Soil total phosphorus deposition and variability patterns across the floodplains of an Iowa river

Abstract

Phosphorus (P) is a major pollutant to freshwaters with large impacts on human and ecosystem health. The goal of this study was to evaluate soil total phosphorus (TP) deposition patterns across the active and higher elevation floodplains of a heavily agricultural watershed, along the longitudinal profile of the Turkey River (TR) in Iowa. Five field sites along the TR and a major tributary, Roberts Creek (RC), were identified to carry out this research. TP concentrations increased in a longitudinal direction from upstream to downstream (p < 0.05). A similar pattern was also identified for the fine particle (i.e., silt and clay) content in floodplain soils, as TP was tightly correlated to the amount of fine particles. The log-transformed TP concentration data were fitted with a mixture of two normal distributions. The investigation of the spatial TP concentration patterns across the floodplains' cross-sections (CSs) coupled with the site-specific flood inundation maps, indicated that TP deposition patterns were primarily governed by the flood characteristics (i.e., magnitude, frequency and duration), which accounts for the bimodal nature of the TP data. Active floodplain terraces, which are frequently flooded, are characterized by statistically lower TP concentrations in soils, compared to higher elevation floodplains (p < 0.05). Active floodplains should be considered as short-term TP storage locations, whereas higher elevation floodplains should be considered as long-term TP storage areas.