The Synthesis And Properties Of Biochar-based Slow-Release Fertilizer Based On The Pyrolysis Byproducts Of Cotton Stalk

Biochar has attracted much attention as a porous material with high specific surface area and excellent performance.However,due to the differences in raw materials and process conditions for biochar preparation,basic research on biochar has been fragmented,diversified,characterized and complicated in different fields.At present,there are two problems that are bottlenecks in the industry and research fields.The first problem is the lack of effective analysis of the interactions between the components of biochar precursors during the pyrolysis process and their effects on the distribution of the pyrolysis products;the second problem is the selection and development of high value-added pyrolysis by-products to reduce costs.The solution of the above-mentioned problems will not only play an important role in promoting the development of biochar,but also provide an important theoretical basis for the development of a series of new biochar-based materials and the high-value utilization of by-products.It is of great theoretical and practical significance to reveal the mechanism of different binder ratios in regulating the slow-release effect of biochar-based fertilizers,so as to coordinate the relationship between soil supply of nitrogen,phosphorus and potassium and crop demand for improving fertilizer utilization and maintaining sustainable agricultural development.A microwave synergistic alkali/ethanol pretreatment method was proposed for the cell wall separation of cotton sticks from the perspective of the separation and structural analysis of the three major elements of cotton sticks,the precursor material of biochar pyrolysis.Based on the characteristics of cotton stalk components,the change patterns and performance evaluation of biochar,wood vinegar liquor and wood gas under the multi-coupling process were systematically studied,while the reasons for the influence of pyrolysis temperature and storage time on the quality of wood vinegar liquor were elucidated.The effects of slow-release fertilizer prepared by using biochar as the slow-release carrier of compound fertilizer and wood vinegar liquor/wood tar/modified starch as the binder on the growth indexes of economic crops and their effects on soil microbial communities were focused on.The main results were as follows.（1）High-purity lignin and cellulose-rich substrates were separated using microwave-assisted alkali/ethanol pretreatment（MAAP）（160℃,15 min）.The results showed that theβ-O-4 structures of AL_1.5（0.39/Ar）and AEL_1.5（0.34/Ar）were lower than that of MWL（0.61/Ar）.The cotton rod lignin obtained by MAAP method had homogeneous fractions with narrow molecular weight distribution and relatively small molecular weight.The molecular weight and Mw/Mn of alkali-ethanol lignin（AELs）extracted by MAAP method were higher than those of basic lignin(AL_0.5),while the relative molecular mass of AEL_1.5 was lower,which indicated that a large amount of lignin and hemicellulose was removed during MAAP pretreatment,resulting in more active sites on the surface of cotton stalk fibers,which led to a large contact surface between enzyme and cellulose and further improved the enzymatic efficiency.Due to theβ-O-4 bond breaking and side chain oxidation reaction,the aliphatic OH was further reduced and the S phenolic hydroxyl group was enhanced in AEL_1.5 compared with AL1.5.（2）The distribution patterns and evolutionary characteristics of biochar（BC）,wood vinegar liquor（PA）and wood gas prepared from cotton straw at different pyrolysis temperatures were investigated using the team’s self-developed pyrolysis multi-coupling equipment.The thermogram analysis of the correlation between the pyrolysis temperature and elements of biochar showed that the contents of C,H and O were highly correlated with the pyrolysis temperature.In addition,when the pyrolysis temperature was higher than 500°C,the specific surface area of biochar had a tendency to decrease gradually.The density of wood vinegar solution decreased from 1.0404 g/cm³ to 0.9955 g/cm³ when the pyrolysis temperature increased from 300°C to 450°C.The production of furan derivatives was inhibited in the wood vinegar solution after 6 months of standing(PA_S-2)compared with the wood vinegar solution after 1 month of standing(PA_S-1),but the conversion of phenolic substances was promoted.The total amino acid content of cotton stalk wood vinegar solution decreased with increasing storage time within a certain range.valine was the highest in PAS-1（36.5 mg/kg）,followed by leucine（26.4 mg/kg）,phenylalanine（18.2 mg/kg）,isoleucine（8.9 mg/kg）,tyrosine（2.3 mg/kg）and lysine（0.5 mg/kg）.All 12 wood vinegars exhibited different degrees of inhibition against the five bacteria,with PA_S-2 showing stronger inhibition activity than PA_S-1,except for Bacillus aeruginosa.at 300°C,the CO₂ volume fraction was higher,reaching at300°C,the volume fraction of CO₂was higher,reaching 48.60 vol%,while the volume fractions of CO,CH₄ and H₂ were 24.44 vol%,1.66 vol%and 1.97 vol%,respectively.When the temperature reached 550°C,the volume fraction of CO₂ decreased by 26.73%and the volume fraction of CO increased by 31.46%.（3）Seven carbon based slow release fertilizers（BSRFs）were prepared using biochar as the carrier and wood vinegar liquid（PA）,bio-oil（BO）and modified starch binder（MSB）as the binder materials.The results showed that PA+BO+MSB treatment could significantly improve the effect of BSRFs on N,P and K slow release.The prepared BSRFs were characterized by SEM,XRD,FT-IR,XPS and EDS,and it was found that PA+BO+MSB contributed to the formation and good pore structure of BSRFs,which improved the slow-release performance of BSRFs on N,P and K.The PA+BO+MSB treatment had the largest canopy size of rape,63.77%and 52.95%higher than CK and BC treatments,respectively.The results of Ritger-Peppas model fitting model indicated that the n values of BSRFs prepared by different binders were in the range of 0.184 to 0.414,indicating that the behavior of BSRFs in releasing nutrient ions was mainly a Fickian diffusion mechanism.The results of economic feasibility analysis showed that PA+BO+MSB had better promotion advantages,and this compound slow-release fertilizer could optimize the slow-release performance of biomass charcoal-based fertilizers and provide theoretical guidance for soil improvement.（4）Theα-diversity,species composition and abundance of soil bacterial communities were analyzed by high-throughput sequencing technology using soils treated with different ratios of PA,BO and MSB binder compounded with charcoal-based fertilizers.The results of abundance clustering at the phylum level for the samples treated with different charcoal-based fertilizers.The relative abundance of BO,PA+BO and PA+BO+MSB treatments showed a decreasing trend as the bio-oil content decreased.However,the relative abundance of MSB,PA+MSB,and PA+BO+MSB tended to increase with the increase of modified starch binder content.The suitable nutrient content and charcoal-based fertilizer produced by the combined effect of wood vinegar solution and bio-oil were more likely to be favored by fast-growing bacteria,such as Pseudoxanthomonas（Pseudoxanthomonas）,Kosakonia（Rice family）,and Sphingomonas spp.（chitinophaga）.The difference in differentiation between PA and BO was0.493-1.451 and between BO and MSB was 0.465-1.451 when PA,BO or MSB binders were applied alone.in addition,soil p H was the main factor affecting bacteria and the change in bacterial diversity was positively correlated with p H and organic carbon.In summary,strengthening the development of new charcoal-based slow-release fertilizer products and the basic research of application,enhancing the sequestration and release of nutrient elements of charcoal-based fertilizers,improving the slow-release ability of charcoal-based fertilizers for nitrogen,phosphorus and potassium,and then coordinating the relationship between fertilizer supply and fertilizer demand,and strengthening the research of agricultural soil and water,economic and environmental effects and comprehensive evaluation index system for wide field applications are the main research directions in the field of biochar-based fertilizer efficiency enhancement technology in the future.