Carbon Cycle

All living things (as far as we know) contain carbon. Your body is 18% carbon. A plant is 50% carbon. The carbon cycle is a series of events that make life possible for us and our garden plants.

Movement of carbon between land, atmosphere, and ocean in billions of tons per year. Yellow numbers are natural fluxes, red are human contributions, white are stored carbon. The effects of volcanic and tectonic activity are not included. (US DOE, Public Domain)

What is carbon?

Carbon (C) is an element. For the most part, the amount of carbon in Earth’s sphere of existence is relatively constant. But carbon can take several forms. The graphite in your pencil is an example of carbon. Apply enough heat and pressure and that pencil lead can and will turn into a diamond. In most cases, carbon pairs up with other elements to form new materials, such as amino acids, fats, and carbohydrates. It can also form potassium bicarbonate, a material used to neutralize acidic soil. Very often, carbon pairs with oxygen to form carbon dioxide

Carbon, in both graphite and diamond form (Robert Lavinsky) CC BY-SA 3.0

Carbon cycle components

Carbon can be found everywhere on, in, and around the Earth. The way carbon moves from place to place, and from form to form, is what makes up the carbon cycle. The components of the carbon cycle are:

• atmosphere - carbon dioxide and methane both absorb and retain heat; carbon can dissolve in raindrops that fall to the Earth in an acidic form [carbonic acid]
• biosphere - all living things, dead things, and soil
• oceans - including living and non-living marine life and dissolved inorganic carbon
• geosphere - sediments, fossil fuels, fresh water systems, and the non-living organic material they contain
• Earth’s mantle and crust

Carbon moves through these changes because of biological, chemical, geological, and physical processes.

The carbon cycle in your garden

The same processes occur in your garden, with similar results. Start with your soil. Soil is a living, breathing complex of minerals, organic matter, insects, microorganisms, worms, and more. Microorganisms break down complex molecules into smaller bits, separating carbon from whatever it happens to be bound to and making it and other minerals available to plant roots as food.

You plant a seed and water it. The seed coat softens, a first root, or radicle, emerges and starts absorbing carbon and other minerals. The carbon then becomes part of the plant. The plant matures and produces fruit or other edible parts, which we harvest and eat. The carbon in those parts then becomes part of us. The parts we don’t eat are added to the compost pile, where the carbon attaches itself to other minerals or is released into the atmosphere as carbon dioxide. 

The sun shines down on Earth’s oceans, waterways, and soil causing evaporation. That moisture condenses and rain occurs, converting any carbon dioxide in the atmosphere into carbonic acid. This form of carbon falls to Earth, enters the soil, and hydrates any unharvested plants. Excess rainwater leaches into ground water where it ultimately makes its way to oceans. The sun’s heat causes more evaporation, water rises into the atmosphere, more rain, and so on.

How do plants use carbon?
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You may have heard that plants take carbon dioxide in and release oxygen during the day, and that that process switches at night. The truth is, carbon dioxide is only taken in, and oxygen is only released back into the environment, while photosynthesis is actively taking place. The act of photosynthesis spilts the carbon dioxide molecule into oxygen and carbon. The carbon is used as a building material, much the way our bodies use carbohydrates. [Carbohydrates are made out of carbon, hydrogen, and oxygen.]

Plants also absorb carbon from the soil, but there has to be the right balance of nitrogen available for plants to use the carbon they need. Nitrogen is the steak and salad equivalent of the human diet. This balance is called the carbon-to-nitrogen, or C:N, ratio. Microorganisms that break down minerals and organic matter into plant-sized bites prefer a C:N ratio of 24:1 to 30:1. If there is too much, or not enough, carbon or nitrogen in the soil, plants will suffer.

Carbon cycling and tap water

Most soils contain a variety of carbon compounds and this can be a problem if your tap/irrigation water is highly alkaline. The chemical reactions that occur between carbonate and bicarbonate ions and alkaline water can make calcium and magnesium less soluble, and harder for plants to absorb. These chemical transformation between these molecules also tends to leave salts behind. Whenever possible, irrigate your garden with rain water.

Carbon sequestering

As we burn fossil fuels and seal the Earth’s surface with concrete, we release an awful lot of carbon into the atmosphere and make it harder for carbon to be absorbed into the soil. Carbon in the atmosphere joins with oxygen to form carbon dioxide. Carbon dioxide (and methane) absorb heat and bounce it back to Earth. This can help prevent another Ice Age and it can lead to global warming, more destructive storms, desertification, rising sea levels, and a harder time for us and our plants. 

Removing carbon from the atmosphere is called carbon sequestering. Naturally, carbon is held in plants and soil. When plants are composted or decompose, some of the carbon they contain is returned to the atmosphere and some enters the biosphere, oceans, and geosphere. 

As we disturb the Earth’s surface, we break the bonds that hold carbon in place. Instead of plowing or rototilling, you can reduce the amount of carbon released into the atmosphere by practicing no-dig gardening. [It’s easier on your back, too.]

Carbon is sequestered into the soil by plant roots. Plant roots secrete carbon in something called exudates. Root exudates feed beneficial bacteria and fungi in the soil which then help feed your plants. These root exudates also promote soil health and reduce erosion.

There are many ways that you can reduce the amount of carbon in the atmosphere:

• protect soil with mulch or cover crops
• plant perennial crops, such as artichoke, rhubarb, and fruit and nut trees
• compost plant debris and kitchen waste properly
• install a rain garden
• use native plants and those suitable to your microclimate
• get a lab-based soil test before adding unnecessary fertilizers
• avoid using potting soil that contains peat
• irrigate your garden with rainwater collected in rain barrels or other means
• buy used items whenever possible, rather than new, and only buy what you actually need
• avoid heating the outdoors with fire pits, BBQs, and patio heaters
• never burn trash, build bonfires, or make biochar
• plant large trees near your home to reduce the need for A/C
• if you have a lawn, allow it to get taller before mowing

Finally, growing your own food is one of the easiest ways to reduce the amount of carbon released into the atmosphere. Not only does it reduce the demand for highly tilled fields around the globe, it reduces the number of trucks, ships, farming machinery, and storage facilities needed to fill your larder. Just go outside and pick what you need!