It's known that erosion can bury carbon in the soil, acting as a carbon sink - but it now seems that a large part of that sink is only temporary.
As much as half the carbon buried in soil by erosion will be re-released into the atmosphere within about 500 years - and possibly even faster, given that climate change can speed the rate of decomposition.
"It's all part of figuring out the global carbon cycle," says Johan Six, professor of plant sciences at UC Davis. "Where are the sources, and where are the sinks? Erosion is in some ways a sink, but, as we found out, it can also become a source."
The team used radiocarbon and optical dating to calculate the amount of carbon captured in soils and released to the atmosphere during the past 6,000 years along the Dijle River in Belgium.
The study's long timescale allowed the researchers to track the gradual reintroduction of buried carbon to the atmosphere. Significant agricultural land conversion - historically the largest source of global erosion - only really got going in the past 150 years, well under the researchers' time frame of 500 years.
Therefore, most carbon sequestered in the soil during the past 150 years of agricultural history has not yet been released, and may become a significant carbon source in the future.
"Our results showed that half of the carbon initially present in the soil and vegetation was lost to the atmosphere as a result of agricultural conversion," says Gert Verstraeten, a professor at KU Leaven in Belgium.
Six points out that erosion can be minimized by no-till and low-till agricultural methods, as well as by cover cropping, which can ensure that soil is not left bare.
"We need to know where and how much carbon is being released or captured in order to develop sensible and cost-effective measures to curb climate change," says team leader Kristof Van Oost, of the Universite catholique de Louvain in Belgium.