August 16, 2020

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Soil Carbon Response to Land Use Changes

Radiocarbon dating provides understanding of how soil carbon is impacted by land-use changes in tropical forests.

The Science

Deforestation of tropical forests leads to the loss of carbon stored in the soil, but afforestation or conversion to plantation forests can result in the addition of carbon to the soil. The research team used radiocarbon and stable isotope measurements of soil organic carbon to disentangle the impacts of land cover conversion on soil carbon storage. They found conversion of forests to pastures or oil palm plantations resulted in the loss of soil carbon. In the pastures, the carbon lost from the soil tended to be very young. In the oil palm plantations, the age of the soil carbon did not change, suggesting young carbon may have been lost during conversion but the oil palms have been able to accumulate new young carbon in the soil. Additionally, the team found conversion of degraded pastures to oil palm plantations or secondary forest may help to gradually increase soil carbon stocks.

The Impact

This is the first study to use radiocarbon dating to understand how quickly carbon is added to or lost from soil following conversion to oil palm plantations. It is important to understand this because of how quickly natural forests are being converted to oil palm plantations, and because there is a very large amount of carbon stored in the soils of tropical forests. These results indicate that soil carbon stocks in cleared forests may recover following reforestation to oil palm plantations to a similar degree as regrowth of secondary forests and that minimizing the time between forest clearing and oil palm plantation can prevent sustained losses of soil carbon.

Summary

Tropical forests contain one-third of the Earth’s terrestrial carbon pool; however, rapid deforestation threatens the stability of this carbon. The team measured radiocarbon and stable isotopes of soil carbon from tropical forests and land that was converted to oil palm plantations in Peru, Indonesia, and Cameroon. Additionally, they looked at the impact of converting tropical forests to pastures in Peru and the subsequent conversion of those lands to oil palm plantations or secondary forest. The team found that even though the amount of carbon in soil decreases from the initial conversion to oil palm plantations, oil palms accumulate new carbon in the soil, potentially offsetting soil carbon losses. In contrast, clearing primary forest for crops or pasture caused losses of both total soil carbon and radiocarbon that persisted for decades following reforestation to secondary forest or oil palm plantations.

Principal Investigator

Karis McFarlane
Lawrence Livermore National Laboratory
mcfarlane3@llnl.gov

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
daniel.stover@science.doe.gov

Funding

Funding for this work was provided by the U.S. Department of Energy Office of Science Early Career Program Award SCW1572 to K. McFarlane.

References

Finstad, K., O. van Straaten, E. Veldkamp, and K. McFarlane. "Soil carbon dynamics following land use changes and conversion to oil palm plantations in tropical lowlands inferred from radiocarbon". Global Biogeochemical Cycles 34 (9), e2019GB006461  (2020). https://doi.org/10.1029/2019GB006461.