Agricultural soil erosion is not a source of carbon dioxide to the atmosphere, according to research published online October 25 in Science. Carbon emissions are of great concern worldwide because they, and other greenhouse gases, trap heat in the Earth's atmosphere and are a major cause of global climate change.
"There is still little known about how much carbon exactly is released, versus captured, by different processes in terrestrial ecosystems," said Johan Six, a professor of agroecology at UC Davis and one of the study's authors. "We urgently need to quantify this if we are to develop sensible and cost-effective measures to combat climate change."
The study was carried out by an international team led by researchers at the Katholieke Universiteit Leuven, Belgium, the University of Exeter, UK, and the University of California, Davis.
The researchers developed a new method to establish the net effect of erosion on exchanges of carbon between the soil and the atmosphere. They found that in landscapes subject to soil erosion, erosion acts like a conveyor belt, excavating subsoil, passing it through surface soils and burying it in hill-slope hollows. During its journey, the soil absorbs carbon from plant material and this becomes buried within the soil in depositional areas. Erosion, therefore, leads to more carbon being removed from the atmosphere than is emitted, creating what can be described as a 'sink' of atmospheric carbon.
The team found that these sinks of CO2 represent the equivalent of around 1.5% of annual fossil fuel emissions. This finding challenges previous assessments that erosion represents an additional source of carbon to the atmosphere equivalent to adding 13% to annual fossil fuel emissions. The finding also challenges the opposite notion that erosion is currently offsetting fossil fuel emissions by more than 10%.
Dr Kristof Van Oost of the Katholieke Universiteit Leuven said: "There is an on-going debate on the link between agricultural soil erosion and the carbon cycle. Academics on one side have argued that soil erosion causes considerable levels of carbon emissions and on the other that erosion is actually offsetting fossil fuel emissions. Our research clearly shows that neither of these is the case."
This new insight into the effect of erosion on the carbon cycle is essential for sound management of agricultural soils. If previous assessments that erosion causes a high level of carbon emissions to the atmosphere had been correct, then erosion control could have been used to offset fossil fuel emissions. If the assessment that erosion created a very large sink of atmospheric carbon had been correct then the environmental benefits of erosion control would have had to be set against the loss of the sink.
"Our results show that erosion control should be pursued for its environmental and agronomic benefits but should not be used to offset fossil fuel emissions," said Professor Tim Quine of the University of Exeter. "Soil erosion is not the silver bullet for offsetting the ever-increasing emission of CO2 to the atmosphere."
The researchers used a by-product of nuclear weapons testing (caesium-137) that is present throughout the world to track the movement of soil around the agricultural landscape. This allowed them to predict how much carbon would be expected to be found in areas of soil erosion and deposition. By comparing these predictions with measured amounts of carbon in 1400 soil profiles they could identify which soils had acted as sinks of carbon and which had acted as sources. They were also able to establish the fraction of carbon that was replaced at sites of erosion, which enabled them to calculate the effect of erosion on the global carbon cycle.
Funding for the study was provided by the European Commission under the Marie Curie IntraEuropean Fellowship Programme, the Fund for Scientific Research, Flanders, and the Kearney Foundation of Soil Science, University of California.
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