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'Plantstones' could help lock away carbon
07 January 2008
From New Scientist Print Edition.
Rachel Nowak, Melbourne

One way to cut greenhouse gas levels in the atmosphere may be to exploit a particular talent some plants have of locking away carbon. All we need to do is choose the right strains of crops to grow, and they will sequester carbon for us for millennia.

That's the idea of two agricultural scientists in Australia, who say the trick is to grow grasses such as wheat and sorghum, which lock up large amounts of carbon in so-called plantstones, also known as phytoliths. These microscopic balls of silica, which form around a plant's cells as they take the mineral up from the soil, may help to strengthen the plant and protect it from disease.

As phytoliths form, they also lock up carbon by trapping scraps of plant material. They are practically indestructible, so once the plant dies they enter the soil where they may sequester carbon for thousands of years, say Leigh Sullivan and Jeff Parr of Southern Cross University in Lismore, New South Wales.

They compared 200-year-old soils in Papua New Guinea with soils buried 400 to 4000 years ago by a layer of volcanic ash, which prevented the addition of any more organic matter. Sullivan and Parr found the longer a soil had been buried the greater the proportion of carbon contained in the phytoliths compared with the rest of the soil, suggesting other carbon sources, such as humus, release carbon dioxide more rapidly than the phytoliths.

They also screened 200 plant types, including 24 cultivars of sugar, sorghum and wheat, and found large differences in their abilities to store carbon in phytoliths. That raises the possibility of farmers earning money in carbon credits for changing to crops that store the most carbon. Sullivan and Parr reported their findings at two meetings last year, in Poitiers, France, and Cologne, Germany.

"Existing cultivars vary greatly in their ability to sequester carbon. If you decide to use the 'Goldrush' sorghum cultivar you can increase the amount of carbon sequestration threefold," says Sullivan.

Under the Kyoto protocol, a change in farming practice that reliably stores carbon can be used to claim carbon credits. So far most have been claimed for growing forests, which lock up tens of tonnes of CO₂ equivalents per hectare per year, until the trees reach maturity.

According to Sullivan, the best sugar cane cultivar tested so far can lock up 0.66 tonnes of CO₂ equivalents per hectare per year, and will do so year in, year out. The practice has the potential to become far more widespread than growing trees, as since food crops are a necessity it would allow farmers to keep earning income from food crops while not tying up land for decades.

"It wouldn't require the farmer to do too much that is different. The amount of carbon [per hectare] is quite small, but it would be significant over a large area of production," says Matthew Warnken of the consultancy Crucible Carbon in Sydney.

The next step is to assess how plants that are better at storing carbon in phytoliths fare in terms of crop yield and quality. "So far our studies of wheat and sorghum suggest that there is no trade-off between yield and carbon sequestration," says Sullivan.

Strains could be bred with enhanced ability to produce phytoliths, or genetically modified to do so.

From issue 2637 of New Scientist magazine, 07 January 2008, page 9

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