Comment on "Managing soil carbon" (II).

Lal et al. have recently argued (1, 2) that restoring soil carbon levels worldwide is important for a number of purposes, including reducing atmospheric CO 2 concentrations. We agree, but we question how much eroded soil C is ultimately delivered to the atmosphere. Previous estimates of the proportion of eroded soil carbon oxidized range from 0% to near 100% (3, 4). We believe that the true value is nearer the extreme low end of that range, because much of this soil carbon is deposited in depressions, water impound-ments, and floodplains, where oxidation rates are lower than in the original soils (5, 6). Therefore, erosion-related emissions of C are probably much smaller than the ϳ1 gigaton (Gt) C/year estimated by Lal et al. We have argued previously (7) that erosion and subsequent deposition represent an apparent sink of ϳ1 Gt/year. Our published estimates suggest that soil erosion globally mobilizes ϳ1.4 Gt carbon annually (7), based on a robust budgetary estimate for the United States of 0.05 Gt C mobilization per year from erosion. Further, we demonstrated that U.S. river discharge of total organic C is about 20% of soil C erosion. We identified storage compartments for bulk sediment on land—large (inventoried) impoundments, smaller (uninventoried) im-poundments, and (by difference) alluvium— and estimated the organic carbon buried in each. The carbon not associated with any storage compartment was assigned to oxidation. Within the uncertainties in our bulk sediment budget, there was no evidence of net carbon oxidation, although we cited the basis on which our estimates could be raised from 0 to 20% oxidation. This estimate was based on budgetary calculations, not an assumption , as stated by Lal et al. (1, 3). Although the Lal et al. value of 20% is not inconsistent with that range, we believe that this high-end value is extreme for the United States and that higher percentages are well outside the constraints imposed by the budget. Indeed, the Lal et al. estimate of 1 Gt yearly carbon loss to the atmosphere is itself based on two unsubstantiated assumptions: 20% C oxidation in the erosion/transport process and 3% C content of soils (8). We have also demonstrated that deposi-tion in artificial impoundments is the largest single sink for eroded soil in the United States (9, 10). The carbon content of this sediment is similar to that of the soil from which it was derived, which in turn indicates …