Factors affecting the effectiveness of riparian buffers in retaining sediment: an isotopic approach

Riparian forest width is a major driver of their capacity to retain sediments from agricultural fields. However, the relationship between forest width and ecosystem service provisioning may vary with local environmental conditions such as relief, soil, and vegetation types. In order to assess the effect of forest width, slope, hydraulic conductivity, and land cover (watershed scale) on the effectiveness of riparian buffers in retaining sediment from pastures cultivated with African C-4 grasses, we used the natural abundance of carbon stable isotopes (delta C-13) in the soil and stream organic sediments as indicators. The study was conducted in small streams of the upper Corumba River basin, state of Goias (Cerrado biome), Brazil. We found that slight increases from 2 to 5% mean slope were sufficient to change SOM to a mixture of C-3 and C-4 carbon sources inside the riparian forests. Therefore, hillslope's steepness and magnitude control soil transport downslope, but after reaching the riparian forest, sediment retention is strongly affected by the forest width. We also found that soil erosion leads to fine sediment deposition in agricultural streams, especially in those watersheds with a high occurrence of degraded pastures. We conclude that sites along the stream course with a combination of steep slopes, narrow forests, and intensive land use are the most vulnerable to sediment inputs and should be the focus of preservation and restoration by landscape managers.