Soil erosion : A carbon sink or source?

The report by K.Van Oost et al., "The impact of agricultural soil erosion on the global carbon cycle" (26 October 2007, p. 626) raises two serious concerns. First, the eroded soil is severely depleted of its soil organic matter (SOM) pool, which is preferentially removed by surface runoff because it has low density and is concentrated in the surface layer. Second, the process of soil erosion by water entails three distinct stages : (i) detachment, (ii) transport/redistribution, and (iii) deposition. (Letters)