Effects Of Oxytetracycline On Nitrogen-fixing And Ammonia-oxidizing Microorganism Community Structures During Composting

Antibiotics are widely used in the modern livestock industry because of its effectiveness at controlling intestinal and respiratory infections. However, veterinary antibiotics are absorbed poorly by animals, where most of the antibiotics might be excreted as parent compounds or metabolites in animal urine and feces, thereby resulting in massive amounts of environmental problems. Thermophilic composting is an effective approach for stabilizing livestock waste via the biological degradation of organic matter, which has been shown to be effective in reducing the levels of persistent organic pollutants such as veterinary antibiotics. The residual antibiotics may had effects on physicochemical property, enzymes activities, microbial community structure and gene abundance. OTC was mixed with cattle manure and wheat straw at different ratios before composting. The enzyme activity analysis method and advanced molecular biological techniques such as polymerase chain reaction-denaturing gradient gel electrophoresis(PCR-DGGE) and quantitative PCR(qPCR) were employed to mainly investigate changes in the structures of the microbial community and gene abundance.The research results were as follows:1. Effects of OTC on physicochemical properties and activities of enzyme involved in nitrogen cycle process during compostingThe results indicated that the temperatures in different composting materials reached its peaks at day 2, but the temperatures under M and H did not met the sanitary requirements for non-hazardous treatment in China. Moreover, OTC accelerated the decline in the compost pile temperature during the cooling stage. OTC had concentration-dependent inhibitory effects on the nitrogenase activity during the early period of composting, but the inhibitory effects weakened along with the composting. OTC could promote urease activities at early composting stage, but had an unconspicuous inhibitory effect on urease activities. OTC had slight effects on urease activities during the composting.2. Effects of OTC on community structure and abundance of nitrogen-fixing bacteria during cattle manure compostingThe results of PCR-DGGE showed that there were apparent differences in the numbers and intensities of the bands. The presence of OTC had a clearer effect on the nitrogen-fixing bacterial community structure during the later stage. The results of PCA and cluster analysis indicated that OTC could change the nitrogen-fixing bacterial community structure, especially during the later stage. The species richness and dominance declined gradually with the preasence of OTC. The results of sequence analysis on nifH gene showed that the obtained nifH gene sequences belonged to Clusters I and III.The results of qPCR showed that OTC had no significant effect on nifH gene abundance during the early period of composting, but had a negative effect on the nifH gene abundance during the later period of composting.3. Effects of OTC on community structures and abundance of ammonia-oxidizing microorganism during cattle manure compostingThe results of PCR-DGGE showed that OTC had effects on the community structures of ammonia-oxidazing bacteria and ammonia-oxidazing archaea. The results of PCA and cluster analysis indicated that OTC could change the community structures and distributions of ammonia-oxidazing bacteria and ammonia-oxidazing archaea. The high concentration of OTC had obvious effects on community structure of ammonia-oxidazing bacteria and ammonia-oxidazing archaea during the later stage. OTC could affect the diversity of ammonia-oxidazing bacteria at early stage, but increased the diversity at later stage. The inhibitory effect of OTC on the diversity of ammonia-oxidazing archaea appeared at early composting stage. The high concentration of OTC suppressed the the diversity of ammonia-oxidazing archaea during the entire composting process. The results of sequence analysis on amoA gene showed that the obtained ammonia-oxidazing bacteria belonged to Nitrosomonas and the obtained ammonia-oxidazing archaea belonged to Crenarchaeote.The two ammonia-oxidizing microorganism played an important role in ammonia oxidation during the composting. The results of qPCR showed that OTC significantly enhanced the amoA gene abundance of ammonia-oxidazing bacteria during the later period of composting, while OTC had no significant effect on the amoA gene abundance of ammonia-oxidazing archaea.