Organic carbon enrichment in sediments: Effects of rainfall characteristics under different land uses in a Mediterranean area

Abstract: The results of an experiment to evaluate the effect of rainfall characteristics on organic carbon (OC) losses and on the type of particles mobilised by erosion under natural rainfall and under different land uses (non-disturbed forested area, and a non-irrigated olive cropland) at plot scale are presented. Labile (particulate organic carbon, POC) and stable (mineral associated organic carbon, MOC) carbon pools were measured in soil and sediments. Based on the product of total rainfall and I30 (mmmmh−1), as a measure of rainfall erosivity, events were divided into three classes (from low to high erosivity). A positive correlation between P*I30 and OC concentration (r=0.54, p<0.01) and P*I30 and OC enrichment ratios (r=0.43, p<0.05) was observed for the forest plot, while no correlation and even a negative trend between both variables were observed in the olive plot. These opposite responses are due to the effect that vegetation cover had on aggregate soil stability and on OC mobilisation and transport through the plot. With high intensity storms the high sediment removal in the olive plot led to subsoil and topsoil becoming mixed reducing the overall nutrient and OC concentration of the eroded sediment. Enrichment ratios of OC (EROC) were higher in the olive than in the forest plot in events of low rainfall erosivity (class 1), which represented almost 50% of the total events occurring in this area, while the differences between plots as regards the total mobilised OC were greater after the high rainfall erosivity events, until up to three times more carbon being mobilised in the olive plot compared with the forest one. POC transport in sediments was higher in the olive plots and was more likely to occur in events of low intensity. MOC was transported in high intensity events: in aggregate form with a greater probability of being sequestered in the sediment (aggregates smaller than 20μm) or in particulate form (after the rupture of aggregates) with a greater probability of mineralisation. [Copyright &y& Elsevier]