Monitoring soil organic carbon erosion with δ13C and δ15N on experimental field plots in the Venezuelan Andes

The question of discriminating sources of organic matter in suspended sediment transported by streams can be addressed by using total organic carbon concentration and stable isotope (δ13C, δ15N) measurements when constant fluxes of organic matter supply can be assumed. However, little is known about the dynamics of organic matter release during soil erosion and the temporal stability of its isotopic composition. In this study, we have monitored soil organic carbon loss and water runoff using natural rainfall events during the 1998 rainy season on three experimental field plots with different vegetation covers (bare, maize and coffee plots), set up on slopes of a tropical mountainous watershed in NW Venezuela (Boconó watershed, 08°57′–09°31′N, 70°02′–70°34′W). Runoff and soil organic carbon losses were considerably higher for the bare field plot than for the cultivated field plots (270 and 2.2–21.0 kg C ha−1, respectively), and these findings are consistent with other soil erosion experiments. Total organic carbon concentrations, total nitrogen concentrations and the δ13C and δ15N composition of fine (<50 μm) size suspended sediments remain constant for high runoff periods (>30 ml s−1) and “high” suspended matter concentrations (>0.5 g l−1), closely reflecting the composition of soil organic matter from which they originate and integrating the high variability of organic carbon in top soil horizons. Because runoff and soil organic carbon loss are closely linked during most of the water flow (at the time scales covered by this study), the contribution of soil organic matter, originating from soils with different compositions, to suspended sediments in runoff can be derived using soil organic carbon erosion rates and stable (δ13C and δ15N) isotope measurements.