Regulation And Internal Biological Mechanism Of Exogenous S-P-Mg Minerals Addition On The Progress Of Aerobic Pig Manure Composting

Livestock and poultry manure can be recycled and converted into sanitary and high-value fertilizers by aerobic composting technology,which is propitious to safe management of livestock and poultry manure and enrichment of nutrient substance.However,along with the organic substance degradation,there are some drawbacks during traditional composting process such as massive greenhouse gases emission,severe nitrogen loss and low degradation rate of organic matter,resulting in the reduction of composting efficiency,weakening the utilization value of compost products and adverse influence on the environment.Scientific regulation of the composting process to promote the safe and stable transformation of organic substances is one of the vital measures to obtain high-quality compost products and to promote the application of composting technology.Our previous study has showed that introducing sulfur（S⁰）into composting system could effectively reduce the ammonia（NH₃）volatilization;but excessive pursuit of reducing NH₃volatilization in composting system led to accumulation of NH₄⁺-N,immaturity of compost products,and phytotoxicity increase.In the engineering practice,it is necessary to further optimize the addition amount of S⁰ and to reduce the phytotoxicity of compost products.For this purpose,pig manure（PM）and sawdust were used as original compost material and bulking agent,respectively,and different proportions of S⁰（0.2%,0.4%,0.6%and 0.8%）,magnesium sulfate（Mg SO₄）（0.2 mol/kg）and monopotassium phosphate（KH₂PO₄）（0.2mol/kg）were added into the compost mixture in five treatments at 7^th day for 70 days composting.The effect and mechanism of S-P-Mg minerals addition on greenhouse gases emmision,nitrogen transformation,organic matter degradation,and microbial community changes during composting were investigated.The main results and conclusions are as follows.（1）During 70-day aerobic composting,the temperature of all treatments achieved compost standard.The durations of thermophilic phase of 0.2%S,0.4%S,0.6%S and 0.8%S were 19,20,18,and 18 days,respectively,which were higher than that of Control（17days）.The p H of all treatments ranged from 6.67 to 8.24 and EC ranged from 1310 to 3750μS/cm at the end of composting,all met the compost maturity standards.The C/N ratio in all treatments were 20.98（Control）,19.61（0.2%S）,18.20（0.4%S）17.21（0.6%S）and 18.77（0.8%S）,respectively,with 0.6%S showed the lowest C/N ratio.The GI values of all treatments were>70%after 70 days.All these results indicated that the addition of S-P-Mg minerals prolonged the thermophilic stage and had no adverse effect on compost maturity.（2）During the composting,the total organic carbon（TOC）was degraded by 16.17%,17.05%,20.46%,24.27%,and 19.35%for control,0.2%S,0.4%S,0.6%S,and 0.8%S,respectively.The treatments with S-P-Mg addition promoted the CO₂ emission and reduced the CH₄emission.The cumulative emission of CH₄ during the composting was in order of0.6%S（5.20 g）<0.4%S（5.84 g）<0.8%S（6.10 g）<0.2%S（7.15 g）<Control（7.48 g）,with0.6%S treatment showed the most CH₄emission reduction.In addition,the total humic acid content of all the treatments decreased,and the HA/FA was higher than 1.6,which met the requirement of compost maturity.（3）The cumulative emission of N₂O was in order of 0.8%S（0.67 g）<0.6%S（0.77 g）<0.4%S（0.88 g）<0.2%S（1.23 g）<Control（1.55 g）.The NH₃ cummulative emission of all treatments were in order of 0.8%S（8.11 g）<0.6%S（10.82 g）<0.4%S（12.28 g）<0.2%S（13.85 g）<Control（19.69 g）.Compared with Control,the cumulative emissions of N₂O and NH₃ decreased gradually with the increase of S⁰ adding amount,especially the 0.8%S treatment.The NH₄⁺-N content of S-P-Mg minerals addition treatments were 0.96 g/kg（0.2%S）,1.05 g/kg（0.4%S）,1.12 g/kg（0.6%S）and 1.21 g/kg（0.8%S）,respectively,which were significantly higher than that of the control（0.29 g/kg）in the final product.The change of NO₃^—-N in all treatments conformed with the evolution of composting process with the content of NO₃^—-N kept in the range of 0.91～2.14 g/kg after composting.Moreover,the levels of TKN were increased by 34.28%（Control）,39.32%（0.2%S）,44.57%（0.4%S）,47.96%（0.6%S）,and 44.09%（0.8%S）,respectively.The content of TKN in the treatments with S-P-Mg addition were higher than that in the Control.The X-ray diffraction（XRD）analysis showed that struvite was formed in the samples treated with S-P-Mg addition,suggesting S-P-Mg minerals addition reduced nitrogen loss and benefited nitrogen conservation.（4）The diversity and abundance of bacterial and fungal communities were increased in the composting process with the addition of S-P-Mg minerals.The dominant phylums of bacteria during composting process were Firmicutes,Proteobacteria and Actinobacteria.The bacterial community succession was significantly correlated with NO₃^—-N（49.0%）,TKN（18.1%）,N₂O（7.5%）,and GI（5.4%）.Ascomycota was the dominant fungal phylum in the composting process.The abundance of Ascomycota was higher in treatments with S-P-Mg minerals addition than that in Control.CH₄（32.7%）,CO₂（16.8%）,NH₃（9.1%）and temperature（8.8%）had significant influence on the succession of fungal community.In general,S-P-Mg minerals addition has synergistic influence on reducing greenhouse gas emissions,preserving nitrogen and improving microbial richness.Combining with maturity indexes,the appropriate ratio of S is 0.6%.S-P-Mg minerals addition is a feasible approach in composting process promotion.