Preparation,Performance Evaluation And Slow Release Mechanism Of Coated Apple Branches Biochar-based Slow-release Nitrogen Fertilizers

The application of nitrogen fertilizer greatly increased crop yield,but also caused environmental problems such as groundwater pollution,soil acidification and water eutrophication.In recent years,biochar-based slow-release nitrogen fertilizers（BSRNFs）have attracted the attention of researchers.They can not only slow down nitrogen release,improve nitrogen use efficiency,but also improve the physical,chemical and biological properties of soil,which has multiple benefits in agricultural production.However,most BSRNFs have weak sustained-release ability,and the mechanism of sustained-release remains to be further studied.Therefore,in this study,apple branch biochar（BC）was used as a slow-release carrier,and the mixture of biochar and urea in the ratio of 1:1 and 2:1（mass ratio of BC to urea）was obtained by dipping method,and then granulated to obtain two kinds of uncoated BSRNFs（1:1 type was labeled BF₁,2:1 type was labeled BF₂）.Using BF₁ and BF₂ as precursors,sodium carboxymethyl cellulose（CMC）and sodium alginate（SA）as coating materials,the coating type BSRNFs（BSRNFs）was prepared.The two fertilizers prepared after CMC coating were denoted as BF_1-1 and BF_2-1,and the two fertilizers prepared after SA coating were denoted as BF_1-2 and BF_2-2,respectively.The physicochemical properties of BC and BSRNFs were characterized by elemental analysis,thermogravimetric analysis,scanning electron microscopy and Fourier transform infrared spectroscopy.The slow-release behavior of BSRNFs in water and soil was studied by hydrostatic leaching and soil column leaching tests.In addition,the kinetic models were fitted to the nitrogen release of BSRNFs,and the slow-release mechanism of BSRNFs was revealed by combining the physicochemical properties of BSRNFS before and after slow release.Finally,the effect of BSRNFs on the growth of spring maize was studied by pot experiment.The main research results are as follows:（1）The physicochemical properties of BSRNFs showed that nitrogen content of BSENFs ranged from 11.56 to 22.54%,and p H value ranged from 8.38 to 10.11.SEM analysis results showed that the surface of BSRNFs was coarser than that of BC,and the coating surface was denser than that of uncoated BSRNFs.The specific surface area analysis results showed that the specific surface area and pore volume of coated BSRNFs decreased,while the average pore size increased.FTIR spectra showed that transmission peaks of C-O and C-C existed at1022 cm^-1 and 1167 cm^-1 of BSRNFs,and a new peak of BF_2-2 appeared at wavelength 1273cm^-1,indicating that the coating of BSRNFs by SA was not a single physical process.（2）The trend of nitrogen release rate of the six BSRNFs in water and soil was roughly the same,but the cumulative nitrogen release of each nitrogen fertilizer in water was greater than that in soil.The results showed that the cumulative nitrogen release rate of each fertilizer at the 74th day was U>BF₁>BF_1-1>BF_1-2>BF₂>BF_2-1>BF_2-2,and the release of nitrogen in BF_2-2 has not reached the release equilibrium at this time.Therefore,BF_2-2 has the best slow-release performance.In addition,the fitting results of the slow-release model showed that the release rate of nitrogen fertilizer in water was more consistent with the first-order kinetic model,and the release rate decreased exponentially.Release in soil is more consistent with the Korsmeyer-Peppas model,following the Fickian diffusion mechanism.The fitting parameters showed that k₁ value and k_kp of BF_2-2 were both the smallest,which further indicated that the release rate of BF_2-2 was lower than that of other BSRNFs in both water and soil.In addition,the characteristic peaks of O-H/N-H,C=O/C=C and C-N functional groups were weakened after sustained release,and these functional groups were involved in the release of nitrogen.After sustained release,the N levels of pyridinic-N,pyrrolic-N and graphitic-N in BF_2-2decreased,and the Na1s peak disappeared completely.The presence of SA slowed down the nitrogen release in BF_2-2 at the initial stage.（3）Compared with sulfur-coated urea,BF_2-2 had no significant effects on plant height,stem diameter,leaf area at ear position and SPAD value of maize,but could significantly promote the development of ear and significantly improve nitrogen utilization rate.The correlation analysis showed that the correlation coefficient between agronomic traits middle area and dry weight per plant was the largest（0.90）.The correlation coefficient between ear dry weight and plant dry weight of each organ was the largest（0.94）.In addition,compared with urea and sulfur-coated urea,BF_2-2 could significantly reduce the content of acid detergent fiber in maize plants.Contribute to the production of quality feed.The results of grey correlation analysis showed that the grey correlation value of BF_2-2 treatment was the largest（0.983）,indicating that the comprehensive quality of maize after BF_2-2 application was the best compared with urea and sulfur-coated urea.In conclusion,in this study,apple branch biochar was used as a slow-release carrier to prepare BSRNFs by water soluble adsorption-granulation-coating process,and BF_2-2 had the best slow-release performance,the possible slow release mechanisms include the encapsulation of SA,the pore adsorption of BC,and hydrogen bonding between urea and BC.Compared with sulfur-coated urea,BF_2-2 can significantly promote the growth of maize and improve the feeding quality of maize,which is expected to be applied in agricultural production.