Preparation And Properties Of Calcium Nitrogen Modified Biochar Based Slow Release Phosphate Fertilizer

Conventional fertilisers release nutrients quickly,especially nitrogen/phosphorus nutrients,which are easily volatilised,leached and fixed,resulting in low use efficiency.Excessive fertiliser application not only increases agricultural production costs and wastes resources,but also causes soil,water and air pollution.Biochar is a porous,aromatised,carbon-rich material obtained by pyrolytic carbonisation of biomass materials such as crop straw and livestock manure under anaerobic or anoxic conditions,which has good adsorption capacity and great potential for saving fertiliser and improving efficiency.This study focuses on the technical problems of low loading capacity and poor bioeffectiveness of loaded nutrients in the existing engineered biochar for phosphorus nutrient enrichment,and systematically investigates the regulation method,phosphorus adsorption and slow release mechanism of calcium and nitrogen modified biochar and its effects on the growth and development quality of tomato seedlings.The study covers the following topics:Firstly,six types of biochar were prepared using bamboo chips as biochar precursors,KHCO₃ as green activator,Ca CO₃ as calcium source and urea as nitrogen source by the methods of"direct doping"and"preliminary carbonization-KHCO₃ impregnation Ca/N co-carbonization".This study examined the growth law and competition coupling relationship of spatial pores,surface groups,and carbon skeleton defects through characterization,both before and after biochar modification,in a multi-scale structure.The results showed that Ca doping is mainly due to the oxidation of the carbon skeleton by Ca CO₃ and the ligand bonding between Ca and the oxidised carbon skeleton on this basis.N doping is mainly because N atoms participate in the carbon skeleton reconstruction,forming four typical N-bond configurations（pyridine nitrogen,pyrrole nitrogen,graphitic nitrogen,and nitrogen oxide）.Activation brings pore expansion and the resulting carbon skeleton distortion,which has a certain effect on the reorganization and oxidation of the carbon skeleton,and can significantly affect the pore and carbon skeleton structure.Secondly,the enrichment behavior of biochar-based carriers on nutrients was analyzed by means of phosphorus speciation experiments and combined with SEM,XRD,XPS and other means.Chemisorption was found to be the key to enrich phosphate in calcium-doped biochar.In addition to chemical precipitation,ligand exchange and electrostatic attraction are two other factors for biochar to adsorb precipitated P and water-soluble P to enhance the adsorption capacity.The high relative content of pyridinic nitrogen and nitrogen oxide in the nitrogen-containing groups and the strong adsorption-binding interaction help the nitrogen-doped structure to promote the adsorption of water-soluble P by the carbon skeleton through anion-πinteraction and hydrogen bond interaction.There was a synergistic effect between biochar spatial pores and surface groups on phosphate adsorption,and the highest biochar adsorption capacity increased to 145.47 mg/g.Finally,the slow-release performance and bioavailability of phosphorus-loaded biochar were evaluated by nutrient release test and tomato planting effect.The results showed that most of the available phosphorus in the fertilizer was dissolved after rinsing with 2%citric acid solution,which proved the effectiveness of the self-made phosphorus slow-release fertilizer.At the same time,biochar-based phosphate fertilizers can improve the growth of tomato plants.Compared with traditional phosphate fertilizers,tomato plant height and stem diameter increased by 12.61%and 4.83%,respectively.And the biochar-based slow-release phosphorus fertilizer had a significant effect on the accumulation and distribution of nitrogen,phosphorus and potassium in various organs of tomato plants.