Effects On Community Structure Of Ammonia Oxidizers And Denitrifying Bacteria From Different Mulch Treatments In Semi-arid Loess Plateau, China

Nitrogen (N) cycling is a major question in ecology, because the availability of N often limits the primary production of plants in many ecosystems. With the rapid development of culture-independent techniques, microbial ecologists have discovered the there are close relationship between N cycling and microbial communities. The nitrification and denitrification which are driven by ammonia oxidizers and denitrifying bacteria, play important roles in the global nitrogen cycle, and therefore receive widespread interest. Mulch technology, can effectively improve crop production in semi-arid Loess Plateau of China. However, long-term mulch treatment changes water content, temperature, and soil nutrients, especially N concentrations, which may-be direct impacts on the sustainability of production. We assessed abundance of functional genes affiliated with nitrification (amoA) and denitrification(nirK) using clone library and real time PCR. The effects of mulch on the community structure of ammonia oxidizers and denitrifying bacteria were investigated in a4-year mulch experiment site in the Loess Plateau of China. Three treatments were tested, non-mulching (CK), half-mulching (HM) and full-mulching (FM). The plastic film was mulched at sowing and reserved in field till the next sowing time. Some main results were obtained:The community structure of ammonia oxidizers in different mulch treatments1. amoA gene clone librarys showed that there are highly difference on the AOB community structure among different mulch treatments.1) The community structure of AOB consisted of Nitrosomonas-like sequences and Nitrosospira-Like sequences, attached to β-Proteobacteria. All AOB sequences fell within six clusters, Cluster3a.1、Cluster3a.2、Cluster3b、Cluster6、Cluster2-related and Cluster11/12。AOB communities were dominated by representatives of clusters3b and3a.2.2) The communities of AOB shifted in different mulch treatments, from a Nitrosomona-cluster6hardly any in FM soils, and increased in CK soil. Moreover, the percentage of cluster6in CK is significantly higher than mulch treatments (FM and HM).3) The Communities of AOB shifted in different mulch treatments at OTUs levels. AOB-CK1-101was absent in FM soil, and with lower proportion in HM soil. However, it accounted for12.31%in CK soil. AOB-CK-78had highly percengage in FM soil(5.51%). However, it was hardly detected in HM and CK soil smples. 2. There was no significantly change in AOA community structure.1)442AOB sequences fell within soil and sediment1and soil and sediment2, Soil and sediment1accounted for30%-58%in each clone library.2) There was no significant shift at OTUs level in AOA community structure. AOA-FM2-8, AOA-HM1-19and AOA-FM3-16was dominance in AOA community structure.3. Differences were also seen in the relative abundances of archaeal and bacterial amoA genes, bacterial amoA abundance (5.66×107～1.64×108copies/g) was significantly higher than Archaeal amoA abundance in all soils (1.16×107～3.12×107copies/g). Except October, the bacterial amoA gene abundances increased with FM, HM and CK. The abundance of bacterial amoA in June of CK was significant higher than FM. The abundance of archaeal amoA gene was no significant change in different treatments.4. Correlation analyses among community structure and environmental factor demonstrate that NO3-and TIN had a huge influence on the community structure of AOB and AOA, NH4+had an influence on the community struture of AOB instead of AOA. Correlation analyses among the abundance of AOA/AOB and environmental factor showed that there were significantly negative correlations between the abundance of AOB and the concentrations of NO3-and TINThe community structure of nirK gene in different mulch treatment1. nirK gene clone library results showed that the diversity of nirK gene was extremely lower, and only10OTUs was obtained from631clones. The dominate OTU was nirK-6, accountting for97.7%.2. Real time PCR results revealed abundance of denitrifying bacteria nirK gene did not change significantly, but had a significant seasonal variation. Correlation analyses among the abundance of nirK gene and environmental factor showed soil moisture should be the influencing factor.In general, the community structure of ammonia oxidizers changed in different plastic-film mulching methods; but no shift in denitrifying bacteria community structure.