Unraveling the complex pathway that carbon takes on its endless migration from atmosphere to soil and back to atmosphere has been one of the outstanding challenges of earth science, made all the more complicated by human activities such as agriculture. A recent study by researchers from Belgium and the U.S. has provided new measurements of how agriculturally-driven changes to the landscape can act as both a source and a sink for atmospheric carbon.
Specifically, the researchers quantified for the first time the long-term net storage, or sequestration, of carbon in the soil due to agriculturally-induced erosion over an entire drainage basin. The results show that the capture of carbon in eroded soils partially offsets the net emission that clearing land for cultivation produces in the first place. But the study also showed that over the course of 500 years, roughly half of the stored carbon is re-released into the atmosphere — a finding that has important implications for understanding both the past and future role of human influence on the climate.
“We know erosion is bad for crop productivity and for ecosystem functioning, but when it comes purely to the carbon balance, then we’re saying that erosion actually helps with carbon being sequestered,” said co-author Johan Six, a professor in the UC Davis department of plant sciences. “But there is also a source that seems to be significant in the longer term.”
Clearing native vegetation for crops has significantly accelerated the rate of erosion by increasing the exposure of soils to the winds and waters that carry it downhill. The impact of such accelerated soil erosion on the carbon cycle has been contested within the scientific community, in part because of the difficulty in measuring all of the processes tying the fate of eroded soils to organic carbon.
Organic carbon in the form of dead plant material accumulates in the topsoil, where microbial decomposition returns it to the atmosphere in the form of carbon dioxide. The rate of decomposition — and, therefore, the stability of the soil carbon — is affected both by the transport of the soil downhill, as well as its burial under successive soil layers. In addition to downhill burial, newly exposed, carbon-deficient soils become available for carbon inputs from new crop plantings.
By analyzing measurements of soil depth and age in the Dilje basin in Belgium, a region that has been continuously farmed for 6,000 years, Kristof Van Oost, from the Catholic University of Louvain, and colleagues tracked the fate of organic carbon in soil by reconstructing the movement of soils in the basin over the entire 6,000-year period. The results, showing a net carbon sink for all of the erosion-related processes measured, appeared in the Nov. 20 issue of the Proceedings of the National Academy of Sciences (PNAS).
Jennifer Harden, a soil scientist with the U.S. Geological Survey in Menlo Park, called the PNAS study “a pioneering effort” which definitively establishes the mechanism of erosion-related carbon sequestration.
“The carbon models even today are not addressing this whatsoever, so this article is going to really, really help,” Harden said.
One of the remaining challenges in quantifying the impact of agricultural erosion on the worldwide carbon cycle is the variability in soils and rates of erosion across different geographical areas. In addition, long-term climate change would also affect the stability of buried soil carbon.
“What we know less about is how the role of lateral distribution of topsoil on the global carbon cycle may be different in different ecosystems, in systems experiencing different types of erosion, and how this can be affected by changing precipitation regimes under the anticipated global climate change scenarios,” wrote Asmeret Berhe, a soil scientist at UC Merced, in an email comment on the Dilje study.
Six said the new results add a new appreciation for the impacts to the carbon cycle that can already be expected in the future, given that the majority of the sequestered carbon from agriculturally-related erosion has happened only within the last 150 years.
“All of that buried carbon — in the future, we’re going to see it coming out,” Six said. “So it has implications for the future. Because now it maybe doesn’t look as bad, but in the future it could look much worse.”
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