France’s agriculture minister Stéphane Le Foll has founded an ambitious international research program, called “4 pour mille” (“4 per 1000”), which aims to boost the amount of carbon-containing organic matter in the world’s soils by 0.4% each year.
The program was launched officially today at the United Nations climate summit in Paris, with the hope to sign up as many nations as possible.
How much carbon do soils store? A lot. At about 2.4 trillion tonnes of carbon, soil is the largest terrestrial carbon pool, and the top 2 metres of the planet’s soils hold four times as much carbon as all the world’s plants. Carbon stored in soil can also stay there for a very long time relative to carbon in plants.
Thanks to recently published maps of global soil carbon stocks, we can work out how much extra carbon needs to be stored in soils (and where) in order to meet the target.
The size of the task
There are roughly 149 million square kilometres of land in the world, so if all the world’s soil carbon were dispersed evenly there would be 161 tonnes per hectare. Hitting the 0.4% target would mean increasing soil carbon stocks by 0.6 tonnes (600 kg) of carbon per hectare per year, on average.
But of course, soils around the world vary widely in carbon storage – tropical peat soils, for example, hold about 4,000 tonnes of carbon per hectare, whereas sandy soils in arid regions may only hold 80 tonnes per hectare. The type of above-ground vegetation and how quickly the soil microbes use the carbon can also affect the amount of storage. Generally speaking, only a quarter of organic matter added to soil ends up being stored as carbon in the long term.
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