Effects And Mechanism Of Exchangeable Calcium On Phosphorus Fixation And Transformation In Calcareous Fluvo-aquic Soils

Phosphorus is one of the irreplaceable essential nutrient elements in the whole growth cycle of crops.However,a large number of phosphorus-fixing substrates in calcareous soil make soil phosphorus easy to be fixed,which limits the absorption and utilization of phosphorus by crops.Therefore,the effects of different forms of calcium in soil on the fixation and transformation of phosphorus in calcareous fluvo-aquic soil were studied,and then the resistance control pathway of soil phosphorus fixation was explored,which provided a theoretical basis for regulating soil calcium behavior and improving the utilization rate of phosphorus fertilizer and mobilizing soil phosphorus pool.In this paper,combining with the dynamic experiment of simulated soil and indoor constant temperature soil incubation with exogenous complexing agent,the effects of exchangeable calcium on phosphorus fixation and distribution and the effects of exogenous complexing agents on soil calcium transformation,soil phosphorus fixation,availability and morphology,and related mechanisms were investigated by inorganic phosphorus fractionation scheme,X-ray Near-edge Absorption Structure Spectroscopy technique（XANES）,scanning electron microscopy（SEM）and infrared spectroscopy（IR）,The main conclusions are as follows:1、In the single mineral system of calcium saturated montmorillonite and calcite,the effect of exchangeable calcium on phosphate adsorption and fixation was greater than that of calcite at the early stage of the reaction,and calcite was the dominant contributors for the inefficiency of phosphate in the system at the later stage of the reaction.The kinetic experiments of the single mineral system showed that when the initial phosphorus concentration was 200 mg/L,the montmorillonite treated by calcium saturation reached the adsorption equilibrium rapidly within 30 min,and the calcite reacted slowly.After 2 h,the phosphorus fixation rate of calcite was accelerated,and the phosphorus absorption amount was more than that of calcium-saturated montmorillonite.The phosphorus absorption amount after reaction equilibrium was 6.5 times that of calcium-saturated montmorillonite treatment.2、In the mixed mineral system of calcium saturated montmorillonite and calcite,phosphate preferentially reacts with exchangeable calcium,and the chemical precipitation process plays an important role and produces more hydroxyapatite,which accelerates the fixation process of phosphate.In the kinetic experiment,the phosphorus uptake of calcium-saturated montmorillonite and calcite system was 3-4 times that of potassium-saturated montmorillonite and calcite system,and the reaction was violent again after 2 hours.But the potassium-saturated montmorillonite and calcite system showed an increase in phosphorus uptake after 8 hours.It can be preliminarily seen from the SEM surface scan that the calcium-phosphorus phase mainly appeared in the area of montmorillonite before the reaction 2 h.The LCF fitting results of XANES spectra showed that after 2 h of reaction,the relative contents of adsorbed phosphorus on the surface of calcite in the three mixed systems with K-Mont and Ca CO₃ ratio of 3,Ca-Mont and Ca CO₃ ratio of 3 and Ca-Mont and Ca CO₃ ratio of 6 were 55.54%,28.34% and21.29%,respectively,and the relative contents of hydroxyapatite were 19.90%,69.17%and 77.52%,respectively.After 12 h of reaction,the relative contents of Ca-P phases in the three mixed systems were 82.48%,91.35% and 83.31%,respectively.It shows that the activity of calcium ions on the surface of montmorillonite is higher,and more apatite is formed by combining with phosphate faster.The presence of exchangeable calcium increases the amount of phosphorus fixation in the system.With the extension of reaction time,the phosphorus fixation of calcite is enhanced.3、After the calcareous fluvo-aquic soil was cultured by exogenous EDTA,the soil exchangeable calcium was transformed into complexed calcium,the content of soil active calcium decreased,the amount of soil phosphorus fixation decreased,and the high concentration（0.3 mol/L）EDTA culture could significantly reduce the soil phosphorus fixation ability in the early stage of culture（before 10 days）,and the content of available phosphorus increased significantly with the increase of EDTA concentration.After high concentration（0.3 mol/L）EDTA culture,the phosphorus fixation capacity of the soil could decrease by 11.49%-30.75% compared with the blank and the available phosphorus could increase by 13.10 mg/kg compared with the blank treatment.However,with the culture time more than 10 days,the phosphorus fixation ability of soil increased and gradually approached the blank,while high concentration EDTA treatment still maintain high available phosphorus content.4、Exogenous EDTA can promote the transformation of ineffective Ca₁₀-P to active and slow-acting Ca₂-P and Ca8-P in calcareous fluvo-aquic soil.At the same time,the content of iron phosphorus and aluminum phosphorus in soil decreased gradually in the later stage of culture.Ca₂-P increased by 44%-79.28%,13.90%-29.51% and2.30%-24.29% at 0.3 mol/L,0.15 mol/L and 0.075 mol/L concentrations of EDTA,respectively.The results of synchrotron radiation fitting also showed that the higher the concentration of EDTA,the more the amount of Ca₁₀-P converted to Ca₂-P.Therefore,exchangeable calcium plays an important role in the process of soil phosphorus fixation.In agronomic measures,the form of soil calcium can be regulated by increasing complexing agents according to the needs,so as to realize the ineffective control of phosphorus.