First use of a compound-specific stable isotope (CSSI) technique to trace sediment transport in upland forest catchments of Chile.

Land degradation is a problem affecting the sustainability of commercial forest plantations. The identification of critical areas prone to erosion can assist this activity to better target soil conservation efforts. Here we present the first use of the carbon-13 signatures of fatty acids (C14 to C24) in soil samples for spatial and temporal tracing of sediment transport in river bodies of upland commercial forest catchments in Chile. This compound-specific stable isotope (CSSI) technique was tested as a fingerprinting approach to determine the degree of soil erosion in pre-harvested forest catchments with surface areas ranging from 12 to 40ha. For soil apportionment a mixing model based on a Bayesian inference framework was used (CSSIAR v.2.0). Approximately four potential sediment sources were used for the calculations of all of the selected catchments. Unpaved forestry roads were shown to be the main source of sediment deposited at the outlet of the catchments (30-75%). Furthermore, sampling along the stream channel demonstrated that sediments were mainly comprised of sediment coming from the unpaved roads in the upper part of the catchments (74-98%). From this it was possible to identify the location and type of primary land use contributing to the sediment delivered at the outlet of the catchments. The derived information will allow management to focus efforts to control or mitigate soil erosion by improving the runoff features of the forest roads. The use of this CSSI technique has a high potential to help forestry managers and decision makers to evaluate and mitigate sources of soil erosion in upland forest catchments. It is important to highlight that this technique can also be a good complement to other soil erosion assessment and geological fingerprinting techniques, especially when attempting to quantify (sediment loads) and differentiate which type of land use most contributes to sediment accumulation.