The Capture and Storage of Carbon Dioxide in Soil

The Earth’s Soil Serves as a Carbon Storehouse

Analysis by S. Tom Bond, Retired Chemistry Professor and Resident Farmer, Lewis County, WV

An exciting new way to reduce atmospheric carbon dioxide by a natural process is being discussed in many places. See here, and see here, and see here, and see here, for example.

To understand it, some background is needed. As the reader will know, most of our energy in this age is derived from burning carbon containing compounds (fossil fuels) in the form of natural gas, oil and coal. They are burned with oxygen in air and carbon dioxide is formed. Less energy is derived from the hydrogen in these compounds atom for atom, and most of the compounds formed from the sulfur, phosphorus. and other elements present in fuels are pollutants.

The process of burning (combustion) oxidizes carbon and the other elements, but some of the carbon dioxide and water from the air are converted to plant life by photosynthesis. Decomposition of dead plants and animals exposed to the air also contributes carbon dioxide. The evidence indicates this has been going on for 2.8 billion years. Sometimes in geological periods past there has been greater or lesser amounts of carbon dioxide in the atmosphere than now. The return to plant life of oxidized carbon through photosynthesis is slow.

Much of plant life grows roots into the soil. When the plant dies its roots are converted to a soil carbon form known as humus to gardeners. This is done by fungus, (actually up to 1.5 million species are present in the soil worldwide according to a recent article in Science, the journal of the AAAS) and other microorganisms. As everyone knows, humus helps hold nutrients in the soil, and water. This helps new plants to grow. Scientists now understand soil based life is an association between plants and special microorganisms acting between soil and plant roots. This helps the plants to get nutrients from the soil and from the soil carbon compounds (which are very numerous and complex) in humus.

The excitement is that in many parts of the world soil carbon is depleted, but with proper management it can be rebuilt. It is a huge reservoir – one source gives 2.7 x 10¹⁸ long tons of carbon in soil compared to 0.78 long tons in the atmosphere and 0.75 long tons in biomass, i.e., living matter.

According to Rattan Lal, director of Ohio State University’s Carbon Management and Sequestration Center, the world’s cultivated soils have lost between 50 and 70 percent of their original carbon stock, much of which has oxidized upon exposure to air. (Notice this is cultivated soils, not all soils.) Some of it goes back 15,000 years to when forests were first cleared for crops.

Regenerating these soils involves agricultural practices such as using year-around cover crops, and what is commonly called rotational pasture and other measures. Top priority would be in restoring degraded and eroded lands, avoiding both deforestation, and farming of peat land. Restoration of mangrove areas along coasts, salt marshes and sea grasses would also play a part.

The good thing about this method of reducing carbon dioxide, in contrast to mechanical sequestration, is that it increases the capacity to produce food for the coming increase in world population by increasing soil productivity, and making soils more resilient to both floods and drought. It is not high tech, but involves techniques already with an advanced state of understanding. It is not capital intensive, but education intensive.

One of the most severe problems is with grassland. Much of the world’s agricultural land produces grass and is not suitable for crops, except for certain small favored spots. It is too dry, too hilly, or too wet for crops which produce parts that can be eaten by humans. Grasslands developed under herds of grazing animals, so they are adapted to each other.

Here is a video of an expert, Allan Savory of Rhodesia (now Nyasaland) in Africa explaining the method in a TED talk. It will be recognized by any Appalachian farmer as “rotational grazing.” It is also called holistic management. The animals are kept close together, stay on one spot long enough to eat the top one-third or one-half the plant, the part that has the highest sugar content, then moved on. (The customary practice now is to keep animals in the same field continuously, in the worst case, as long as any of the plant growth remains there.)

The claims of holistic management have their detractors, too. But if you are “into” research on extreme hydrocarbon energy exploitation (shale drilling, mountaintop removal, deep water drilling, etc.) you have learned to look at who is financing what. Don’t skip the last line. Established environmental groups can be slow to change, too! They can’t argue that grazing doesn’t reduce grass fires, though, a benefit important near habitation.

Ohio State University maintains the Carbon Management and Sequestration Center, which lists four measures for reestablishment of soil carbon: afforestation, wetlands management, no-tillage of soils and close management of grazing. (1) Reforestation is the name that has been used for decades when the objective was to get the resulting timber. It is now recognized as a good way to build soil, too, because trees have the same kind of relations with microorganisms and humus as grass. Our Appalachian soils developed under forest cover. It is also recognized that the carbon in timber (since it is protected by roofs, etc., last decades to centuries after the tree is cut, before returning to carbon dioxide.

(2) Wetlands often preserve carbon for a long time, since the remains of plants are cut off from oxygen at the water level. Peat is a residue built up from wetland plants that is almost entirely organic. At one time it was cut and dried for fuel, a practice which has largely been discontinued. However peatlands can be drained and farmed, resulting in oxidation of the peat to carbon dioxide. The emphasis is now on preventing this from being done. (3) “No tillage” means not plowing to plant crops which have been traditionally handled this way. Plowing and tillage are primarily weed control activities. Today the best method for corn, soybeans and such like is to plant a cover crop, frequently a legume, which gets good growth before winter and keeps the ground covered and crowds out baby weeds. This prevents oxidation of the carbon in the soil, adds to it, and helps fertilize the main crop. It also helps control some insect pests and encourages wildlife.

So holistic management of soil can remove some carbon dioxide from the air and do it for decades to come. This can provide quite a few other benefits to society, such as food, timber, erosion and flood control, improved wildlife habitat, and scenic values. It is not  high tech nor does it does it require rare or expensive materials. It employs and educates many people, including the poorest, all over the world. Well now, it looks pretty good, doesn’t it?

This article was prompted by a recent extended review from Yale University.

Tagged as: carbon dioxide, carbon storage, cattle, climate change, Earth, farming, food, fossil fuels, soil