Influence Of Ferrous Sulfate-Biochar Mixture Addition On The Spectral Dynamics Of Dissolved Organic Matter And Microbial Succession In EAC

In China,the sheep farming industry is transitioning from traditional scattered farming to a more large-scale and industrialized approach.However,this shift has resulted in significant environmental threats from the resulting manure and wastewater.Aerobic composting is an effective method for safely disposing of livestock waste,but traditional methods suffer from issues such as incomplete organic matter degradation,long ripening periods,and greenhouse gas emissions.Previous research has found that electric-field-assisted aerobic composting（EAC）can help address these issues.By applying a stable direct current electric field to both sides of the compost pile,the natural electric potential of the pile can be regulated,and the life activities of anaerobic zone electroactive bacteria can be enhanced.This reduces secondary pollution from the compost and significantly improves its humification degree,making EAC a promising method for the safe and resourceful disposal of livestock waste.In order to further enhance the composting effectiveness of EAC,this study investigated the optimal ratio of a mixed additive consisting of biochar and ferrous sulfate,providing a theoretical basis for the production and application of EAC mixed additives.The control group was subjected to a stable 2 V direct current electric field applied to the two sides of the aerobic composting pile,and a full-factorial experiment with two factors and three levels was conducted to determine the optimal ratio of ferrous sulfate（1%,3%,5%）and biochar（10%,15%,20%）in the pre-mixed sheep manure and straw materials.The physicochemical properties,aromaticity of dissolved organic matter（DOM）,relative molecular weight,and fluorescence intensity of humic substances for each ratio were measured.Furthermore,the influence of the optimal ratio（Group E:3%ferrous sulfate and15%biochar）on DOM functional groups and microbial communities in EAC were further explored.The main results are as follows:（1）Different ratios of ferrous sulfate and biochar can affect the physicochemical properties of EAC substrates.Among them,the temperature of the heap in group E was the highest（77℃）,the duration of high temperature was the longest（11 days）,and the absolute amount of total kjeldahl nitrogen（TKN）on the 35th day increased by 7.97%compared with day 0.On the 35th day,the dissolve organic carbon（DOC）content and germination index（GI）value of group E were 0.25±0.01 g/L and 174.70±27.34%,respectively.Compared with group EAC,the DOC content of group E decreased significantly by 143.50%（p<0.05）,and the GI value increased significantly by 42.82%（p<0.05）on the 35th day.The influence of ferrous sulfate on the maximum temperature and p H of the composting pile was greater than that of biochar,while the impacts on GI and TKN were found to be the opposite.（2）The SUVA₂₅₄ values of DOM in the substrates on day 35 were 3.18±0.11,3.69±0.06,and 3.20±0.01 for groups B,E,and H,respectively,which were significantly higher than those of group EAC by 26.81%,47.14%,and 27.46%（p<0.05）.The SUVA₂₈₀ values of DOM in the substrates on day 35 were 2.68±0.09,3.14±0.05,and 2.68±0.01 for groups B,E,and H,respectively,which were significantly higher than those of group EAC by26.55%,48.53%,and 26.58%（p<0.05）.The impact of ferrous sulfate on the aromaticity and relative molecular weight of the composting DOM was greater than that of biochar.（3）Four-component EEM-PARAFAC models were established,and the corresponding organic matter types for each component were C1（fulvic acid-like）,C2（humic acid-like）,C3（tryptophan-like）,and C4（tyrosine-like）.On day 35,the maximum fluorescence intensity（Fmax）of fulvic acid-like substances（C1）in the DOM of composting substrates in groups B,C,E,F,G,H,and I were 0.83±0.01 R.U.,0.91±0.01 R.U.,1.11±0.01 R.U.,0.88±0.00 R.U.,0.81±0.01 R.U.,0.94±0.01 R.U.,and 1.05±0.01 R.U.,all significantly higher than that of the EAC group（p<0.05）.The Fmax values of humic acid-like substances（C2）in the DOM of composting substrates in groups B,C,E,G,H,and I were 0.64±0.01 R.U.,0.63±0.01 R.U.,0.77±0.01 R.U.,0.58±0.00 R.U.,0.66±0.01 R.U.,0.70±0.01 R.U.,and 0.76±0.01 R.U.,all significantly higher than that of the EAC group（p<0.05）.The effect of ferrous sulfate on the maximum fluorescence intensity of humic substance components in composting DOM was greater than that of biochar.（4）Five functional groups and compound sources were identified by consensus from the Fourier transform infra-red（FTIR）spectra curves in the 4000～400 cm^-1wavelength range of the EAC and E group.The peak areas corresponding to the 3315 cm^-1（O-H and N-H stretching vibration）,1650 cm^-1（C=C stretching vibration）,1417 cm^-1（C-O-H,O-H and-COO^-deformation vibration,C-O asymmetric stretching vibration）and 1135 cm^-1（C-OH stretching vibration）ranges of the E group’s 35th day DOM FTIR spectra were 219.83,17.91,17.57 and 86.92,respectively,which were significantly higher than those of the EAC group by 172.17%,199.00%,153.90%and 171.12%,respectively.The 2D-FTIR-COS analysis results showed that the response order of carboxylic acids and aromatic substances in the electric field composting process was changed by the E group.（5）SUVA₂₈₀ and C4＿Fmax are critical factors influencing the variations in humic substances（C1＿Fmax and C2＿Fmax）within the EAC group.The dominant bacterial taxa at the genus level demonstrate a significant positive correlation with DOC,GI,and C2＿Fmax.The phyla Chloroflexi,Planctomicrobium,and Niabella are the key microorganisms responsible for aromatic transformation,humification,and molecular condensation of DOM.Within the E group,TKN,SUVA₂₈₀,A_226⁴00,and S_R emerge as determining factors affecting the changes in humic substances（C1＿Fmax and C2＿Fmax）.The dominant bacterial taxa among the top 20 exhibit significant positive associations with TKN,SUVA₂₅₄,SUVA₂₈₀,and C2＿Fmax.The phyla Actinobacteriota,Deinococcota,as well as the genera Truepera and Chryseolinea,play crucial roles in the aromatic transformation,humification,and molecular condensation of DOM.