Transformation And Coupling Mechanism Of Phosphorus And Iron During Wet-dry Rotation In Acidic Soils In Southern China

As we all know,the strong binding of phosphorus(P)and minerals is widespread in nature,causing P surface adsorption or precipitation with the minerals.The highly weathered soil itself is a collection of many secondary minerals,which has a huge binding capacity.The phosphorus in southern soils is mostly adsorbed and fixed by iron/aluminum oxides,and it forms mineral-bound phosphorus.Through continuous coating by iron,which is further transformed into occluded phosphorus,then resulting in a significant reduction in the availability of phosphorus.In addition,the alternate drying-wetting rotation phenomenon is artificially formed in the rice paddy field rotation system in southern China.Under the alternate drying and wetting,the soil has undergone physical,chemical,and biological changes,which have a significant impact on the structure and function of the soil ecosystem.After the soil is flooded,a reduction effect occurs,which will decompose iron oxide and its bound phosphorus,leading to the release of part of phosphorus,and thus increasing the phosphorus supply level of the soil-plant system.However,since the content of iron oxide in various types of soils is not the same,it is not clear how iron oxide content affects the transformation of phosphorus form and the main control process of P transformation behavior during flooding.Therefore,it is essential to study the relationship between phosphorus and iron transformation in the alternating drying and wetting process,which has become an important research hotspot.This study took acid Yellow-Brown soil in the Yangtze River basin and red soil in southern China as the research object,carried out indoor incubation experiments under simulated drying and wetting alternation condition.Chemical Sequential Extraction of P,Diffusive Gradients in Thin-films technology(DGT technology)and X-ray Absorption Near-Edge Structure(XANES)spectroscopy were used to investigate the occurrence and transformation process of phosphorus in acidic soils with different iron oxide contents during the alternating drying and wetting process,as well as the fix and release of phosphorus and the changes in bioavailability during this process.The main conclusions are as follows:(1)Under the condition of soil flooding,p H increases and Eh decreases.The p H of Yellow-Brown soil and Red soil increased by 1.12 and 0.61 units respectively to the end of flooding,and tended to be neutral with the time of flooding.The Eh of the two soils first decreased and then increased to stability,and the flooded 56 d decreased by 34.5 m V and 20.15 m V,respectively.(2)The amorphous iron oxide content and iron activation degree of Yellow-Brown soil are higher than red soil,and the adsorption and phosphorus fixation capacity is stronger than red soil.Under flooded conditions,the generation of amorphous iron oxide in red soil is higher than that of Yellow-Brown soil and the adsorption capacity of phosphorus.Fe(II)of Yellow-Brown soil is higher than red soil,reduction effect is stronger than red soil,reducible iron phosphate releases Fe(II)and phosphate.During drying,decreased amorphous iron oxide and Fe(II)in both soils and decreased adsorption and reduction.(3)The available phosphorus during wet and dry rotation is affected by various environmental factors,the main factor of soil p H,Eh and iron oxide in soil.In Yellow-brown soil,with high phosphorus,The Olsen-P increased significantly.After drainage,Olsen-P did not necessarily decrease,increase of microbial phosphorus release or decomposition of organophosphorus mineralization to produce inorganic phosphorus.In Red soil,Olsen-P changes are not obvious,and may be associated with low P content or strong occluded.(4)Alternately dry and wet causes changes phosphorus forms in two soil.The phosphorus in Yellow-Brown soil is mainly Ca-P,Fe-P and O-P,decreasing Al-P by32.18% under alternating dry and wet conditions,Ca-P by 2.67%,O-P by 23.04% and Fe-P by 55.00%.Acid red soil is mainly O-P and Fe-P,with Al-P decrease of 68.92%,O-P increase of 37.05%,and Fe-P decrease of 15.99%.Fe-P increases significantly and O-P decreases,indicating the transformation process of O-P to Fe-P,low P/Fe is easier to generate O-P,so the formation of O-P is significantly related with the iron oxide content of soil.During drying,amorphous iron oxide decreased in both soils,but the crystallinity of iron oxide increased,O-P slightly increased in Yellow-Brown soil,and Fe-P significantly increased.In ed soil,O-P continued to increase,and Fe-P continued to decrease,indicating that drying was favorable to the formation of O-P,and high P/Fe in Yellow-Brown soil was favorable to the formation of Fe-P.(5)DGT results showed a continuous release of soluble active phosphorus in both soils and increased phosphorus availability during flooding.The release of phosphorus in high P/Fe is higher than low P/Fe.Iron bound phosphorus release with redox sensitivity,significantly increasing the phosphorus supply level of Yellow-Brown soil.In red soil,the P content decreases and the iron release increases,indicating that iron significantly affects phosphorus release and P/Fe is low,so the release process is controlled by the adsorption of iron oxide.The Linear Combination Fitting of XANES shows the trend of adsorption Fe-P to precipitation Fe-P during the whole flooding.It shows that the adsorption is greatly affected by reduction and redox sensitivity Fe-P always transforms to stable Fe-P.Two results indicate that the release of P exists concurrent with precipitation,a small amount due to reduced dissolution,but also that phosphorin remains a constant fixation process and associated with P/Fe.From the above conclusions,it is of great scientific significance to explore the transformation mechanism of iron oxide to phosphorus in the soil to improve the availability and utilization of phosphorus in the soil,to regulate agricultural non-point source pollution and to alleviate the pressure on phosphate rock resources.