World’s Soils Have Lost 133bn Tonnes of Carbon Since the Dawn of Agriculture

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Author: Daisy Dunne | Published: August 25, 2017

The world’s soils have lost a total of 133bn tonnes of carbon since humans first started farming the land around 12,000 years ago, new research suggests. And the rate of carbon loss has increased dramatically since the start of the industrial revolution.

The study, which maps where soil carbon has been lost and gained since 10,000BC, shows that crop production and cattle grazing have contributed almost equally to global losses.

Understanding how agriculture has altered soil carbon stocks is critical to finding ways to restore lost carbon to the ground, another scientist tells Carbon Brief, which could help to buffer the CO2 accumulating in the atmosphere.

Soil as a carbon sink

The top metre of the world’s soils contains three times as much carbon as the entire atmosphere, making it a major carbon sink alongside forests and oceans.

Soils play a key role in the carbon cycle by soaking up carbon from dead plant matter. Plants absorb CO2 from the atmosphere through photosynthesis, and pass carbon to the ground when dead roots and leaves decompose.

But human activity, in particular agriculture, can cause carbon to be released from the soil at a faster rate than it is replaced. This net release of carbon to the atmosphere contributes to global warming.

New research, published in the Proceedings of the National Academy of Sciences (pdf), estimates the total amount of carbon that has been lost since humans first settled into agricultural life around 12,000 years ago.

The research finds that 133bn tonnes of carbon, or 8% of total global soil carbon stocks, may have been lost from the top two metres of the world’s soil since the dawn of agriculture. This figure is known as the total “soil carbon debt”.

Around two-thirds of lost carbon could have ended up in the atmosphere, while the rest may have been transported further afield before being deposited back into the soil.

And since the industrial revolution, the rate of soil carbon loss has increased, says lead author Dr Jonathan Sanderman, a scientist at the Woods Hole Research Center in Massachusetts. He tells Carbon Brief:

“Considering humans have emitted about 450bn tonnes of carbon since the industrial revolution, soil carbon losses to the atmosphere may represent 10 to 20% of this number. But it has hard to calculate exactly how much of this has ended up in the atmosphere versus how much has been transported due to erosion.”

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