Let’s start with a definition of carbon sequestration:

Carbon sequestration is the long- term storage of carbon in oceans, soils, vegetation (especially forests), and geologic formations…Soils contain approximately 75% of the carbon pool on land …Therefore, soils play a major role in maintaining a balanced global carbon cycle

Ecological Society of America

Carbon cycling: how does it really work?

As plants are growing [on the land] they absorb carbon dioxide from the air. Through the process of photosynthesis this carbon dioxide and water are used to form simple sugars that are composed of carbon, hydrogen, and oxygen. These sugar compounds are the foundational building blocks to build all the rest of the plant compounds, such as complex carbohydrates and polysaccharides, proteins and amino acids, and plant lipids. All of these compounds contain an average of roughly 40 percent carbon.

In exchange for nutrients supplied by the soil system, plants release large amounts of these substances [exudates] into the soil to feed the soil biology. These root exudates contain a variety of organic compounds. The healthier a plant becomes the greater the amount of root exudates and the higher the quality. According to Horst Marschner, in his book titled Mineral Nutrition and Higher Plants, healthy plants can release as much as 60 to 70 percent of their total sugar production back into the soil as root exudates.

This can only occur, however, if these are healthy plants, in other words, they have an energy surplus. That can be a lot of carbon sequestration because a healthy plant will have at least as much root biomass below ground as there is plant biomass above ground. So if we have 100 pounds of plant biomass above ground, and an additional 100 pounds belowground, this still represents only approx. 40 percent of this plant’s total energy production (i.e. for maintaining itself).  Which means then ~60% is given off as exudates…mainly carbon. Which feeds the soil life and is ever expanding, so long as there are living plants.  This is the real secret to building soil carbon effectively and efficiently. We can readily see why forage-based livestock agriculture and perennial polycultures are the most efficient method of building soil organic matter and stable humic substances. As well, it needs to be said that generally, organic farming has a primary goal of building soil health unlike industrial farming.

Adapted from NOFA Newsletter January 2016 – The Best Ways to Sequester Carbon

What’s key?

Absolutely critical to carbon sequestration is keeping your soil covered in living plant material for as many days out of the year as possible. If your gardens/land aren’t photosynthesizing, the soil carbon battery isn’t charging. So, keep all your soil pumping carbon by using as many perennial plants or cover crops in between annual plants, for example.  Also, the more diversity of plants, the more diversity of soil life, which means extra resilience.

Avoiding bare soil can also be done by spacing plants close together and also by mulching heavily when not growing in it.

Diverse Biology

Soils cannot recharge by chemistry alone, they must have a reserve of biological activity to keep energy flowing into plants. Every disturbance of the soil impacts its biology – from rototilling to hoeing, the difference is only of degree. Some farmer-teachers, including Jean Martin Fortier (The Market Gardener) of Quebec, believe that bare soil itself, passive as it seems, is an unnatural state that impacts soil health. Given that we are constantly disturbing our soil in the routine acts of gardening and farming, we should make sure our plants have the particular soil life that they need to thrive.”

Making good quality compost adds that and so can microbial inoculants, typically added directly to the roots of plants when planting (like Root Rescue or Myke).

Nature’s nutrient cycling system – where and how carbon gets stored

Learn more here

Studies and Resources

The Organic Center, Washington, DC: Organic Soils Sequester Carbon and Advances in Agronomy

UC Davis: Compost Key to Sequestering Carbon

U of Illinois: Synthetic Nitrogen Destroys Soil Carbon

Organic Agriculture Centre of Canada (OACC): Organic Science Cluster III

Movie: Kiss the Ground (currently available on Netflix)