Influence of geomorphic setting on distribution of nutrient stocks in lake Champlain basin floodplains

Spatial distribution of sediment and nutrient stocks in floodplains is of interest to environmental managers evaluating the source or sink role of riparian areas. Human disturbances within catchments and floodplains over recent centuries have altered natural sediment routing and storage patterns. We evaluated phosphorus (P) and carbon (C) content in floodplain soils of glacially-conditioned catchments of northwestern Vermont. Soil pit samples were excavated in years 2007 – 2010 at 60 floodplain locations distributed across 7 watersheds ranging in area from 8.0 to 2,230 km2. Pedon site samples (n= 347) were collected from multiple horizons up to depths of 240 cm and analyzed for texture, nutrients, major elements and extractable cations. Floodplain textural composition was not well predicted by mapped soil series, suggesting the need for soil mapping at more detailed scales and extended depths in these dynamic settings. Total P (< 2 mm fraction) ranged from 69 to 1,874 mg/kg, with a mean of 690 mg/kg. Potentially-desorbable P, measured as oxalate-extractable P (P_ox), ranged from 13.3 to 1,078 mg/kg, with a mean of 270 mg/kg. Total P concentration and the P_ox/TP ratio were each positively correlated with the percentage of combined clay and silt (< 50 µm, r = 0.697, p<0.001). P_ox/TP ratio was also positively correlated with aluminum, iron and magnesium (p<0.001). C content ranged from 0.04 to 38 % wt. Covariance of log-transformed C with other constituents was generally insignificant, except for N (r = 0.730) and log-transformed P_ox and Fe_ox (r = 0.347 and 0.415, respectively). A subset of 47 pedon sites in similar valley settings (unconfined, gradients < 1%) was further examined to evaluate effects of channel-floodplain (dis)connectivity on sediment texture and nutrient characteristics. Results suggest that percentage of fines, and concentrations of P, P_ox, and C are somewhat elevated in floodplains which are vertically well connected to the channel, relative to floodplains that have become disconnected from the channel as a result of historic incision. Ongoing analyses incorporating additional geomorphic, land cover, and fluvial connectivity metrics are expected to reveal additional factors that contribute to spatial variability of sediment and nutrient characteristics in floodplains.