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Carbon burial

We all know about global warming. The cause is the extra amounts of greenhouse gases that humans are pouring into the atmosphere. The two most significant of these are carbon dioxide (CO₂) and methane (CH₄). This article focuses on CO₂.

Various strategies have been proposed to reduce the amount of CO₂ in the atmosphere. Many focus on reducing our output, such as by moving from coal-fired power stations to solar energy, and replacing oil with biomass. Others focus on removing the CO₂ that is already in the air, such as by reafforestation on a huge scale. This removal from circulation is called sequestration.

Can agriculture, horticulture and forestry make a significant contribution to carbon sequestration?

The carbon cycle

The carbon cycle is the name given to the movement of carbon in its various forms between the air, soil, water, life and rocks. The diagram above, taken from Wikipedia, shows the amounts of carbon that moved around the cycle in 2004. It shows that burning fossil fuels adds about 5.5 gigatonnes of carbon per year into the atmosphere.

Plant a tree

It seems obvious that planting trees will remove carbon from the atmosphere. But is it that simple?

Every tree we plant removes CO₂ out of the atmosphere while it grows. But what happens at the end of its life? The answer is that it decays, and the carbon is released once again. So planting trees as the sole strategy to remove carbon is ultimately not a long-term solution. Nevertheless, it is an important short-term solution that also helps maintain biodiversity, lower saline water tables and reduce soil loss.

To have any long-term effect, we need to keep the timber out of the carbon cycle. The production of houses and furniture is one solution.

A more radical solution is to bury the trees deep enough that they will not decay and release methane, which is 23 times as powerful a greenhouse gas as CO₂. This strategy, proposed recently by Ning Zeng, an atmospheric scientist at the University of Maryland in College Park, could completely offset the world’s production of greenhouse gases ... if we buried half of all trees that grow, and used a workforce of 2 million people. One of the biggest problems with this proposal is finding the energy to power the machinery that would harvest the trees and dig the holes without generating more CO₂ emissions. It also removes habitat, buries nutrients and supplies a food source for termites, which would release the carbon back into the atmosphere.

Black earth

A more workable technology to sequester carbon in the soil is terra preta, or “black earth”. This technology was developed by Amazonian people hundreds of years ago. At a relatively low temperature – around 350 °C – plant waste is smouldered in the absence of air. This leaves almost pure carbon and plant nutrients. The pure carbon is resistant to oxidation and can remain untouched in the soil for over a thousand years. The soil fertility is greatly enhanced too. Patches of these soils are still producing crops today.

Malcolm Fowles, at the Open University in England, believes that this technology can be adapted to both the industrial scale and the home scale and can make a significant dent in carbon emissions.

Deep rooting

One technique by which agriculture, horticulture and forestry can sequester modest amounts of carbon in soil is to grow deep-rooted plants. A news story from the ABC quotes Tim Wiley, a pasture agronomist from the WA Department of Agriculture and Food, as saying “Drought-tolerant perennial pastures could make a big dent in Australia’s greenhouse [gas] emissions by helping soils to soak up carbon.” He has found that naturally drought-tolerant pasture species “appear to have an exceptional ability to build up carbon in the soil.”

Naturally deep-rooted plants – perhaps specially bred for their deep-rooting ability – will leave carbon behind in the deep soil as the roots die. Careful irrigation scheduling – infrequent but deep watering – will also encourage plants to send their roots deeper in search of water.

Just how much carbon can be sequestered this way, however, remains to be seen.

Soil improvers – a short-term solution

Composted organic wastes – manure, food scraps, sawdust and so on – will improve soil structure and fertility, and consequently productivity. But they do not provide a long-term means of carbon sequestration. The reason is that the soil is a dynamic, living ecosystem, in which microorganisms (principally bacteria and fungi) and microfauna (such as worms and various insects) rely on carbon sources as food. All organic carbon in the soil will ultimately be broken down and released as CO₂ or methane.

But as a short-term solution to carbon emissions and a basis to soil and plant health, the addition of composted organic matter to soil is vital.

Research by SESL

SESL is currently working with a couple of clients to test how much carbon can be sequestered by cropping techniques. If you would like to explore the possibilities further, give us a call.