Monitoring soil organic carbon erosion with isotopic tracers : two case studies on cultivated tropical catchments with steep slopes (Laos, Venezuela).

Soils of tropical regions account for ca. 30 % of the terrestrial organic carbon pool. Present-day fires, deforestation and land use changes enhance soil erosion and soil organic matter mineralization, increasing as much the CO2 flux to the atmosphere and reducing the efficiency of organic carbon storage in soils. This chapter presents the results of several isotopic studies carried out on two cultivated watersheds with steep slopes located in tropical regions (Venezuela, Laos) in order to better: 1) link the erosion status and the organic carbon content of cultivated soils by coupling 137Cs and total organic carbon measurements; 2) constraint the source of suspended organic matter during flood events by monitoring the composition of suspended loads in runoff and stream flows with d15N and d13C measurements and; 3) evaluate the impact of a water reservoir set up at the outlet of a watershed on suspended organic carbon fluxes generated by erosion. The redistribution pattern of fallout 137Cs and the soil organic carbon content in the first 30 - cm layer of cultivated soils of the Houay Pano catchment (Laos) are significantly linked. A common process apparently relates the soil erosion status with the amount of organic matter contained in the topsoil horizons. However, the budgets based on soil organic carbon depletion trends are higher than those derived from 137Cs soil erosion rate models. This difference provides evidence for additional organic carbon losses, most likely controlled by mineralization of soil organic matter and dissolved organic carbon release by runoff. The d15N composition of fine-sized suspended organic matter transported by runoff and stream flows during flood events on the Rio Boconó watershed (Venezuela) highlights the contribution of particulate organic carbon derived from incisive erosion of organic matter bearing rocks (“fossil” organic carbon) of the geological basement (i.e.: shales and argillaceous rocks of the river banks). The extent of “fossil” organic carbon release depends on the intensity of the flood event. During an intense event with a return period of 10 years, the contribution of “fossil” organic carbon to suspended sediments was approximately equivalent to that of soils sensu stricto Monitoring the composition of fine (<50 µm or