Phosphorus Frcations Movement Characteristics Of Typical Soils In The Upper Reaches Of The Minjiang River,China

Phosphorus?P?plays an important role in the growth of plants,but it is also one of the main factors of water eutrophication.Its concentrations and distribution characteristics in soils can not only reflect the geochemical information,but also show the migration characteristics.In recent years,there has been an increasing interest in the non-point source pollution caused by the movement of soil P.The movement of soil particles to the water body will reduce the accumulation of soil P and increase the burden on the water environment.P transport is mainly dominated by surface migration,which it flows into the receiving water body along with slope runoff and sediment.The fractions and fixation capacities of soil phosphorus in six kinds of typical soils,including paddy soil?PS?,fluvo-aquic soil?FS?,cinnamon soil?CS?,yellow-brown earth?YE?,dark-brown earth?DE?and subalpine-meadow soil?SS?,therefore,were assessed in this study to explore the characteristics of P movement in the upper reaches of Minjiang River.The main research results were as follows:?1?The soil NaOH-P and HCl-P were the main fraction in soils,with an average concentration of 280.57 mg kg^-1 and 172.66 mg kg^-1,respectively.Altitude dramatically affects the concentrations of soil P in the study area.The concentrations of NaOH-P in soil increased considerably with the altitude increasing?P<0.05?,while the concentrations of HCl-P decreased conspicuously with the altitude increasing?P<0.01?.The climate dramatically affects the concentrations of soil P in the study area.As the temperature increasing,the concentrations of NaOH-P reduced significantly?P<0.05?,while the concentrations of HCl-P enhanced conspicuously?P<0.01?.With the increasing precipitation,the concentrations of NaOH-P increased firstly and then decreased gradually,while the HCl-P concentrations decreased firstly and then increased gradually.The parent material is important for the distribution of soil P fractions in the upper reaches of Minjiang River.In lake sediments,the concentrations of NaOH-P were remarkably higher than those in other parent materials?P<0.05?,while in limestone the concentrations of HCl-P were the highest?P<0.05?.The change of land use considerably affects the concentrations of soil P.When the land use pattern changed from cultivated land to forest land or grassland,the NaOH-P concentrations gradually accumulated?P<0.05?.Therefore,the impact factors including altitude,temperature and precipitation,parent material and land use have an influence on the concentraion of soil phosphorus frations,but the mechanism of the impact remains to be further studied.?2?The Langmuir,Freundlich and Temkin models could better to describe the isothermal adsorption process of soil P in the study area.The maximum adsorption amounts of P obtained by Langmuir model in PS,FS,CS,YE,DS and SS were 576.61 mg kg^-1,730.92 mg kg^-1,339.88 mg kg^-1,470.26 mg kg^-1,1012.89 mg kg^-1 and 821.82 mg kg^-1,respectively.The adsorption kinetics experimets showed that the amount of P adsorbed by soil exceeded 50%of the total amount of adsorption within 2 h,and it exceeded 80%of total adsorption amout within 8 h.The rapid adsorption occured within the initial 2 hours,and the 2-8 h time periods are summarized as slow adsorption stage,while the 8-24 h time periods are summarized as balance phase.Therefore,the adsorption process of P in typical soils in the upper reaches of Minjiang River fits well both the quasi-first-order kinetic equation and the quasi-second-order kinetic equation,but the latter is better with a higher R².In addition,this study also shows that the typical soils in this region have a greater ability to buffer exogenous P.?3?The maximum buffer capacity value of soil P?PMBC?indicated that the ability of buffering exogenous P in PS,CS,and YE in the study area was weaker than that in FS,DS,and SS.The value of equilibrium phosphorus concentration at zero sorption of soil P?PEPC₀?ranged from 17.05 to 25.68 mg L^-1,which was much larger than the eutrophication threshold of the water body.The values of phosphorus sorption index?PSI?in PS,CS,and YE were lower than the critical value of 25,indicating that the risk of P movement in PS,CS,and YE was higher,which may indicate the tidal soil and dark brown soil in this area.And the loss of migration of subalpine meadow soil was smaller.Correlation analysis showed that the P movement risk in PS,CS and YE were higher than those in FS,DS,and SS.Principal Component Analysis indicated that the risk assessment factor could be summarized as buffered exogenous phosphorus intensity factor and water eutrophication potential factor.Redundancy analysis further suggested that NaHCO₃-P and NaOH-P play an important role in enhancing the ability of DS and SS to buffer exogenous P.In conclusion,the risk of P movement in PS,CS and YE at the soil-water interface was higher than that in FS,DS,and SS in the upper reaches of the Minjiang River.And in the absence of serious soil erosion,the source of P enrichment in water is not mainly from the soil P and P fertilizer application.Furthermore,the H₂O-P and NaHCO₃-P in those soils were easily lost during the soil erosion process.When the soil particles finally enter the water body,NaOH-P and HCl-P will act as P sources for eutrophication of the water body.