Legacy Sediment Floodplains: Linking Soil Physical Characteristics to Phosphorus Mobility

Abstract

Legacy sediment floodplains (LSFs) are becoming more prevalent globally yet little is known about them. These relatively recent riparian landscapes often contain decades of accumulated soil nutrients and metals that may contribute to adjacent aquatic ecosystem degradation. LSFs form in the same space as natural riparian zones but they may not be functioning in the same capacity as biogeochemical transformers. Four studies were completed in an immature bottomland hardwood forest LSF located along the Illinois River in eastern OK. The studies were completed to gain a better understanding of LSF soil properties and soil phosphorus (P) dynamics of a forested LSF. The first study focused on characterizing the soils of a forested LSF. Soil samples (n=153) were collected, processed, and analyzed for multiple physical parameters including soil moisture content (SMC), soil organic matter content (SOM), bulk density, particle size distribution, and porosity. Infiltration tests were completed both in the field and the laboratory to determine saturated infiltration rates. The study revealed that soils were mainly silt loams. Sand% decreased and clay% increased with depth while silt% remained the same. Only silt% and clay% increased significantly with distance and only at 20-30 cm below ground surface (bgs). Both in-situ and benchtop infiltration rates were generally more rapid than those associated with clayey silts. The second study focused on LSF soil phosphorus distribution. The study aimed to quantify and visualize LSF soil P distribution both laterally and vertically, determine if P concentrations changed with depth or distance, and discern the interrelations between P forms and metals. Water-extractable P (WEP), Mehlich-3 P (M3P), total phosphorus (TP), and total metals concentrations were determined from soils (n = 128) collected from 44 locations throughout the site. WEP and M3P were tightly correlated at all depths and WEP concentrations were not significantly different between depths. TP increased with depth and was moderately correlated with WEP and M3P. The third study evaluated P release from LSF soils under flooded conditions by conducting two experiments. In the flooded soil column experiment, fresh soil cores were collected from four site locations. The experiment was separated between ponding and active flood trials. Throughout the experiment there was minimal variability in WEP release and minimal flux after 48 hours. The second experiment was a soil WEP serial extraction experiment. Twelve air-dried LSF soil samples were randomly selected for the serial extraction experiment. The soils underwent five serial WEP extractions to assess WEP concentration trends over the serial extraction. The experiment demonstrated that soils may maintain soil WEP concentrations over numerous flood events. The final study that investigated soil P sorption capacity of LSF soils. Soil samples (n=128) collected from throughout the research area were analyzed for WEP, M3P, and Mehlich-3 metals (M3-Al, M3-Ca, M3-Fe, M3-K, and M3-Mg). Analyte results were then used to calculate soil phosphorus saturation ratio (PSR) and soil phosphorus storage capacity (SPSC) using agricultural (PSR = 0.1) and environmental thresholds (PSR=0.14). Sorbed P was also measured for all soils as a phosphorus sorption index (PSI) to determine the remaining capacity of LSF soils to sorb P. Sorbed P, PSR, and SPSC results point towards the soils (0-30 cm bgs) of the Lake Frances LSF as not being a source but a sink for P. The research presented herein may assist with decision-making on legacy sediment management strategies. More specifically, it may assist water resource managers, environmental scientists, local and federal governments, and private stakeholders in developing best management practices particularly as it pertains to long-term LSF management after dam removal or reservoir depletion.