CARBON DISTRIBUTION AND LOSSES - EROSION AND DEPOSITION EFFECTS

Because of concerns about the eventual impact of atmospheric CO2 accum ulations, there is growing interest in reducing net CO2 emissions from soil and increasing C storage in soil. This review presents a framewo rk to assess soil erosion and deposition processes on the distribution and loss of C in soils. The physical processes of erosion and deposit ion affect soil C distribution in two main ways and should be consider ed when evaluating the impact of agriculture on C storage. First, thes e processes redistribute considerable amounts of soil C, within a topo sequence or a field, or to a distant site. Accurate estimates of soil redistribution in the landscape or field are needed to quantify the re lative magnitude of soil lost by erosion and accumulated by deposition . Secondly, erosion and deposition drastically alter the biological pr ocess of C mineralization in soil landscapes, Whereas erosion and depo sition only redistribute soil and organic C, mineralization results in a net loss of C from the soil system to the atmosphere. Little is kno wn about the magnitude of organic C losses by mineralization and those due to erosion, but the limited data available suggest that mineraliz ation predominates in the first years after the initial cultivation of the soil, and that erosion becomes a major factor in later years. Soi ls in depositional sites usuary contain a larger proportion of the tot al organic C in labile fractions of soil C because this material can b e easily transported. if the accumulation of soil in depositional area s is extensive, the net result of the burial (and subsequent reduction in decomposition) of this active soil organic matter would be increas ed C storage. Soil erosion is the most widespread form of soil degrada tion. At regional or global levels its greatest impact on C storage ma y be in affecting soil productivity. Erosion usually results in decrea sed primary productivity, which in turn adversely affects C storage in soil because of the reduced quantity of organic C returned to the soi l as plant residues. Thus the use of management practices that prevent or reduce soil erosion may be the best strategy to maintain, or possi bly increase, the worlds soil C storage, (C) 1998 Elsevier Science B,V , All rights reserved.