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The Loam Ranger – Soil carbon and carbon trading

Dear Loam Ranger,

Will I be able to earn income by selling carbon credits to store carbon in the soil in my paddocks?

A lot has been said in the media lately about soil carbon sequestration and how this might help offset the rise in atmospheric carbon from human activities. Several of our clients have expressed interest in selling carbon credits to big business on the basis that they can sequester (that is, lock away) carbon in their soil. In theory, this has flow-on benefits of improving the fertility, cation exchange capacity and water-holding capacity of the soil. In practice, it is not straightforward.

First, it is important to understand the different forms of carbon in the soil, and what they mean for carbon sequestration. Carbon occurs in soils in two forms:

Inorganic carbon

In some soils, carbon is present as calcium carbonate, or lime. Such soils are said to be limey or alkaline and usually occur in drier environments. Often the lime is inherited from the geology (limestone and limey marine deposits), but sometimes it forms as a secondary mineral as a result of various soil chemical processes. Long-term cropping typically acidifies soil (through the removal of nutrient cations and their replacement with hydrogen ions; and through a build-up of organic matter, which is acidic). As soils become acidified, lime is converted to carbon dioxide and lost to the air.

Organic carbon

This originates from the decay of plant material. The most obvious pool is the fossilised carbon of peat, brown coal and black coal, which started life as organic carbon deposited in peaty waterlogged soils, where the carbon is protected from atmospheric oxygen and does not decay. Peat can be thought of as a young fossil.

In contrast, organic carbon entering aerated soils starts to decay as microbes and other soil organisms break it down as a food source. The great majority of such carbon is released back into the atmosphere as carbon dioxide. However, in stable ecological systems a more resistant form can accumulate. This organic matter is composed of complex cross-linked polyphenolic compounds resistant to microbial decay, but it can still be lost if disturbed by fire or farming. The compounds accumulate under cold, damp conditions, but not under low rainfall or high rainfall warm conditions. This explains why the great grassland steppes of Asia and North America have fertile soils with a high organic matter content. Rainforest soils and desert soils, on the other hand, accumulate very little of such compounds and are notoriously prone to loss of fertility if farmed. Australian soils, on the whole, have a very low organic matter content.

Under continued and well-managed grazing or occasional well-managed cropping, farming systems can maintain a stable organic carbon content. However, in Australia, and internationally, traditional land clearance and over-cropping have depleted much soil organic matter. How do we restore it?

Returning carbon to the soil

Compost is an appropriate way of returning carbon to soil in the restricted circumstances of small-scale, intensive horticulture of high-value crops. Most of the added carbon in compost is lost through decay (around 90% within 12 months and 99% in the next 5 years), but over all there is a net increase in stored carbon. Unfortunately, this approach is prohibitively expensive for low-value, extensive agriculture such as grain farming or pasture.

Another widely discussed solution is charcoal carbon, or biochar. This involves the slow combustion of plant wastes at low temperatures, largely in the absence of oxygen. Volatiles are lost to the air, but most of the carbon is retained as biochar. Incorporated into the soil, biochar retains nutrients and moisture, improving soil fertility. Native South Americans in Brazil have used this method for over 1500 years (some still do), creating pockets of fertile soil, called terra preta, that continue to produce good crops today.

However, the benefit of the reduction of atmospheric carbon dioxide must be weighed against the cost in fossil fuel consumption of transporting and spreading such carbon on farming soils. In Australian farming systems, the low density of plant matter probably means that biochar production is not economically viable in broadscale agriculture. And so far it is untested under Australian conditions.

Good farming practices with proper rotations, attention to soil fertilizer and sustainable yield hold the best promise of long-term improvement, but much research remains to be done.

Carbon offsets

Any farmer considering entering into a contract with a carbon emitter to sell carbon credits and sequester carbon in soil must seriously consider the likelihood of failure to achieve the contracted goal.

Also, all of this depends on the assumption that certain “good” farming practices can lock up carbon indefinitely. As we have seen above, composting is not practical in broadscale agriculture, most added carbon is lost in a few years, and biochar is a yet-unproven technology. In other words, the research basis does not yet exist for governments or industry to make reliable estimates of increases in stored soil carbon through any given farming management practice.

Currently the only accepted method of carbon sequestration at the agricultural scale is not agricultural at all but forestry-based: planting of forests for construction timber or even for burial. And that brings its own financial considerations if you don’t plan to harvest for 50 years.

Future topics

In coming issues we plan to discuss forms and measurement of soil organic matter, and soil carbon at the paddock scale.

Further information

http://en.wikipedia.org/wiki/Carbon_sequestration

http://en.wikipedia.org/wiki/Terra_preta

http://www.abc.net.au/science/news/stories/2007/1946410.htm

http://www.newscientist.com/article/mg19826542.400-burying-biomass-to-fight-climate-change.html

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