Dynamic Changes Of Tetracycline Resistance Genes In Aerobic Compost Of Livestock And Poultry Manure

With the rapid development of economy and society,large-scale and intensive livestock and poultry breeding industry has been rapidly developed,and a large number of livestock and poultry excrement is in urgent need of treatment.Antibiotic resistance genes(ARGs)are usually high in animal feces due to the large use of animal antibiotics.In recent years,it has been found that aerobic compost has excellent performance in removing antibiotics and ARGs pollution from livestock and poultry manure.However,there are few studies on the dynamic changes of ARGs in aerobic compost process,which cannot provide sufficient theoretical basis for improving the removal efficiency of ARGs by aerobic compost.Therefore,in this study,pig manure,chicken manure and common crop straw were used as test materials,carbon source type(corn straw,wheat straw,peanut straw and golden mushroom residue)and initial C/N ratio(16,21 and 26)were adjusted,and tetracycline(TC)and tetracycline resistant bacteria(TRB)were added for composting test.By analyzing the absolute abundance and dynamic changes of transcription expression level of resistance genes in the composting process,factors affecting the abundance change and reduction efficiency of resistance genes in the composting process were explored.The main research results are as follows:(1)Aerobic compost can significantly remove tetracycline and its tetracycline resistance genes(TRGs)in livestock and poultry manure.The initial C/N ratio and carbon source type affect the composting process and the removal efficiency of TRGs.When the initial C/N ratio was 16,the contents of organic matter and total nitrogen in compost products treated with enoki residue as carbon source were the highest(432.99 and26.35g/kg),while those treated with peanut straw as carbon source were the lowest(341.91 and 19.94g/kg).The type of carbon source affected the removal efficiency of aerobic compost for TRGs and class I integron(int I 1).The absolute abundance of int I 1 after composting was generally reduced by more than 80%,and the removal rate of int I 1 was 96.85% when peanut straw was used as carbon source.The absolute abundance of tet C decreased by 56.32% and 54.01% when wheat straw and peanut straw were used as carbon sources,but increased by 33.54% and 417.12% when corn straw and enmulina vellum residue were used as carbon sources,respectively.The removal rates of tet X,tet M and tet O reached 97.01%,99.28% and 99.72%,respectively.(2)During the composting process,the absolute abundance of TRGs and int I 1 generally rebounded twice,mainly in the second half of the high temperature period and the rot period.In all treatments,TRGs and int I 1 showed an obvious decreasing trend within 0-3d,but the abundance of tet X,tet G,tet C and tet M all showed a rebound.Initial C/N ratio affected the occurrence of rebound,and low C/N ratio was more severe than that in compost.Setting the initial C/N ratio of 21 can improve the removal efficiency of TRGs,and the total removal rate of TRGs reaches more than 93.80%.When the initial C/N ratio of corn straw was16 and 21,the removal efficiency of aerobic compost was 91.56% and 95.98%,respectively.When the initial C/N ratio was set at 21 and 26,the removal efficiency of TRGs by aerobic compost was higher,reaching93.80% and 92.50%.(3)Although the trends of transcriptional abundance and absolute abundance of TRGs and int I 1 were the same,they were not completely consistent.The carbon source and the initial C/N ratio affected the transcription expression of TRGs and int I 1.When the initial C/N ratio was 16,the transcription abundance of TRGs in peanut straw compost was generally higher.The abundations of tet X,tet C and tet W in the compost samples were 7.12×107,3.56×106 and 2.20×107 copies/g,respectively.When corn straw and wheat straw were used as carbon sources,the transcriptional abundance of TRGs and int I 1 was the lowest when the initial C/N ratio was 26.(4)In the aerobic composting experiment with the addition of tetracycline and resistant bacteria,about87% of TC in livestock and poultry manure was removed by aerobic composting,and the initial concentration of TC had no significant effect on the removal rate.TC significantly increased the total amount of microorganisms at the early stage of aerobic composting,and significantly increased the absolute abundance of TRGs and int I 1 in the whole process of composting,and the promoting effect increased with the increase of TC concentration.TC supplementation increased the risk of rebound of tet X,tet C and tet G in postcomposting and increased the absolute abundance of TRGs in composting products.The absolute abundance of tet X,tet C and tet G in high-concentration TC supplementation was 7.65 times,8.75 times and 5.08 times of that in control composting products,respectively.In the presence of TC,aerobic compost could not effectively remove tet X,tet C and tet G from livestock and poultry manure.Meanwhile,residual TC significantly reduced the removal rate of TRGs from aerobic compost.The removal rates of tet W,tet M and int I 1 in high concentration TC treatment were 93.51%,88.56% and 21.95%,respectively,which were lower than those of control treatment.(5)Exogenously added TRB were rapidly removed at high temperature without significantly changing the trend of increase or decrease of TRGs and int I 1 and the removal efficiency.However,when TC and TRB were added together,the removal rate of TC was reduced.The absolute abundance of int I 1 and tet G increased by 165.45% and 124.22%,respectively,after 31 days of composting in M-T1 treatment(90mg/kg TC was added according to the dry weight of pig manure and 250 m L of inactivated TRB bacteria solution).Free TRGs are more difficult to be removed,which significantly affects the removal efficiency of aerobic compost on TRGs and int I 1.