Study On Interaction Between Biochar-based Phosphate Fertilizer And Phosphorus-dissolving Bacteria RS-198 And Its Effect On Transformation Of Released Phosphorus

Biochar-based phosphate fertilizers（BPFs）are prepared by pyrolysis of biomass and phosphorus containing minerals.They have broad application prospects because of their strong sustainability and high nutrient utilization.However,the traditional pyrolysis method has high energy consumption and cannot control the nutrient content in BPFs.In addition,the prepared BPFs contain high ineffective phosphorus,such as precipitated phosphorus and organic phosphorus.In this study,BPFs with controllable nutrient structure were prepared by using cotton stalk（CS）,K₃PO₄and Mg O as pyrolysis raw materials and microwave radiation as pyrolysis mode.Then,biochar-based insoluble-phosphate fertilizer（NBPF）was prepared by regulating the proportion of cotton stalk,K₃PO₄and Mg O,and combined with phosphorus-dissolving bacterium Enterobacter hormaechei RS-198 to explore the mechanism of RS-198absorption and transformation to phosphorus in NBPF and enhance the effectiveness of precipitated phosphorus and organic phosphorus in NBPF.The specific results are as follows:1)BPFs was prepared by microwave co-pyrolysis of CS,K₃PO₄and Mg O.By measuring the heating curve and calculating the activation energy under different conversion rates of BPFs,it is found that K₃PO₄and Mg₂P₂O₇,Mg₃（PO₄）₂and Mg HPO₄reduced the activation energy of pyrolysis reaction and accelerate the reaction rate.Therefore,BPFs were synthesized within 15 min.The coats-Redfern method was used to speculate the possible reaction between K₃PO₄,Mg O and CS during pyrolysis.It was concluded that the reaction between K₃PO₄and CS and Mg O made the synthesized BPFs had multi-level nutritional structure,including water-soluble P,precipitated P and organic P.The content of these three forms of phosphorus could be regulated by controlling the proportion of K₃PO₄and Mg O.Combined with slow-release test and spectroscopy,it was concluded that the multi-level nutritional structure was released in the order of water-soluble phosphorus>precipitated phosphorus>organic phosphorus.The addition of Mg O also improved the pore structure and zeta potential of BPFs to capture precipitated phosphorus and adsorbed water-soluble phosphorus,which prolonged the release period of P.2)NBPF prepared with the mass ratio of CS,K₃PO₄and Mg O of 1:0.5:0.5 had a high proportion of precipitated phosphorus（65.9%）and organic phosphorus（24.3%）.Precipitated phosphorus was attached to the surface of NBPF to make it rough,and micro-level macropores in NBPF provide rich sites for the attachment of RS-198.In addition,the low content of free radicals in NBPF had no toxic effect on RS-198,and its high content of liable carbon accelerated the colonization rate of RS-198.RS-198 was well attached to the surface of NBPF and activates the precipitated phosphorus and organic phosphorus in NBPF.Then,RS-198 stored activated P in the form of Poly P and released under the condition of external phosphorus starvation,which transformed and released the phosphorus in NBPF.3)NBPF increased the fixation of soil to RS-198 because of the strong van der Waals and electrostatic forces between NBPF and RS-198.RS-198 dissolved precipitated phosphorus and mineralized organic phosphorus by secreting organic acid and soil alkaline phosphatase in co-culture experiment of NBPF and RS-198.Therefore,the content of soil available phosphorus gradually increased in the first 60 days of co-culture.However,the soil available phosphorus content decreased from 60 to 90 days of co-culture.Through the fractionation extraction of phosphorus in soil,solid ³¹P NMR analysis of soil samples and scanning electron microscope observation of separated NBPF,it was found that:（1）RS-198 absorbed and stored soil available phosphorus for its own metabolism;（2）RS-198 formed biofilm on the surface of NBPF,slowed down the outward diffusion of P in NBPF,and adsorbed excess available phosphorus in soil.Therefore,co-pyrolysis of biomass and phosphorus minerals by microwave radiation can prepare biochar-based phosphate fertilizer with controllable nutrient content.In addition,loading phosphorus-dissolving microorganisms on biochar-based phosphate fertilizer can improve the effectiveness of phosphorus nutrients.