Underestimation of carbon dioxide emissions from organic-rich agricultural soils

Organic-rich agricultural soils, including drained peatlands, are hotspots for biogenic CO2 emissions, contributing to elevated atmospheric CO2 levels. Due to microbial mineralisation, the organic carbon (OC) content of these soils transitions to that of mineral soils, but it remains unclear how the residual OC content controls the rate of CO2 emission. We show that area-scaled CO2 emissions from soils with >6% OC are not controlled by OC content and OC density. National greenhouse gas inventories assign area-scaled CO2 emission factors to soils with >12% OC, but soils with 6-12% OC are disregarded or treated with lower emission factors than soils with >12% OC. In this respect, our results suggest that CO2 emissions from organic soils could be underestimated by 40% in the Danish National Inventory Report to the United Nations Framework Convention on Climate Change (UNFCCC). We conclude that a global underestimation of CO2 emissions from 6-12% OC soils occurs in countries with large proportions of organic soils in transition from organic to organo-mineral soils due to agricultural management. Refining CO2 emission estimates for 6-12% OC soils is critical for the accuracy of national inventories, but also for recognising the climate benefits of emerging initiatives to rewet drained organic soils.