Effect Of Biochar On Mechanism Of Aerobic Composting Of Biogas Residue And Evaluation Of Fertilizer Efficiency

With the rapid development of large and medium biogas project,the high value utilization of anaerobic fermentation residue has become a hot topic in recent years.After anaerobic fermentation,the important nutrients of biogas residue are effectively preserved and often used as soil amendments.The traditional treatment of biogas residue is mainly for direct return to the field,but when the biogas residue is returned to the field with livestock and poultry manure as the fermentation raw material,there are potential environmental risks such as NH₃volatilization,soil salt accumulation,heavy metal enrichment,pathogen and PAHs accumulation,and affect the normal growth of crops.Aerobic composting of biogas residue is a safe and effective way to increase the value of biogas residue,and it also endows the biogas fertilizer with high quality and high added value.However,the biogas residue has low C/N and cellulose,which makes it less humic and loss of carbon and nitrogen when composting alone.Therefore,it is urgent to explore the additives which can accelerate the process of composting and ripening of biogas residue to improve the composting efficiency of biogas residue,and to clarify the mechanism of additives to promote the composting of biogas residue and the mechanism of composting products on soil and crops after application to reduce environmental risks.In this study,biogas residue from biogas engineering was used as raw material for composting.By adding different qualities of biochar in the composting process,the law of decomposition and transformation of organic matter,emission reduction of major greenhouse gases,balance of carbon and nitrogen elements transformation and heavy metal inactivation in the composting process was explored respectively.The effects of compost biochar-based organic fertilizer on the content and speciation of heavy metals in soil and crop systems were systematically studied in simulated heavy metal polluted soils.At the same time,pot experiment was conducted to explore the effects of biochar-based organic fertilizer combined with chemical fertilizer on soil physical and chemical properties,enzyme activity,plant yield and fertilizer utilization rate.This study can provide a reference basis for the high-value utilization of biogas residue and the application of biochar in the composting process At the same time,it provides reference ideas for the extension and the improvement of economic benefits of biogas engineering,and the acceleration of the protection and utilization process of cold black soil region.The main research results of this paper are as follows:(1)Biochar with mass of 2.5%(T1),5.0%(T2),7.5%(T3)and 10.0%(T4)were added into the composting material to investigate the dynamic mechanism of humification in the composting process of biogas residue,and principal component analysis(PCA)was used to comprehensively evaluate the maturity of biochar.The results showed that the addition of biochar shortened the time for the reactor to reach the high temperature period.Compared with CK(without biochar),the peak temperature of the reactor increases by 3.4?at most,and the duration of the high temperature period increased by 1 to 4 days.The electric conductivity(EC)of the reactor was significantly decreased by 2.44%?9.76%at the end of composting.The germination index(GI)of T2 was 22.73%higher than that of CK,and the change trend of the germination rate and GI of all treatments was similar.Biochar can promote the degradation of lipids,alcohols and ethers in organic compounds,and was beneficial to increase the stable aromatic ring substances in the composting products.The amounts of bacteria,actinomycetes and fungi decreased significantly before the end of composting,and the degradation rate of lignocellulosic was hemicellulose>cellulose>lignin.The accumulation of humic acid(HA)by biochar was significant in the late composting period.In addition,the degradation rate of fulvic acid(FA)was significantly increased during the composting process.Cluster analysis(CA)showed that T3 and T2 had the best decay promoting effect among all treatments.Combined with PCA comprehensive evaluation of the composting ripening degree of all treatments,T3 had the highest comprehensive score,which could better promote the composting ripening process of biogas residue aerobic compost.(2)Biochar had a significant effect on reducing major greenhouse gas emissions in the composting process.Compared with CK,the cumulative release of CO₂,CH₄,NH₃and N₂O decreased by 12.14%-30.60%,29.48%-42.65%,21.49%-44.83%and 17.41%-34.15%,respectively.Correlation analysis results showed that greenhouse gas emission was significantly correlated with reactor temperature,NO₃^--N and HA content.CO₂and NH₃emissions were the main forms of loss of carbon and nitrogen,respectively.The addition of biochar effectively reduced the loss of carbon by 15.06%?30.57%and the loss of nitrogen by 23.61%?41.08%.In the process of composting,biochar can promote the conversion of Cu,Zn,Cd and Pb from exchangeable state to residue state,thus inhibiting the bioavailability of heavy metals.The passivation rates of exchangeable heavy metals are 7.41%-11.75%,7.41%-15.26%,28.06%-63.89%and 25.16%-45.02%,respectively.Compared with CK,the content of residual heavy metals increased.After the end of composting,the potential ecological risk index of heavy metals in the composting products of each treatment was 94.32?107.17,which was lower than that of CK(128.87).(3)The application of biochar-based organic fertilizer in simulated heavy metal contaminated soil had a great difference in the speciation distribution of heavy metals,which was reflected in the significant decrease of the content of weak acid extracted state in heavy metals and the increase of the content of residue state in heavy metals,and promotes the transformation of heavy metals from weak acid extracted state to residue state in soil.The accumulation of heavy metals in the aboveground and underground parts of cabbage plants was different,and Cu,Zn,Cd and Pb were mainly accumulated in the underground parts of cabbage plants.Compared with CK(organic fertilizer without biochar added),the total amount of Cu,Zn,Cd and Pb in plants after application of biochar organic fertilizer decreased by 4.30%-13.96%,14.77%-53.49%,7.64%-47.64%and7.83%-39.00%,respectively.The results showed that biochar-based organic fertilizer could inhibit the accumulation of heavy metals in plants.(4)In pot experiment of combined application of biochar-based organic fertilizer and chemical fertilizer(F1:biochar organic fertilizer alone;F2:chemical fertilizer alone;F3,F4 and F5 were the proportions of biochar-based organic fertilizer to chemical fertilizer 1:2,1:1 and 2:1,respectively;CK:no fertilization).Compared with F2,soil physical and chemical properties were improved by combined application of two fertilizers,including:soil bulk density decreased by1.80%-4.50%,soil total porosity increased by 2.86%-6.73%,and soil cation exchange capacity(CEC)increased by 1.15%-3.44%.There were differences in the proportion of combined application on soil enzyme activity.F3 significantly increased soil catalase activity,F4significantly increased soil invertase activity,F5 significantly increased soil urease activity,and the comprehensive soil enzyme index of each treatment was F3>F5>F4>F1>F2>CK.Compared with F2,plant yield of F3 and F4 increased by 2.35%?6.58%.Correlation analysis results showed that the yield increase was due to the improvement of soil structure,nutrient supply and catalase activity of biochar-based organic fertilizer combined with chemical fertilizer.Comprehensive yield and PCA results showed that F3 had a better effect on soil physical and chemical properties and enzyme activity while maintaining a higher yield,and was a more reasonable organic and inorganic fertilization mode for black soil.