Effects Of Land Use Patterns On Diversity And Community Structure Of Soil Microbial In Leymus Chinensis Steppe

In this study, the research is in the purpose of Leymus chinensis steppe of Inner Mongolia. Sampling was collected in August 2009 when vegetation was growing at its best condition. Samples were taken at depths of 0–0.20m. Phospholipid fatty acid analysis was employed to assess the structure of micorobial community of the Leymus chinensis steppe soil ecosystem under different land use patterns. The active diazotrophic and ammonia-oxidizing bacteria community was investigated in the three treatments of mowed, grazed and fenced Leymus chinensis steppes in Hulunbeier grassland of Inner Mongolia by using approaches of polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) and sequence analysis.The results of the research showed as follow:The results revealed that soil microbial community structure changed evidently. Microbial characterization of the total PLFA content at mowing plots were significantly higher than the fencing and grazing plots, and the fencing plots had the lowest total PLFA content. Correlation analysis showed the total concentration of PLFA was signifcantly positive correlated with microbial biomass carbon determined by the fumigation–extraction method. Mowing significantly increased the content of soil bacteria, actinomycetes of fencing plots were the highest, soil's fungus content wasn't change significantly under different land use patterns. Bacterial characterization of PLFA in mowing plots was the highest which was 29.6 nmol·g^-1, bacterial content of grazing and fencing plots was lower than the mowing plots 27.8% and 49.3%. Dominant PLFA type of mowing plots were a15:0,15:0 and i16:0, accounted for 64.6% of total PLFA; dominant PLFA type of grazing plots were i15:0,i16:0 and 18:1ω9t, accounted for 46.3% of total PLFA; dominant PLFA type of fencing plots were a15:0,15:0 and i16:0, accounted for 45.6% of total PLFA.Grazing activity significantly reduced the number of species and quantities of nitrogen-fixing microorganisms, as well as the nifH gene diversity. However, fencing plots had the lowest diversity of nifH gene. While the highest one found in mowing plots. A total of 30 sequences representing 25 different sequence types were recovered from the DGGE gels after phylogenetic reconstructions. The results also revealed that most sequences were coming from Alphaproteobacteria of Proteobacteria, and characterized by sequences of members of Rhodobacter, Bradyrhizobium, Mesorhizobium, Rhodopseudomonas, Xanthomonas, Azospirillum, Gluconacetobacter, Methylobacterium and Methylocystis. Thirteen sequences were similar with those nitrogen-fixers existed in genbank, similarity was about 88%—90%, which indicated that there was certain affinity between these sequences and the existing microorganisms but not completed same. It suggested that those sequences would be unique in this region. We had found three common bands of Methylocystis Rhodopseudomonas and Methylobacterium in each test plot. Gluconacetobacter was just existed in fencing plots. The two specific diazotrophic communities were Rhodobacter and Mesorhizobium in mowing plots. Symbiotic nitrogen-fixers existed in grazing, mowing and fencing plots accounted for 21.4%, 47.3% and 31.3% respectively in their dominant nitrogen-fixing bacteria. The result of principal components analysis showed that the influence of different land use patterns on nitrogen-fixing microbial communities composition can be ordered as grazing plots> fencing plots > mowing plots. Nitrogen-fixing microbial communities in Leymus chinensis steppe were significantly(P<0.05)influenced by the levels of nitrate nitrogen, total phosphorus, available phosphorus contents and pH value when canonical correspondence analysis was employed to identify relationship between nifH gene and soil physico-chemical factors under different land use patterns. The result obtained from correlation analysis showed that there was a signify- cant(P<0.05)negative relationship between the nitrate nitrogen and the total phosphorus content, furthermore, the available phosphorus content was strongly correlated(P<0.01)with the pH value.The results showed that grazing activity significantly increased the number of species and quantities of ammonia-oxidizing bacteria, as well as the diversity. A total of 21 sequences representing 21 different sequence types were recovered from the DGGE gels after phylogenetic reconstructions. The results also revealed that most sequences were coming from Nitrosospira and Nitrosomonas ofβ-Proteobacteria, accounted for 72.4% and 28.6% respectively. Cluster 4 of Nitrosospira existed in grazing, mowing and fencing plots accounted for 25.0%, 27.2% and 26.3% respectively in their dominant ammonia-oxidizing bacteria. And Cluster 6 of Nitrosomonas existed in grazing, mowing and fencing plots accounted for 33.1%, 18.1% and 21.1% respectively in their dominant ammonia-oxidizing bacteria. Ammonia-oxidizing bacteria communities in Leymus chinensis steppe were signify- cantly(P<0.05)influenced by the levels of ammonium nitrogen when canonical correspondence analysis was employed to identify relationship between ammonia-oxidizing bacteria and soil physicochemical factors under different land use patterns.