a good soil, one high in organic matter, you are improving air quality?  This is done by reducing carbon dioxide, one of the main culprits implicated in global warming, through a process called “carbon sequestration”.
Carbon dioxide is added to the atmosphere in many ways.  Prior to the industrial revolution, the main source was breakdown of organic matter from soils.  Now it is mainly produced from burning fossil fuels.  Yet carbon dioxide is taken from the atmosphere by plants.  Although plants respire as we do, adding some carbon dioxide back to the atmosphere, the net result is a reduction if the plants end up as organic matter.  Although the largest amount of carbon is dissolved in oceans, the second largest amount is stored in soils.  These are often called carbon “sinks”.
The carbon in the carbon dioxide is tied up into stable forms, or “sequestered”, in soil organic matter which is 57 percent carbon.  This process begins by the plant, through the process of photosynthesis, turning carbon from carbon dioxide into various compounds for growth.  As plants decompose with the help of soil microorganisms, forming organic matter, the carbon is held in a short term manner.  At this early stage, if organic matter is disturbed or broken up, the carbon can be oxidized and released back to the atmosphere.  Otherwise, it can end up in longer term and more stable storage as in “humus”.  This is merely the dark brown, spongy, and most stable form of organic matter that can last for many years, even centuries.  You might think of the initial organic matter as a bank account for carbon, the humus as a long-term investment account.
Keeping these few facts in mind, it makes sense that the more plants you have, the more carbon dioxide will be taken from the air.  Dense biomass such as from cover crops in your garden, and dense landscape plantings, result in more carbon trapped.  Cover crops and mulches also prevent soil erosion, a main means of soil exposure and so loss of carbon.
Preventing deforestation, and planting trees, also are seen recommended to add plant mass and reduce carbon dioxide.  Two cautions are to only plant trees where they will aid the soil, and where they naturally occur.  In wetlands, grasses might be more appropriate and enrich soils more quickly.  Trees can absorb much water and nutrients from some soils, creating a poor habitat for soil microorganisms, resulting in less organic matter than if smaller plants with more leaf litter were grown.
Once carbon is trapped in plants, there is the need to turn it into organic matter in the soil, either through compost, mulches, or even leaving grass clippings on lawns or leaf litter in beds.  Otherwise, carbon can be oxidized and lost back to the air, not to mention the fossil fuels spent blowing leaves, and carting residue off to a landfill.  These fossil fuels of course add more carbon dioxide back to the air. 
Zero or minimum tillage of soil results in the least oxidation of the carbon, minimizing the amount lost back to the air.  Minimum tillage, as well as methods to reduce compaction,  results in less disturbance to the earthworms and soil microorganisms that actually take the plant debris and make it into organic matter and humus bound into soil.  Minimum or zero tillage has the added benefit of reducing use of fossil fuels if powered equipment is used.
Another aspect to be aware of is that these microorganisms need some nitrogen to convert plant debris to organic matter, roughly in a ratio of ten pounds carbon to one pound of nitrogen. 
So if insufficient fertility in soils, or even in a compost pile, less if any organic matter will be made.  This means little of the carbon will be held, but can return to the air as carbon dioxide. Legume crops such as clover, alfalfa, and soybeans can be used to help provide this nitrogen. Legumes and their associated bacteria take from the air or “fix” nearly 17 million tons of nitrogen a year, which is worth about $8 billion.
One scientist estimated that from a garden in Maine just under a half acre, with high organic matter (7 percent, which is about double the usual), that about 19 tons of carbon had been sequestered from the atmosphere over a ten year period.  With the average American releasing six or more tons a year, this one garden offset the emissions from an average American for a three year period.  On the larger scale, one estimate is that up to 20 percent of targeted reduction in carbon dioxide emissions could come from soil sequestration by agriculture.
Grow plenty of crops and plants, and recycle their debris back to your soils.  By building your soil organic matter and treating it properly,  you’ll be rewarded with better plant growth as well as knowing it is helping reduce carbon dioxide from the atmosphere.

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