Fates of Eroded Soil Organic Carbon: Mississippi Basin Case Study

We have developed a mass balance analysis of organic carbon (OC) across the five major river subsystems of the Mississippi (MS) Basin (an area of 3.2 3 106 km2). This largely agricultural landscape undergoes a bulk soil erosion rate of ;480 t·km22·yr21 (;1500 3 106 t/yr, across the MS Basin), and a soil organic carbon (SOC) erosion rate of ;7 t·km22·yr21 (;22 3 106 t/yr). Erosion translocates upland SOC to alluvial deposits, water impoundments, and the ocean. Soil erosion is generally considered to be a net source of CO2 release to the atmosphere in global budgets. However, our results indicate that SOC erosion and relocation of soil apparently can reduce the net SOC oxidation rate of the original upland SOC while promoting net replacement of eroded SOC in upland soils that were eroded. Soil erosion at the MS Basin scale is, therefore, a net CO2 sink rather than a source.This paper is part of ongoing studies by the coauthors to determine the role of landscape erosion and deposition in material fluxes and biogeochemical cycling. Parts of this work have been supported by internal institutional support at CICESE, Emporia State University, Miami University College of Arts and Science, and the Kansas Geological Survey, and by a Kansas NASA EPSCoR grant awarded to R. W. Buddemeier and R. O. Sleezer.We thank the numerous individuals who contributed technical assistance or conceptual support to these efforts. Three reviewers have provided useful critical comments on versions of this manuscript. Of these, we would like to single out Jon Cole, who thoroughly grasped the big picture of what we were advancing and whose summary comment seems worth quoting: ‘‘The idea that soil erosion is a large net sink of atmospheric CO2 is very interesting, well supported by the arguments and data in this paper, and likely to be a huge controversy. This controversy is a good thing, as Martha Stewart might say.’