Phosphorus Fractions And Its Sorption-Release Characteristics Of The Wetland Soils In The Yellow River Delta

Soil samples were collected from the natural salt wetland and ecologically restored wetland with freshwater input in the Yellow River Delta National Nature Reserve. The distribution characteristics of soil phosphorus and its correlation with soil physicochemical properties beneath Phragmites australis community, Suaeda salsa community and bare land were monitored in different soil depths (0-10,10-20, 20-30,30-40,40-50 cm). Phosphorus adsorption-desorption characteristics of different soils were compared and analyzed. The risk of phosphorus release from surface soil was assessed by using phosphorus release risk index (ERI).Results demonstrated that the total phosphorus (TP) average contents was 541.58 mg/kg. The contents of TP in natural wetland soils were higher than restored wetland soils. Phragmites australis covered soils showed higher TP contents than Suaeda salsa covered soils and bare land soils. In all zones, different forms of phosphorus and the proportions of TP followed the order which was inorganic phosphorus (IP) (355.10 mg/kg,65.57%)> residual phosphorus (RP) (134.95 mg/kg,24.92%)> organic phosphorus (OP) (51.63 mg/kg,9.53%). There was no significant difference in the average contents of IP and RP between restored wetland and natural wetland, while OP contents were higher in natural wetland. The contents of IP and OP in soils from sampling sites with vegetation were higher than that of bare land. With increasing depth, the contents of IP and OP showed no significant difference in soils from most of sample sites, while the content of RP in soils decreased with increasing soil depth. Correlation analysis suggested that organic matter content, electric conductivity (salinity), soil pH as well as aggregate composition were major factors affecting soil phosphorus contents in wetland. The maximum phosphate sorption capacity of soil (Qmax) ranged from 139.40 mg/kg to 224.06 mg/kg and the maximal buffer capacity (MBC) ranged from 46.00-307.28. The Qmax and MBC of natural wetland soils were higher than restored wetland soils, and the Qmax and MBC of Phragmites australis covered soils were higher than Suaeda salsa covered soils and bare land soils. It showed that the natural wetland soils (especially Phragmites australis covered soils) had better buffering capacity of phosphorus. The phosphorus desorption rates of soils ranged from 3.34%to 16.85%, the phosphorus desorption capacity and rates of soils in sampling sites with vegetation were higher than bare land soils, the average desorption rate of phosphorus in natural wetland soils was higher than restored wetland soils. Correlation analysis suggested that the soil moisture and the content of active iron and active aluminum in soils were key factors influencing soil phosphorus adsorption. The results of ERI showed that the risk of eutrophication of water eutrophication in the Yellow River delta wetland was high (ERI> 25%). Phosphorus release risk of restored wetland soils was higher than natural wetland soils. Therefore, measures should be taken to improve the phosphate sorption capacity of soil and reduce the potential risks of wetland eutrophication in the Yellow River Delta, especially in the restored wetland.