Divergent patterns of soil phosphorus discharge from water-level fluctuation zone after full impoundment of Three Gorges Reservoir, China

Phosphorus (P) discharged from soils in the water-level fluctuation (WLF) zone becomes increasingly important to the water quality control of the Three Gorges Reservoir (TGR) as the decrease in P input from upstream reaches and point-source pollution. To investigate the amount of soil P discharge from the WLF zone since the full impoundment of the TGR in 2010, soil and sediment samples were collected along the altitudinal gradients (140, 150, 160, 170, and 180m above sea level) in three transects in the middle reaches of the TGR. Soil P composition was determined by a sequential extraction procedure. Different amounts of P discharge from the WLF zone were found among three soil types because of their difference in the initial P content before impoundment, with an order of yellow earth (171.1gm(-2)), fluvo-aquic soil (141.7gm(-2)), and purple soil (73.8gm(-2)). An altitudinal pattern of soil P discharge was observed with the maximum at the 170-m sites. The downward transport of exchangeable P and clay-bound P with runoff was the major path of the soil P discharge at the 170-m sites with a slope gradient >15 degrees. Considerable P discharge with erosion at the upper section of the WLF zone was facilitated by the longer exposure period compared with that at bottom section (150-m sites) because of the annual anti-seasonal impoundment-exposure cycles of the TGR. The transformation of Al/Fe-P and subsequent release to water was a main mechanism of the soil P discharge during the impoundment period. The altitudinal pattern of P discharge was a result of joint effects of slope gradient, soil P forms, and the anti-seasonal hydrological regime of the TGR. The results highlight the critical role of the upper section (165-175m) in controlling the P output from the WLF zone into the water of the TGR.