Effect Of Combined Application Of Biochar And Phosphate Fertilizer On Phosphorus Transformation In Saline-alkali Soil And Its Microbiological Mechanism

Soil salinization seriously affects agricultural production and ecological environment health,and the low utilization efficiency of phosphorus in saline-alkali soil is the key problem.Biochar is often used as a soil amendment to improve soil quality,however,how the interaction between biochar and phosphate fertilizer affects soil phosphorus availability and its related mechanisms are still unclear.In this study,biochar was prepared from waste Phragmites australis.The optimum preparation conditions was screened and applied to saline-alkali soil improvement.The combined effects of biochar with different types of phosphate fertilizers on the content and transformation of soil phosphorus fractions were investigated,and the microbial mechanisms of the effect of combined application of biochar and phosphate fertilizer on the transformation of phosphorus fractions were revealed.The results can provide a scientific basis for the efficient utilization of phosphorus in saline-alkali soil and provide a reference for the application of combined application of biochar and phosphate fertilizer.The main results of this research were as follows:Phragmites australis-derived biochar（RBC）at different temperatures,analyzed its physical were prepared and the chemical characteristics of RBC were investigated.The improvement effect of RBC on saline-alkali soil was evaluated through pot experiments.The results showed thatthe yield and H/C ratio of RBC decreased gradually with the increase of pyrolysis temperature,and the specific surface area and pore size increased.The application of biochar improved soil structure and reduced soil bulk density.RBC can improve soil nutrients and promote nutrient cycling,increase the content of available phosphorus,alkaline hydrolysis nitrogen,available potassium,organic matter and microbial biomass carbon,and promote urease activity and sucrase activity.Meanwhile,RBC highly increased plant growth and crop yield of Brassica chinensis L..Principal component analysis and soil quality index evaluation showed that RBC significantly improved the quality of saline-alkali soil,and RBC which was prepared at 400℃（RBC 400）possessed the optimum improvement effect on saline-alkali soil.Correlation analysis showed that the positive correlation between crop yield and available phosphorus was the strongest,in which the correlation coefficient was0.823.In order to explore the effect of combined application of biochar and phosphate fertilizer on the content and transformation of phosphorus fractions in saline-alkali soil,RBC 400 was chosed to mix with soluble phosphate fertilizer,insoluble phosphate fertilizer and organic phosphate fertilizer to cultivate saline-alkali soil.The results showed that RBC 400 could promote the activation of phosphorus and improve phosphorus availability in saline-alkali soil.The combined application of RBC 400 and phosphate fertilizer significantly increased the content of available phosphorus（AP）,total phosphorus（TP）,organic phosphorus（OP）and inorganic phosphorus（IP）in saline-alkali soil,and increased the soil phosphorus activation coefficient（PAC）.At the same time,the combined application of RBC 400 and phosphate fertilizer significantly increased the contents of labile phosphorus,moderately labile phosphorus such as dicalcium phosphate（Ca₂-P）,octacalcium phosphate（Ca₈-P）,aluminum phosphate（Al-P）,labile organic phosphorus（LOP）,moderately labile organic phosphorus（MLOP）and moderately resistant organic phosphorus（MROP）.The combination of RBC 400and phosphate fertilizer decreased the contents of resistant phosphorus such as hydroxyapatite(Ca₁₀-P)and highly resistant organic phosphorus（HROP）.Correlation analysis showed that available phosphorus possessed the strongest correlation with Ca₈-P.Path analysis showed that RBC 400 could accelerate the solubilization of inorganic phosphorus and the decomposition of organic phosphorus,reduce the content of fixed phosphorus,and increase the contents of slowly effective phosphorus and effective phosphorus.The results also showed that Fe-P and MROP directly affected available phosphorus in saline-alkali soil of combined application of biochar and phosphate fertilizer,and the reduction of the content of both contributed to the accumulation of available phosphorus in soil;LOP and Fe-P were the main important determine factors and main limiting factors,increasing LOP content and limiting Fe-P content could effectively promote soil available phosphorus content.The effects of combined application of RBC 400 and phosphate fertilizer on enzyme activity,bacterial community,fungi community and pho D functional microbial communities in saline-alkali soil were explored,and the microbiological mechanisms of phosphorus transformation were analyzed.The results showed that the combined application of biochar and different types of phosphorus fertilizers had different effects on phosphatase activity.Compared with the corresponding single application of phosphate fertilizer,the combined application of biochar and soluble phosphate fertilizer（BK）reduced the activity of acid phosphatase（ACP）and neutral phosphatase（NEP）,and increased the activity of alkaline phosphatase（ALP）.The combined application of RBC 400 and insoluble phosphate fertilizer（BC）reduced the activity of ACP,ALP and NEP,and the combined application of RBC 400 and organic phosphate fertilizer（BM）increased the activity of ACP and NEP and reduced the activity of ALP.The combined application of RBC 400 and phosphorus fertilizer affected the community structure and abundance of fungi,bacteria and pho D functional microorganisms.BK and BC increased the abundance of Basidiomycota,Mortierellomycota and Proteobacteria,and decreased the abundance of Firmicutes and Bacteroidota.BM increased the abundance of Basidiomycota,Mortierellomycota and Firmicutes,and decreased the abundance of Bacteroidota and Proteobacteria.The correlation analysis between the different species of pho D functional microbial community and different phosphorus fractions showed that Rhodopseudomonas was significantly correlated with many phosphorus fractions,positively correlated with Ca₂-P,Ca₈-P,Al-P,LOP,MLOP and MROP,and negatively correlated with Ca₁₀-P,and RBC may change the abundance of Rhodopseudomonas and affect the phosphorus cycle.The prediction of bacterial microbial function showed that the combined application of biochar and phosphate fertilizer could enhance the function of nitrate ammonification,nitrite ammonification and aromatic compound degradation in soil,promote carbon cycle and nitrogen cycle,and improve the ability to inhibit pathogenic microorganisms.