Agricultural soil erosion does not contribute to global warming

Agricultural soil erosion does not contribute to global warming, a new EU-funded study published in the journal Science reveals.

‘Agricultural soil erosion is recognised as an important impact on agricultural quality, but only recently have we identified its significance for the global carbon cycle,’ says Professor Tim
Quine of the University of Exeter in a podcast on the Science website.

However, previous studies into the matter gave conflicting results. Some suggested that agricultural soil erosion acts as a carbon source, effectively adding 13% to annual fossil fuel
emissions, while others appeared to show that erosion is a carbon sink, offsetting fossil fuel emissions by more than 10%.

‘There is an ongoing debate on the link between agricultural soil erosion and the carbon cycle,’ said Dr Kristof Van Oost of the Catholic University of Leuven. ‘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.’

In fact the scientists found that agricultural soil erosion acts as a small carbon sink, with a value equivalent to around 1.5% of annual fossil fuel emissions.

The scientists arrived at this conclusion by using caesium-137, which was deposited around the world as a by-product of nuclear weapons testing, to track the movement of soil around the
agricultural landscape. This allowed them to predict how much carbon once could expect to find in areas of soil erosion and deposition.

They then compared these predictions with the amounts of carbon found in 1,400 soil profiles to determine which soils had acted as sinks of carbon and which had acted as sources. Furthermore,
they were able to calculate how much carbon was replaced at sites of soil erosion.

Their analyses revealed that soil erosion acts like a conveyor belt, picking up subsoils, transporting them through surface soils and burying them in hill-slope hollows. During its journey, the
soil absorbs carbon from plant material. This carbon is subsequently buried within the soil in the deposition areas. Erosion therefore leads to a net removal of carbon from the atmosphere,
making it a carbon sink, albeit a small one.

‘This has rather significant implications for the way we design policy for agricultural land,’ said Professor Quine. If erosion had turned out to be a major source of carbon emissions, then
erosion control could have been used to cut fossil fuel emissions. On the other hand, had erosion proved to be an effective carbon sink, then the wider 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,’ concluded Professor Quine.

EU funding for the work came from the Marie Curie Intra-European Fellowship Programme.