Effects Of Soil Iron Oxides On Inorganic Phosphorus Forms And Flooding Release In Pengxi River Basin Fluctuation Zone Of The Three Gorges Reservoir Area

Under the influence of water level adjustment in the Three Gorges Reservoir（TGR）,the soil in the water-level fluctuating zone（WLFZ）experienced periodic flooding and outcrops,and the soil redox environment also changed periodically.Iron oxide is one of the components of the soil in the WLFZ.It has the characteristics of variable surface charge,large number of hydroxyl sites and large specific surface area.It is easy to change with the change of the redox environment and has an important influence on the phosphorus form and adsorption and release of phosphorus in the WLFZ soil,but there is still no research on the effect of iron oxides on phosphorus in the WLFZ soil.Therefore,it is of great importance to study the influence of iron oxide on phosphorus in the WLFZ soil and to comprehensively understand the influencing factors of the phosphorus cycle in the WLFZ soil.The paper carried out indoor simulation experiments on the soil of the WLFZ of the Pengxi river basin in the TGR area and explored the effects of the addition of exogenous iron oxide and the removal of soil iron oxide on the phosphorus form,adsorption and desorption of the soil in the WLFZ,and analyzed the changes of iron oxide and phosphorus forms in the WLFZ soil under different redox flooding conditions,illustrate the effect of iron oxide on the form and release of phosphorus in the soil.The paper mainly obtains the following conclusions:（1）After the addition of exogenous iron oxide,the Olsen-P content in the WLFZ soil decreased by 12.89～56.69%.The Olsen-P content decreased with the increase of iron oxide addition,but the downward trend gradually slowed down.When the soil iron oxide content is high,more phosphorus is fixed by the iron oxide,and the soil phosphorus activity and release risk are reduced.After adding iron oxide,the total IP content of the soil in the WLFZ remains unchanged.Fe-P is positively correlated with the amount of iron oxide added,and the percentage of total IP increases with the increase of the added amount,because of the transformation of Ex-P,Al-P and Ca-P to Fe-P,it is negatively related to the amount of iron oxide added.The maximum phosphorus absorption Q_max and the maximum buffer capacity MBC are positively correlated with the amount of iron oxide added,indicating that the slope a of the desorption capacity is negatively correlated with the amount of iron oxide added.The increase of iron oxide content in the soil of the WLFZ zone can improve the adsorption capacity of soil to phosphorus,inhibit the desorption of phosphorus and reduce the risk of phosphorus release.The effect of exogenous iron oxide on soil phosphorus in the WLFZ reveals the difference in soil iron oxide content in the WLFZ of TGR,which can lead to different soil phosphorus forms and phosphorus adsorption capacity.（2）The removal of iron oxide has little effect on soil pH and OM in the WLFZ,and the contents of Fed and Feo decreased by 88.31%and 90.20%,respectively.After the removal of iron oxide,the iron oxide-bound phosphorus is released,which increases the soil Olsen-P content.Because the higher iron oxide content of the soil at the S6 site binds more phosphorus,the increase in Olsen-P is greater after the iron oxide removal.After removing iron oxide,the percentage of Fe-P and Oc-P in the soil decreased by4.66%and 10.92%due to the reduction of iron oxide.The iron oxide-bound phosphorus was released and then adsorbed,so the percentage of Ex-P and Ca-P increased by 1.68%and 14.74%respectively.After the removal of iron oxide,the reduction in iron oxide content caused a decrease in the phosphorus fixation capacity of the soil,resulting in a decrease in the soil phosphorus absorption intensity factor K,the maximum phosphorus adsorption amount Q_max and the maximum buffer capacity MBC of 66.7%,39.26%,and 79.8%,respectively.XRD analysis and specific surface area analysis of soil particles show that after removing iron oxide,the composition of other phases is basically unchanged except for the reduction of iron oxide in the soil of the WLFZ.The specific surface area of the soil in the WLFZ has decreased by 27.07%,hence the iron oxide affects the specific surface area of the WLFZ soil,and the important factor of phosphorus adsorption and desorption ability.After removing iron oxide,the soil phosphorus adsorption index PSI decreased by 73.5%,the phosphorus adsorption saturation DPS increased by 31.7%,the soil phosphorus loss risk increased,and it was easy to release into the overlying water body.（3）After the WLFZ soil was submerged in water,the Eh,pH,OM,Fed content in the soil decreased,while the Feo content increased.The change of soil physical and chemical properties brings the drop of Eh and pH of the overlying water.Compared with the result before submerging,the release of phosphorus in the soil of the WLFZ to the overlying water increased the TP content of the overlying water in the nitrogen-filled group by 0.14～0.20 mg/L,which was higher than the 0.08～0.12 mg/L in the nitrogen-free group.At the same time,the Fed content in the nitrogen-filled group decreased by 29.52%,a decrease greater than the 20.47%in the non-nitrogenated group,the Feo content increased by 10.07%,and the increase was less than the 47.42%in the non-nitrogenated group.After flooding,the crystalline iron convert to amorphous iron.The reduction of Fe³⁺oxides in the WLFZ soil in the reduced flooded environment reduced the Fe-P and Oc-P content of the nitrogen-filled group after flooding by 14.95%and 1.88%,and the decrease was greater than the 13.62%and 1.36%of the non-nitrogenated group.At the same time,the release of phosphorus combined with iron oxide increased the content of Olsen-P in the nitrogen-filled group by 15.68%,which was higher than that of the un-nitrogen-filled group by 13.28%.After submerging,soil phosphorus activity and release risk increase,and soil inorganic phosphorus forms changeCompared with the non-nitrogen filled group,the nitrogen-filled group has a stronger reduction under flooding environment.After submerging,the soil Fed,Olsen-P,and inorganic phosphorus forms change more,and the TP content released to the overlying water body is higher,revealing that the soils at low-altitude under long-term flooded submerged may have a greater risk of phosphorus release under flooded conditions due to the stronger reduction of the flooded environment.