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  • Writer's pictureClare Snowdon

Soil Carbon

Did you know that the soil contains at least 3 times more carbon than exists in the atmosphere? There are believed to be around 2 trillion tons of carbon stored in the world’s soils. However, human activities have released vast quantities of carbon dioxide from the soil already and man-made climate change is making it even more challenging. It is activities such as the extraction and drainage of peat and intensive agricultural practices that are particularly problematic, as well as our increasing use of land for building.

There’s a fantastic article here that explains more about this, but I will offer a short summary of the importance of soil in the carbon cycle and what is needed for the soil to be helping us tackle the climate crisis.

Soil is a lot more than just dirt. It is not the soil in isolation that is the carbon hero. It is soil with healthy plants growing (and dying). The plants capture the carbon dioxide from the atmosphere and use it to make sugars in a process known as photosynthesis. Some of the sugars are used for respiration (energy) – exactly as in human respiration, so some of the carbon is released back to the atmosphere as carbon dioxide. Some of the carbon is used as part of the structure of the plant and some may be shared with soil organisms such as the fungi that are associated with the plant roots in the mycorrhizal networks in the soil.

When the plant dies, the plant matter may decompose, and the carbon is passed to the various decomposer organisms. This generally results in some release of carbon dioxide. However, in anaerobic conditions (without oxygen), the plant matter may be decomposed more slowly and methane released. Methane is a potent greenhouse gas. Matter that is not decomposed becomes hummus – that lovely dark soil. Peat is an especially rich land-based carbon store because it is formed by organic matter covered by water, often in acidic conditions. It has a particularly high proportion of the matter that has not decomposed.

So the key “ingredients” here are plants in soil and healthy soil organisms. Pesticides and fertilisers kill the soil organisms. Digging disturbs the soil structure and releases carbon dioxide and removal of the plants and organic matter from the soil means that the carbon is not captured. If peat is dried out or burned or dug up, the carbon that was stored is released as carbon dioxide.

In the garden, we can help by minimising digging and disturbance, not being too tidy – let the dead stuff go full cycle and return nutrients to the soil, using peat-free compost (ideally make your own) and gardening organically. The more of the garden you can have as healthy soil with plants in, the better from a climate (and biodiversity) perspective. Also, having plants that will live to a ripe, old age is great – the longer the plant lives, the longer the carbon is locked up in the plant and the larger the plant gets, generally the better. If you can, whatever you prune keep in the garden – perhaps as a dead hedge or compost heap. Avoid burning it.

Sadly climate change makes it all the more challenging for nature to capture and store the carbon, so it is important that we do all we can to tackle the causes of climate change and to make choices (such as buying food from sustainable agricultural practices or growing our own) that help.

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