Spatial variations of soil phosphorus forms and the risks of phosphorus release in the water-level fluctuation zone in a tributary of the Three Gorges Reservoir

The dynamics of soil phosphorus (P) in the water-level fluctuation zone (WLFZ) generally poses a great threat to the ecology of dam-formed reservoirs worldwide. Our study explored the spatial variations of soil P forms and the risks of P release in the WLFZ in a tributary of the Three Gorges Reservoir (TGR) in the upper reaches of the Yangtze River, China. Soil samples from multiple altitudes (145, 155 and 165 m) in the WLFZ, upland soils (175 m) as well as reservoir sediment samples were collected along seven transects in a tributary of the TGR (Pengxi River) in August 2016. Hedley's fractionation method was used to characterize the P forms, and the physical and chemical properties of the samples were determined. The distributions of the total P (TP) across the transects decreased in the order of sediments (675.9 ± 124.8 mg/kg), upland soil (658.9 ± 191.1 mg/kg), and WLFZ soil (613.9 ± 100.7 mg/kg). Similar distributions of the bioavailable P (Bio-P) were observed. Longitudinally, the TP and Bio-P of the WLFZ soil gradually decreased from the estuary to the upstream of the tributary. The spatial variations of P in the WLFZ soil in the tributary of the TGR was influenced by the soil particle size distribution (PSD) that, in turn, was a result of joint effects of the anti-seasonal hydrological regime, rainfall erosion and mainstream backwater. Long-term flooding increased P release from soil and decreased the soil degree of P saturation (DPS) throughout the WLFZ, especially in the lower portion of the WLFZ. According to Water-P, molar Al:Fe ratio and DPS, the current risk of soil P release throughout the WLFZ are very low. Monitoring should be prioritized at an altitude of 165 m and in the estuary of the WLFZ considering the high DPS and/or low Al:Fe ratio in these areas.