Study On The Regulation And Mechanism Of The Composite Addition Of Biochar And Manganese Sulfate On The Aerobic Composting Process Of Pig Manure

Aerobic composting technology is an effective means of achieving resource utilization and harmless treatment of livestock and poultry manure.However,there are some problems in the biochemical transformation of organic materials,such as greenhouse gas emissions,carbon,and nitrogen losses,low humification,and long fermentation cycle.Adding additives in compost can optimize the microenvironment,accelerate the humification and improve the product utilization.Early studies found that adding Mn SO₄ to compost can use Mn²⁺to adjust microbial species diversity and community structure,and promote the humification process of compost.However,as a water-soluble chemical,adding a large amount of Mn SO₄ to compost will increase the concentration of water-soluble ion in compost,and reduce product quality and safety.Biochar（BC）is a porous carbonaceous material with good biocompatibility and salt ion adsorption capacity,it is widely used as an additive in compost.This study utilized the unique structure of BC and the characteristics of Mn SO₄ to regulate the composting process.Six treatments were set up in the experiment:Control,T1（5%BC）,T2(0.25%Mn²⁺+5%BC),T3(0.25%Mn²⁺),T4(0.50%Mn²⁺+5%BC)and T5(0.50%Mn²⁺).Through the 70d aerobic composting experiment,its regulatory potential and mechanism for pig manure composting process were explored,and the following main results were obtained:（1）All treatments rapidly heated up within the first three days and entered a thermophilic stage（>50℃）,with a duration of more than 14 days for each treatment,meeting the requirements for harmless composting.The initial p H value of all treatments ranged from 6.80to 7.58,which fluctuated with the composting process and droped to around 8.5 at the end,meeting the composting maturity requirements.At the end of composting,the electric conductivity（EC）of each treatment was 1.28,1.18,1.58,1.44,1.66 and 1.63 m S·cm^-1,which were lower than the limit value of 4.0 m S·cm^-1,meeting the safety standard of organic fertilizer;In addition,the GI value of T4 treatment was the highest（1.02）,and all treatments were greater than 0.7,which met the compost maturity requirements.（2）At the end of aerobic composting for 70 days,compared to Control,the cumulative emission of NH₃ in T4 showed the largest decrease（38.76%）;N₂O emissions were mainly concentrated in the 35th–50th day,where T4<T2 and T5<T3;The NH₄⁺-N content reached the peak at the 3rd day,T4 treatment has the highest content(3.07 g·kg^-1).At 70d,the NH₄⁺-N content of each treatment was less than 0.4 g·kg^-1,which met the composting requirements.In addition,after 35 days of composting,the NO₃^--N content of each treatment increased gradually.Compared with the Control,the increase of NO₃^--N content was T2>T3>T4>T5>T1in turn,indicating that the combined addition of 0.25%Mn²⁺and 5%BC had a significant effect on the nitrification process of composting,but the high concentration of Mn²⁺（0.50%）had a slight inhibition on the production of NO₃^--N.Compared with the Control,the TKN of T4 increased the most（22.33%）.（3）The auxiliary addition of BC and Mn²⁺was beneficial to promote the metabolism of organic materials in compost by microorganisms.At the end of composting,T4 had the highest cumulative CO₂ emissions（3134.00 g）and had good CH₄ emission reduction capabilities.In addition,the degradation rate of total organic carbon（TOC）of T4 was the highest（20.67%）in all treatments,and the content of dissolved organic carbon（DOC）of T4 was relatively low at the end of composting,which indicated that increasing the concentration of Mn²⁺based on introducing 5%BC could effectively promote the mineralization and degradation of organic materials during composting.From the analysis of UV-visible absorption spectrum and Fourier transform infrared spectrum,T4 treatment had obvious advantages in humification of organic matter.It could increase the number of functional groups such as hydroxyl and carboxyl groups in compost products,effectively improve the complexity of humus structure,aromatization degree and stability of products,and accelerate the process of carbon conversion.At the end of 70d composting,T4 treatment had the highest HA production(81.26 g·kg^-1)and the lowest C/N（17.27）,which also proved that the combination of 0.50%Mn²⁺and 5%BC was beneficial to further control the humification process of compost.（4）T4 treatment supplemented with 0.50%Mn²⁺and 5%BC significantly promoted the activities of urease,dehydrogenase,and laccase,with the highest activities reaching 18.12mg·g^-1·d^-1,6.53 mg TPF·g^-1·d^-1 and 65.44 U·g^-1,respectively.High-throughput sequencing of microbial communities found that bacterial and fungalαdiversity index in T4 was significantly higher than that of the Control treatment,and the relative abundance levels of Firmicutes and Proteobacteria in the thermophilic stage were relatively high,54.62%and38.05%,respectively.At the same time,the relative abundance of Actinobacteria and Ascomycota in the maturity stage of the treatment also reached the maximum（15.60%and58.91%）,which further proved that the composite addition of 0.50%Mn²⁺and 5%BC could effectively improve the microbial community structure of pig manure compost.（5）In the 70d composting experiment in which Mn²⁺participated,the concentration of high-valent manganese(Mn³⁺and Mn⁴⁺)in T2-T5 treatment gradually increased,and the increment of T4(1.36 g·kg^-1)and T5(1.08 g·kg^-1)treatments was relatively large,indicating that there was a transformation from low-valent Mn²⁺to high-valent Mn³⁺and Mn⁴⁺during the composting humification process,and the highest conversion rate could reach 26.77%（T4）.The X-ray energy spectrum analysis（XPS）of the compost sample showed that the characteristic peaks of Mn³⁺and Mn⁴⁺appeared in T4 and T5 treatments,which was consistent with the X-ray diffraction analysis（XRD）results,that is,at the end of composting,the part of Mn²⁺added in the initial material was converted into Mn³⁺and Mn⁴⁺,and Mn₂O₃ and Mn O₂were the main phase components,oxidative Mn³⁺and Mn⁴⁺could be used to effectively decompose organic matter,thus improving the maturity of pig manure compost.In summary,biochar and manganese sulfate were effective and reasonable auxiliary additives for livestock manure composting.In this study,0.50%Mn²⁺and 5%BC were used as the optimal doses,which could effectively promote greenhouse gas emission reduction,reduce carbon and nitrogen losses,improve the compost microenvironment and product quality,and provide new ideas and scientific basis for further improving the resource utilization and harmless treatment of livestock manure.