Agricultural soil erosion is thought to perturb the global carbon cycle, but estimates of its effect range from a source of 1 petagram per year −1 to a sink of the same magnitude. By using caesium-137 and carbon inventory measurements from a large-scale survey, we found consistent evidence for an erosion-induced sink of atmospheric carbon equivalent to approximately 26% of the carbon transported by erosion. Based on this relationship, we estimated a global carbon sink of 0.12 (range 0.06 to 0.27) petagrams of carbon per year −1 resulting from erosion in the world's agricultural landscapes. Our analysis directly challenges the view that agricultural erosion represents an important source or sink for atmospheric CO 2 .

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Is the cover crop practice suitable for soil and water conservation in olive tree cropping? Rainfall, runoff, sediments, nutrient and organic carbon losses from 8 x60-m plots were measured during four hydrological years (2002 to 2007) in a field trial, in which two different soil management systems were used to confirm this hypothesis: a cover crop, (CC), and conventional tillage, (CT). The plots were located in a private olive tree farm on a sandy-loam soil, near Seville, southern Spain. The cover crop, as compared to conventional tillage, efficiently reduced runoff and sediment yield down to tolerable levels, 5.68% of the rainfall being converted to runoff, and the soil loss reaching 0.04 kg m-2 year-1 , as the average of four years. Additionally, in the cover crop treatment, the values of the nutrient export either dissolved in the runoff water or adsorbed in the sediment, were lower than the analogous values of the conventional tillage treatment: 0.631 and 0.065 kg m-2 year-1 of organic carbon and nitrogen, respectively, 0.175 and 0.0333 kg m-2 year-1 of soluble K and P, respectively, and 0.010 and 0.002 kg m-2 year-1 of available K and P, 2 respectively. The adoption of a cover crop as a soil management practice can be a feasible way to reach sustainability in many olive-cropped soils of southern Spain, although this method is not always easy to implement due to technical problems such as seed selection, its maintenance, and the choice of the correct killing date to avoid water competition. These difficulties could explain the slow rate of its adoption by many farmers. Further exploration of these aspects is required, as well as a specific agricultural extension campaign.

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Many field investigations have documented the existence of significant spatial variability of the saturated hydraulic conductivity Ks in typical catchments, and several modeling studies have examined the effects of this variability on Hortonian surface runoff. However, these modeling studies had limitations, and some could not deal adequately with possible runoff/run-on phenomena where areas of low saturated hydraulic conductivities K s are upstream of areas of higher K s . The kinematic wave equations with interactive Smith-Parlange infiltration are solved by a finite-difference scheme on a characteristic computational net. It is shown that runoff hydrographs are strongly affected by trends in hydraulic conductivity, particularly for small runoff events. The results also show that Smith and Hebbert [1979] mislabeled outflow hydrographs, resulting in incorrect conclusions about the effects of linear trends in K s. A simple technique is introduced to account for flow concentration in rills and the resulting reduced effective wetted areas subject to infiltration after rainfall ceases. It is shown that this mechanism has a major impact on runoff peaks, volumes, and time to peak for cases where Ks increases downslope.

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