Impact of deposition on the enrichment of organic carbon in eroded sediment

A substantial part of eroded material can be deposited along the runoff pathway. This deposition process may alter the composition of the transported material. Topography- and vegetation-induced deposition processes were examined under laboratory conditions and at the hillslope and watershed scale. The laboratory experiments showed that the enrichment ratio of the specific surface area, ER_SSA, of the transported sediment followed an exponential increase with decreasing sediment delivery ratio, SDR, regardless of the type of deposition process. However, the increase in ER_SSA with decreasing SDR values was lower than expected. The upper limit of the ER_SSA was estimated to be 1.66, which is much lower than the calculated theoretical upper limit of 5.22. This difference can be attributed to the transport of the eroded material in micro-aggregated form. It was also found that the specific surface area, SSA, is a good predictor of organic carbon, OC. The observations on field plots confirmed the results of the laboratory experiments. Measurements at the watershed level indicated that the intensity of the erosion process had a more important influence on sediment enrichment, while the impact of deposition tended to be rather limited. However, sediment monitoring over a longer period is required to reveal the importance of the different erosion processes with regard to OC losses at the field and watershed level.