This age indicated to the researchers that they were looking at carbon from deeper parts of the soil—stuff that was once stable but, as the trees are cleared and fields are plowed, is being surfaced and lost. “Most of our deforested catchments had lost their tree cover more than two decades before the study, so post-deforestation erosion is a major factor,” study author Travis Drake told Earther in an email. “The seasonal tilling of croplands followed by heavy rains likely erodes the surface soils, exposing the older [carbon] we measured in the streams.
To do so, the researchers took samples of river water from 19 sites in the Kivu region of the eastern Democratic Republic of Congo, representing a gradient from pristine forest to completely deforested. (An estimated 927 square miles of forest were lost in the region from 2000 to 2010. ) They looked at the radiocarbon age and chemical makeup of dissolved carbon, which leaches into the rivers from the nearby soils, providing a good indicator of what’s being lost from the system. The findings were stark.