| Composting is a technology which has been widely used in the disposing of agricultural waste. The lossing of nitrogen during the process of composting will result odor and reduce the quality of compost products. Studying the transfer and transformation of nitrogen during composting has important practical significance. Denitrification is the dominating process in the nitrogen cycle during the composting, which converts oxidized nitrogen compounds (NO3-, NO2-) back into gaseous forms of nitrogen (NO, NO2, N2) by heterotrophic microorganisms under anaerobic conditions. So denitrification plays an important role in the nitrogen cycle. We choose the denitrification genes as the object of the research, using the real-time polymerase chain reaction technology to study the diversity of denitrifiers in the composting, and explore the relationship between the species and composting parameters. The aim of this study was to provide theoretical guidance for optimizing the composting process.In this paper, aerobic composting was processed with agricultural waste such as straw. Composts were sampled in different locations of the composting pile during different composting periods. The changes of pile temperature, pH, moisture content, soluble NH4+-N N03--N and WSC were measured. The results showed that the temperature of top, center and bottom zones were significantly different and the maximum temperature reached55℃,62℃and60℃respectively, which can reach the requirements to kill pathos. The temperatures above50℃were maintained for10days. The pH ranged from6.5～8.5and turned into alkaline in the end which was beneficial for microbes. Moisture content was adjusted artificially and ranged from50%to62%during the whole process. The content of NH4+-N raised from the initial composting, reaching the maximum at6d and then had a downward trend. The N03--N showed a significant increase in the first3days. Then the content decreased sharply during the thermophilic stage. After that the nitrate content gradually increased to549.0,468.3and445.6μg g-1dry compost sample in the top, center and bottom zones respectively. The WSC displayed similar change trend in each zone and increased in the first4days. After then, it was decreased to around30%of the initial value in the end. The denitrification genes(nirK, nirS and nosZ) abundacnes of different locations during the composting were determined by real-time PCR. The results showed that the three genes were all detected in the present study, and NosZ gene has the largest copy number and the nirK gene was more abundant than nirS gene. The nirK and nirS genes existed throughout the whole process while the nosZ gene was absent in the beginning of the composting. All the three genes showed similar change trends and rose to maximum in the cooling stage. The ANOVA testing results indicated that there were significant differences among the three locations of the composting pile, which was less significant for nosZ than nirK and nirS.Redundancy analysis (RDA) was applied to explore the correlations between the abundances of nirK, nirS and nosZ and composting physic-chemical parameters. It suggested that water soluble carbon (WSC), pile temperature and nitrate had the most impacts on the abundances of the three genes. The t-value biplots for each environmental variable indicated that nirK was positively related with temperature and pH (p<0.05), nirS was positively related with temperature (p<0.05), nosZ was positively related with NH4+-N while negatively with WSC. |
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