EcoVet: Seeing the carbon for the trees

The recent climate change debate has descended into a farce and not only just in Australia. There has however been considerable comment recently about carbon bio-sequestration and soil carbon in particular. The Wentworth group have a new publication “Optimising Soil Carbon in the Australian landscape” which is well worth reading, in this monograph it is clearly demonstrated that the potential annual sequestration of soil carbon is many times greater than the Australian production of green house gasses. The problem remains however that the only way to economically suck carbon out of the atmosphere is via photosynthesis which is through the sun providing the energy to build up complex carbon compounds which are incorporated into plant tissues that ultimately end up in other living things or remain sequestrated in an organic form in the soil. Humans are able to short circuit this process in a number of ways with one of the most effective being the production of “bio-char” – a charcoal material resulting from pyrolysis of organic material. The only problem is that currently the cost of all of these alternatives, especially pyrolysis, remains too high.

That currently leaves revegetation and other forms of natural accumulation of carbon back into the soil. Slow but when well managed very efficient and effective. The cost fits within the range of currently proposed carbon prices with enough margin to drive a new land use industry but only on low value land. That said, this is no silver bullet and only when combined with other carbon pollution reduction mechanisms will it have a significant impact.

Soil carbon quantities are often equivalent or greater than the total combined within plants growing on the same area. While the equation varies considerably depending on climate, vegetation system and soil type, the 50:50 rule of thumb gives some idea of the value of carbon stored. An average semi-open woodland system contains around 400 tons per hectare of carbon at climax with a similar quantity stored in the soil. It is important to realize that the carbon exists not just as dead decaying vegetable material but more importantly within a host of dynamic living entities, especially fungi and microorganisms. The largest known living thing was recently discovered in Canada – a single subterranean fungus that covers several hectares and is estimated to weigh 10,000 tons.

When a forest is cleared not only are the trees removed but the subterranean habitat is destroyed and soil carbon is subsequently lost at about 25 per cent per year. Our pasture lands have therefore changed dramatically over the last 200 years. Changing from open grassy woodlands with high humus, friable soils to depauperate systems that require enormous effort to retain their productivity.

Over the years realisation of the importance of soil carbon has grown to the point that bio-char as an input subsidised by a correctly priced carbon pollution reduction scheme may be an invaluable part of our production systems. An interesting thought is that the brown coal that we burn for power production in Victoria, “Like burning wet socks” a friend of mine says, may be more valuable as a soil additive than its current use.

A barrier is the current exclusion internationally through the Kyoto protocol of soil carbon as a recognised form of carbon storage, indeed only vegetation over 2 metres tall is accepted. This is however likely to change as our ability to accurately and easily measure soil carbon is improved.

So what of assisted natural vegetation systems as at least an interim part of the carbon equation? And don’t forget that if bio-char becomes part of the equation that we will need vast amounts of raw materials for the process.

It has been said that ‘If land won’t grow grass it won’t grow trees” while this is true in the production forestry sense our natives can with the right help grow in some of the most hostile situations. Just think of the growth that can be achieved with the restoration of mined land which are little better than moonscapes in many cases.

Natives generally do not like even modest levels of soil nutrients with especially phosphorus and nitrogen being quite toxic. So depauperate land which in many cases is uneconomic for primary production would seem like a good place to start. The problem is that these same lands have lost structure and most of their soil carbon which even for natives are critical requirements for effective growth.

The healthy forests which will absorb the most carbon require a highly biodiverse system to assist in their metabolism. It has been clearly demonstrated by recent research that a healthy soil flora and fauna is essential, with many organisms especially fungi acting in close symbiotic relationships with growing plants. Restoration of existing vegetation systems is therefore easier as many of the important pieces of the biodiversity mosaic are already present. Revegetation of otherwise bare land is by comparison very difficult. There is no remaining seed bank except for weeds, previous fertiliser regimes may have left a toxic legacy, none of the biological building blocks remain and loss of soil structure has destroyed water retention .

Success under these circumstances is hard to achieve but paradoxically the use of modern agricultural techniques can make the job easier:

  • Soil testing to determine toxic conditions and then treatment or species selection for correction.

  • Herbicides to control weed species and reduce competition. The use of selective and pre- emergent herbicides is also being actively trialed

  • “Fertilising” with high yield carbohydrates such as sugar to promote microbial growth and lock up nitrogen and other nutrients. Currently cost makes this very interesting technique experimental only.
  • Producing native seed stock in “orchards” using conventional horticultural techniques to provide consistent quality, high germinability and reduce price.

  • Coating seed to reduce seeding rates, reduce predation and control especially fungal diseases.

  • Treatments to reverse dormancy.

  • Use of advanced sowing equipment which will “scalp” seed rows to remove the weed seed bank while at the same time accurately placing a wide variety of species at their correct sowing depth.

The next decade will see our production landscapes incorporate “carbon plantings” which while starting to correct carbon pollution through new business opportunities will also deliver ecosystem services that in many subtle ways will improve agriculture.

We can also look forward to the time when the economics for the production of bio-char have been sorted out and even the true cost of human induced climate change is fully recognised which, if applied as an offset, will further improve the productivity of our agricultural systems through the incorporation of vast amounts of soil carbon.

David Butcher

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