Transformation Of Phosphorus Speciation In A Calcareous Fluvo-aquic Soil And Effects Of Exchangeable Calcium On Phosphate Fixation

Phosphorus?P?,one of the macronutrients,is indispensable during crop growth and development.Most of the P uptaked by plants is derived from soils.Many factors,such as phosphate fertilizer application,organic matter,microorganisms,calcium carbonate,and exchangeable calcium all affect the speciation,transformation and availability of phosphorus in calcareous soils.In recent years,researches on the forms and transformation methods of phosphorus in soil have attracted much attention.However,the transformation process of phosphorus in calcareous fluvo-aquic soils and the effects of different forms of calcium on phosphorus fixation have not been well-recognized.Therefore,in this study,long-term farmland investigation and laboratory experiments were combined to study above questions.We preliminarily explored the form and transformation process of phosphorus in the calcareous fluvo-aquic soil and the effects of exchangeable calcium on phosphate fixation with inorganic phosphorus fractionation method,TEM and XANES.The main conclusions are described as follows:?1?Ca-P is dominant P pool in the calcareous fluvo-aquic soil.The proportion of Ca-P accounts for 72.4%of the total inorganic phosphorus,followed by the percentage of O-P.The proportion of Fe-P and Al-P is the lowest.In the composition of Ca-P,Ca₁₀-P is the most important form of Ca-P,accounting for 78.7%of Ca-P content,followed by Ca₈-P,accounting for 21.0%of Ca-P.Ca₂-P accounts for the least proportion,only 0.3% of Ca-P.It shows that phosphate fertilizer is mainly fixed by calcium in the calcareous fluvo-aquic soil,and the proportion of Ca₁₀-P is the largest,which reduces the availability of phosphate fertilizer.?2?As the amount of phosphate application increases,the percentage of Ca₂-P and Ca₈-P in the soil also increase,of which the proportion of Ca₂-P increases from 0.2%to 4.2%, and the proportion of Ca₈-P increases from 15.2%to 33.9%;the percentage content of Al-P and Fe-P are basically stable at 6%;the proportion of O-P basically maintains at 15%-20%.However,the percentage of Ca₁₀-P to the total amount of inorganic phosphorus is opposite to that of the other two forms of Ca-P,that is,as the amount of phosphate fertilizer increases,the percentage of Ca₁₀-P gradually decreases from 57.1% to 33.5%.?3?Ca₂-P,Ca₈-P,and Al-P are the main sources of available phosphorus in calcareous soil. After the application of phosphate,the three inorganic forms of phosphorous in the calcareous fluvo-aquic soil increase significantly,and the fixed phosphorus existed in the form of highly available inorganic phosphorus for a period of time,indicating that the application of phosphate fertilizer can increase the potential phosphorus supply capacity of the calcareous fluvo-aquic soil. ?4?Organic matter can activate stable Ca₁₀-P in the calcareous soil in a short time,so that Ca₁₀-P gradually transforms to Ca₂-P and Ca₈-P,increasing the content of available phosphorus.Moreover,decreasing microbial activity and weakening the mineralization of organic phosphorus can reduce the content of inorganic phosphorus in the soil.?5?Increasing the exchangeable calcium content can increase the adsorption capacity of montmorillonite and the calcareous soil for phosphate.The maximum adsorption capacity of montmorillonite for phosphate increases by 107%,and the maximum adsorption capacity of this calcareous soil for phosphate increases by 32%.After removing a certain amount of exchangeable calcium,the amount of phosphate adsorbed by the calcareous soil is significantly reduced by 24%-38%.It can be seen that under alkaline conditions,the activity of soil exchanged calcium is high,which promote the fixation of phosphate.The exchangeable calcium content in this soil accounts for about 11%of the total calcium content.Therefore,the exchangeable calcium in the calcareous fluvo-aquic soil plays an important role in the fixation of phosphate.