Distribution and Release Potential of Soil Phosphorus Fractions in Water-level Fluctuation Zone of the Tributary Bay, Three Gorges Reservoir

Abstract

After repeated wetting-drying cycles, the migration and transformation of phosphorus in the water-level fluctuation zone (WLFZ) are accelerated, and the eutrophication of tributary bays in the reservoir becomes increasingly serious. Soils in the WLFZ of a typical tributary of the Three Gorges Reservoir were collected, and then phosphorus forms and the degree of phosphorus saturation (DPS) were determined to analyze the phosphorus release potential. The results showed that, ① the average contents of total phosphorus (TP), inorganic phosphorus (IP), and organic phosphorus (OP) of soils in the WLFZ were 771.80, 485.33, and 166.30 mg·kg-1, respectively. The non-labile IP and OP were the dominate speciation of IP and OP, respectively. ② The distribution of P forms of soils in the WLFZ was affected by wetting-drying cycle. The contents of Ex-P and NaHCO3-Po of soils in the WLFZ were significantly higher than those in the contrasted soils, whereas the contents of Fe-P, HCl-Po, and Fulvic-Po decreased significantly along the elevation (P<0.05). The alternative wetting-drying cycle promoted the generation of labile P and the release and accumulation of moderately labile P of soils in the WLFZ. ③The content and proportion of bioavailable phosphorus (Bio-P) of soils in the WLFZ were in the range of 49.19-148.78 mg·kg-1 and 7.17%-24.78%, and the degree of phosphorus sorption (DPS) was in the range of 5.85%-22.00%. At present, the phosphorus release risk of soils in the WLFZ was low. The soil at 170 m elevation requires further attention. Fe-P, HCl-Po, and Fulvic-Po contributed significantly to phosphorus release. The increase in pH promoted the release of Fe-P. Alkaline phosphomonoesterase (ALP) was an important participant in the transformation of OP. Additionally, organic matter (OM) was the main source of OP.