New insight into basics of soil carbon

You may have heard a lot about soil carbon, "storing" carbon in soil, or "carbon markets." The October 1^st Soils Matter blog discusses the basics of soil carbon, and why it's an important topic.

One important type of "soil carbon" is organic matter, which is made of decayed materials from living things like plants, animals and microbes. How does organic matter have carbon in it? The term "organic" in chemistry means chemical compounds that contain carbon (and usually hydrogen) along with other elements. Organic compounds are prevalent in nature, especially in living things. You may recall that plants "breathe in" carbon dioxide, and "breathe out" oxygen. Plants use the carbon dioxide to make new cells so stems, roots, and leaves can grow. The carbon part of carbon dioxide is even used to make flowers, fruits, nuts, and vegetables (along with many other elements like nitrogen and oxygen).

A feature of healthy soil is a buildup of carbon in the form of organic matter, and even waste of earthworms, insects and other creatures that live in the soil. Even human bodies have organic carbon compounds in them! Humans are mostly made of hydrogen, oxygen, carbon, and nitrogen.

Another form of soil carbon is frozen. Frozen carbon dioxide is found in abundance in the frozen ground of the tundra. Warming temperatures of the earth can cause that frozen carbon dioxide to be released into the atmosphere. As global warming continues, soil carbon is being lost at faster and faster speeds, as the frozen carbon dioxide thaws. In addition, increases in temperatures of the tundra also increase the activity of soil microbes. As they "eat" more organic matter, this stored carbon is lost.

The carbon cycle is an important concept tied to soil carbon. As plants grow and die, they leave behind sugars and carbohydrates (both contain carbon) in the soil. This is used by soil microbes and other soil life as a food source. This makes a wonderful carbon cycle, with plants pulling carbon from the air in the form of carbon dioxide and storing it in the soil as organic matter for other living things to use.

As long as this carbon cycle stays balanced – either by keeping organic matter quantities stable or increasing – the atmosphere is safe. But when human disrupt this cycle by building cities where forests once were, or other anthropogenic changes, it disrupts the carbon cycle. The carbon that was stored as organic matter can easily be put back into the atmosphere as carbon dioxide. Clearing land for agriculture also made soils lose carbon to the atmosphere. Melting tundra soils and ice also contribute to this lost carbon.