Gardening, Healthy Soil, and Carbon Sequestration

by Adrian Ayres Fisher

Starting right now, we, all us humans on the planet, have a short window of opportunity to solve global warming, keep global average temperature rise below 3.6° F (2°C), and avoid more and worse instances of the kind of extreme weather disasters that have recently pummeled every region of the US (and the world). To accomplish this, total emissions must peak and rapidly decline into negative territory. Impossible, you say? Amazingly enough, it can be—and is being—done, using currently available technologies. The general consensus is that success will come through leaving fossil fuels in the ground, switching to renewable energy, and doing everything possible to reduce emissions in a variety of ways.

Soil carbon sequestration is a proven climate change solution.

Not often discussed is another, crucial part of the solution: carbon sequestration, aka pulling carbon dioxide out of the air and storing it. The best – in fact the only – available methods to do this are carbon-focused land-management techniques, many already in use, applicable on every scale, regional to hyper-local, and in every kind of ecosystem. Soil carbon sequestration is so important that if employed globally, it could enable us to evade the carbon emissions trap.

To this end, earth’s remaining natural areas serve as vitally important carbon sinks. Conservation and restoration combine necessary environmental benefits with climate change mitigation. In North America, temperate woodlands, and prairies of the kind that once sprawled across 140 million acres in the U.S., can draw down approximately 1000 pounds of carbon per acre per year.

Carbon Storage and Soil Health

But what about the billions of acres used for human purposes? Here, too, huge amounts of carbon can be stored by nurturing soil health, as regenerative organic farmers and ecological gardeners everywhere are already doing.

Long-term, deep soil carbon storage comes about through the creation of humus, the result of relationships between actively growing plants, fungi and soil microbes and other critters, in a matrix that includes mineral soil and organic material. This process, humification, builds topsoil while storing carbon in a stable form that can stay put for hundreds of years.

A visible difference: The same soil at 1% carbon (left) and 5% carbon (right). Photo: Rodale Institute

How does this work? During the process of photosynthesis plants break down atmospheric carbon dioxide into oxygen, released back into the air, and carbon, which gets combined with water and converted into carbon sugars the plant uses to fuel itself. However, some of this “liquid carbon,” travels down to the roots, and, as it fuels their growth, a portion leaks out of the roots into the soil. Plants are trading these “root exudates,” to mycorrhizal fungi, bacteria and other microbes in return for nitrogen, phosphorus, other nutrients and micro-nutrients. In the process, vast networks of mycorrhizae form in the soil, connecting plant roots with nutrients they couldn’t otherwise access. Plants don’t seem negatively affected by this loss of carbon sugars. Rather, a more complex soil biome, means healthier soil and plants; in healthy soil, plants get 85-90% of nutrients they need through this exchange.

The story doesn’t end there. The mycorrhizae themselves, having utilized the carbon sugars and supplied plants with nutrients, in turn exude a gluey, sticky protein called glomalin. With other gums and glues produced in the carbon-nutrient exchange, glomalin aids in the formation of soil aggregates by sticking together particles of sand, clay and silt, creating the larger clumps that that collectively we call humus. Glomalin is thirty to forty percent carbon and is incredibly stable and long-lasting; humus itself is about sixty percent carbon. In 1996, when glomalin and its role in humus production was discovered, it became possible to accurately measure soil carbon levels, important for assessing our carbon-storage efforts.

Carbon sequestration happening here. Photo: Adrian Fisher

Gardeners have always appreciated humus, since its presence guarantees that soil is fertile and has good tilth – porous, “fluffy,” with air pockets, room for water penetration and good water holding capacity, among other virtues. Humus isn’t something you can separate out of healthy soil. Structurally it is the soil and as every good gardener knows, humus-rich loam is the best medium for growing flowers or vegetables. What is new is the discovery of the relationships that build humus and how all that carbon gets stored – and also what disturbs the system.

Practices Matter

A number of conventional farming and gardening practices are now scientifically understood to harm soil health by preventing the formation of carbon-rich, healthy soil. Over-application of synthetic NPK fertilizer shuts down soil production of nitrogen and slows down or even halts humus formation and carbon storage; on a diet of NPK fertilizer, plants cease to produce liquid carbon, relationships break, and the soil begins to deteriorate. Also, plowing, tilling, or extensive digging slices up soil aggregates, breaks up the vast fungal networks, and, by exposing the soil to air, releases carbon dioxide and nitrous oxide. Soil structure declines, and so does its biological health. Finally, leaving soil bare in winter means depriving the soil biome of the benefits that growing plants provide, by interrupting vital relationships and starving the soil critters. These three practices can result in compacted, poorly textured, low carbon soil that is infertile, unable to manage water or grow plants, and prone to blowing away.

Conversely, regenerative, carbon-focused farming, gardening, and landscaping build topsoil and store carbon by fostering good soil health.  Avoiding synthetic inputs, adding organic material, minimizing soil disturbance, and “armoring” the ground with cover crops or by growing perennials will, over time, increase measurable soil carbon.  Starting with a baseline of 1-2%, soil carbon can increase over ten years to 5-8%. While 1% of carbon in soil equals about 8.5 tons per acre, after ten years there can be 25-60 tons of carbon per acre. Worldwide, possibly half of human emissions could be sequestered in this way.

On non-agricultural land in the U.S., both private and public, the potential is huge. There are, famously, over 40 million acres of lawn and untold millions more of gardens and landscaping. Properly managed, all this land could provide wildlife habitat, improve water management, and help solve climate change, while adding beauty to the world.

About the Author

Adrian Ayres Fisher is sustainability coordinator for Triton College, in River Grove, Illinois, where her duties include managing the college’s natural areas and bioswales. In her private life, she monitors rare plants in Cook County Forest Preserves and is active with Wild Ones native plant landscaping association. She also grows vegetables and maintains a native plant “pollinator reserve” in her backyard.


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