Effectiveness of soil in vegetated buffers to retain nutrients and sediment transported by concentrated runoff through deep gullies

Little research has evaluated naturally vegetated buffers to retain pollutants in soil from concentrated runoff through deep (2-14 m) gullies. Soil enrichment in the flow path of 11 naturally vegetated gullies in southern Alberta, Canada, was used as a long-term signature of filtering during concentrated flow. Soil was sampled at three depth intervals (0-2.5, 2.5-5, and 5-10 cm) along two 50-m transects inside and outside the flow path of the vegetated gullies in each of 3 yr (2011-2013). The influence of soil type, flow path (inside vs. outside), distance into vegetated flow path, depth, and their interactions on enrichment of nutrients (NH4-N, NO3-N, soil test P (STP), total P) and particle size fractions (clay, silt, and sand) was determined. Significantly (P ≤ 0.05) greater enrichment of nutrients and specific particle size fractions inside than outside the flow path of the vegetated gully suggested that greater deposition occurred inside the concentrated flow path. In contrast, there was little evidence for enrichment of nutrients and sediment at the front or inlet of the buffer (except STP), or for infiltration of more soluble nutrients into the subsoil. Soil enrichment in buffers may reveal long-term filtering processes that may not be shown with short-term runoff experiments.