Effects Of Elevated CO₂,Phosphate-Solubilizing Enterobacter Sp. And Montmorillonite On Phosphorus Release In Soil And Phosphate Mineral

Phosphorus(P)is a crucial element for life,and therefore is critically important in maintaining the structure and functions of ecosystems.It has been estimated that there is a total of 8.4×108 Tg P on the Earth.However,most P exists in the insoluble forms of compounds with Fe,Al,Mg,Ca and their oxides.Silicates also limit the availability of P in soil.Plant growth is often limited due to lack of available P,particularly in the tropical regions.Additionally,large amounts of P entering aquatic ecosystems can cause environmental pollution(e.g.,eutrophication).Therefore,it is of great significance to understand the mechanisms that control phosphates solubility and P biogeochemistry for agricultural sustainability and environmental conservation.P release in soil is influenced by soil chemical,physical and biological properties.For example,soil respiration in the rhizosphere increases the concentration of CO2 in the microenvironment.Elevated atmospheric CO2 also increases plant root activities and soil CO2.The increased CO2 can create a weak acidic microenvironment which facilitates the dissolution of inorganic phosphate.In addition,phosphate-solubilizing bacteria(PSB)are able to promote the conversion from insoluble inorganic phosphates to available P via secreting organic acids.As an important inorganic component of complex soil,clay mineral is with high specific surface area and cation exchange capacity.Clay mineral hence affects the physical and chemical properties of soil and growth of microorganisms,and may further affect the P transformation.How elevated soil CO2,clay mineral and PSB individually or interactively affect transport and transformation of P is still unclear.In this study,we investigated effects of elevated CO2,PSB and montmorillonite on P release and transformation from apatite.Ca and P concentrations,organic acid secretion,pH,CO2 emission rate were analyzed.In addition,attenuated total reflection infrared spectrum(ATR-IR),X-ray diffraction(XRD)and scanning electron microscopy(SEM),transmission electron microscope(TEM)and modeling were also applied to explore the compound formation and morphology changes.Major results and conclusions are as follows:1.Elevated CO2 and saline ions added into the solution promoted the dissolution of phosphate mineral,likely by acidizing environment and enhancing ion exchange,respectively.The release of water-soluble P from phosphate under high CO2 and NaCl reached 2.14 mg L-1 which was around 50 times as high as that in ionic water.In the soil system,elevated CO2 promoted water-soluble P release in both saline-alkali soil and acidic red earth soil,and the effect was stronger in the saline-alkali soil.2.Phosphate-solubilizing Enterobacter sp.increased the release of P from natural apatite via reducing the pH of solution Pb2+ was used as a tracer to monitor released P(in the form stable pyromorphite)and Cl-was essential for the formation of pyromorphite.3.Phosphate-solubilizing Enterobacter sp.promoted the release of P via secreting formic acid to dissolve apatite into nanoparticles.Montmorillonite was able to adsorb Enterobacter sp.onto its surface and inhibited their growth and phosphate-dissolving ability.The maximum adsorption capacity of available P on montmorillonite is only 0.197 mg P g-1 montmorillonite.However,TEM analysis indicated the adsorption of abundant phosphate nanoparticles on montmorillonite.In summary,combination of clay mineral and phosphate-solubilizing Enterobacter sp.can significantly regulate P release from apatite and subsequent transformations or absorption.