Phosphorus Fractions And Phosphate Adsorption-desorption Characteristics In The Soil Of Water-level-fluctuating Zone In Pengxi River,Three Gorges Reservoir

With regard to ecotone between the water and terrestrial ecosystems,the Water-Level-Fluctuating Zone（WLFZ）of the Three Gorges reservoir is an active area of nutrients,energy transfer and transformation in ecosystems.The“storing the clean and discharging the muddy”dispatching mode makes the soil of WLFZ undergo frequent wet-dry alternation causing the change of soil physical and chemical properties and environmental conditions in the WLFZ,which has an important effect on the phosphorus cycle in the soil.Due to the soil at different elevations undergoing the wet-dry alternation of different frequency,the soil exhibits different phosphorus forms distribution characteristics and adsorption-desorption characteristics,but so far there are few studies on the phosphorus in the soils at different elevations.Therefore,the comprehensive and systematic study of soil phosphorus forms distribution characteristics and adsorption-desorption characteristics at different elevations is of great significance to deeply understand the migration and transformation of phosphorus in the soil of the WLFZ.In this paper,the physical and chemical properties of the soil in the WLFZ were analyzed,the concentrations of different phosphorus fractions of the soil at different elevations and the soil before and after the flooding were investigated,the thermodynamics test of phosphorus isothermal adsorption of the soil was carried out to explore the effect of dry-wet alternation on the phosphorus adsorption characteristics of the soil.At last,the phosphorus release activity of soil under external phosphorus pollution was studied by Diffusive Gradients in Thin-films（DGT）.The results show:（1）The average concentration of total phosphorus（TP）in the soil of the WLFZ was 599.00 mg/kg.In addition,the TP and inorganic phosphorus（IP）concentrations both decreased with the decrease of altitude.The concentrations of active phosphorous（Ac-P）in the soil of the WLFZ was significantly less than that at the riverside where is located at the altitude of 180 m.Furthermore,Ac-P concentrations at the elevation of 155 m,145 m were closed to the Ac-P concentration of the sediment at the elevation of 140 m but lower than the concentration detected in the samples obtained at the altitude of 165 m.The distribution of Ac-P concentrations at different altitudes indicated that the frequent dry-wet alternations resulted in the loss of Ac-P in the soil.There was no significant change in soil phosphorus content in the WLFZ after flooding.Organic matter concentration positively correlated with the organic phosphate concentration,indicating that the input of organic matter affected the concentration of organic phosphate.（2）Compared with the riverside soil,the EPC of the soil in the WLFZ was higher,and it increased with the decrease of the elevation,and increased after flooding.The maximum adsorption amount of phosphorus(Q_max)in the WLFZ soil was greater than that of the soil at the riverside,and the Q_maxax of soil gradually increased with the decrease of the elevation.After flooding,the phosphorus adsorption capacity of soil changed little.After the phosphorus adsorption,the phosphorus adsorbed on soil was mainly transformed into active phosphorus forms such as EX-P,Al-P and Fe-P.The specific surface area test of soil particles showed that the specific surface area of the soil at the elevation of 155 m was significantly larger than that of soil at the elevation of 180m.XPS analysis showed that metal oxides played an important role in soil phosphorus adsorption.FTIR analysis showed that the hydroxyl on montmorillonite and CaCO₃ in soil participated in the process of phosphorus adsorption.The particle size,organic matter and Fe content of soil showed a significant positive correlation with EPC and Q_maxax of the soil,indicating that they would play an important role in the adsorption of phosphorus.Compared with the riverside soil,the WLFZ soil had a higher ratio of clay/silt particles,OM and Fe content,which led to the better adsorption property of the soil in the WLFZ.（3）The R value of the WLFZ soil was 0.106⁰.286,which mean that the soil belonged to the partially sustained soil.It showed that the desorption rate of phosphorus in the soil was lower than the consumption rate of phosphorus in the interstitial water.Compared with the riverside soil,The R value and the distribution coefficient（K_d）of the WLFZ soil was lower and the response time（T_c）was higher,indicating that the phosphorus release activity of the WLFZ soil was less than that of the riverside soil.Thus the WLFZ soil with strong adsorption capacity had smaller phosphorus release capacity.Compared with the riverside soil,the WLFZ soil had a better phosphorus buffer effect,the external phosphorus can be fixed to the WLFZ soil,which had a positive effect on reducing external P loading of the reservoir.