Phosphorus Characterization and Contribution from Eroding Streambank Soils of Vermont's Lake Champlain Basin

Streambank erosion is an important contributor to sediment and nutrient export. This study determined total P (TP), soil-test P (Modified Morgan's, MM-P), and the degree of P saturation (DPS) in eroding riparian soils along four Lake Champlain Basin stream corridors. We investigated the relationship between these data and soil texture and with a series of GIS-derived landscape metrics. We also quantified the potential P load from eroding streambanks using remote sensing. Soil samples were taken from 76 erosion features to a depth of 90 cm on four streams in Chittenden County, Vermont. Mean concentrations of TP and MM-P were similar among the watersheds and through depth. Neither TP, MM-P, nor DPS were well related to texture. Metrics from available spatial databases for parent material, soil series, and landscape position were somewhat useful in predicting TP and MM-P. Eroding streambank soil from 2004 through 2007 in the four streams was estimated to contain from 0.5 to 3.9 Mg of TP and 1.4 to 10.9 kg MM-P. The mean DPS in each watershed was <18% and, along with low MM-P concentrations, suggests that eroded streambanks may act as sinks rather than sources of P. The portion of total nonpoint P export potentially contributed by streambank erosion ranged from 6% in the stream with the lowest erosion rate to 30% in the stream with the highest erosion rate. Based on TP values, the P contribution of these streambanks could be considerable, and more information is needed on their actual contribution to bioavailable P in receiving waters.