Effects Of Different Management Models On Abundance And Structure Of Soil Nitrogen-fixing Bacterial Community In Phyllostachys Pubescens Plantations

Moso bamboo(Phyllostachys pubescens) is an important non-timber forest product in the forest ecological system of subtropical China. The total area of bamboo has been more than 6.01 million ha,and moso bamboo is the most popular bamboo species accounting for 4.43 million ha, by 2013. At present, extensive and intensive management are the two main management models for moso bamboo.Moso bamboo is an N favorite species. Nitrogen-fixing bacterial community plays a major role in biological nitrogen-fixation in the ecological system. Topsoils(0~20 cm) and subsoils(20~40 cm)were collected from five soils with increasing cultivation time. Polymerase chain reaction-denaturing gradient gel electrophoresis(PCR-DGGE) and quantitative real-time PCR(qPCR) were employed to characterize the changes in soil nitrogen-fixing bacterial community structure and abundance in soils based on nifH gene, respectively. Factors affecting nitrogen-fixing bacterial community structure in soils were analysed using redundancy analysis. The objective was to find out the characteristics and evolution of soil nitrogen-fixing bacteria in moso bamboo stand soils. The main results obtained were as follows:Extensive managed moso bamboo forests. Soils were collected from four plantations with increasing cultivation time(5a, 9a, 15 a and 18 a, respectively) and a soil with Pinus massoniana(PM)planted with the similar ecological background(defined as control). The results of topsoils showed that after the transformation of Pinus massoniana(PM) stand into Phyllostachys pubescens, the Shannon indices of soil nitrogen-fixing bacterial community increased significantly, and decreased with increasing cultivation time but still higher than the control. Principle component analysis(PCA)showed that cultivation time clearly affected soil nitrogen-fixing bacterial community structures in Phyllostachys pubescens plantations with the community structures of 5a and 9a differed clearly from control with PC1 and PC2, respectively. However, differences between the community structures of15 a and 18 a and control were weak. Statistical analysis showed significant difference among them(r=0.901, p=0.001). Gene copies of nifH were highest in 5a but lowest in 9a and generally unchanged with increasing cultivation time as revealed by q PCR. Redundancy analysis(RDA) showed that the soil available potassium, available phosphorus, organic matter and pH were significantly affecting nitrogen-fixing bacterial community in soils under Phyllostachys pubescens plantations with increasing cultivation time. In the subsoils, the evolvements of nitrogen-fixing bacterial communityare similar to surface soils, but the highest values of diversity and abundance were processed in 15a;for the copy number of nifH gene, it was less than topsoils; RDA also showed that the soil available potassium, available phosphorus, alkalytic nitrogen, nitrate nitrogen and organic matter were significantly affecting the bacteria.Soil samples were collected from long-term intensive management moso bamboo forests, which included a station that received no fertilization(CK), and four level cultivation years, ten years(10a),fifteen years(15a), twenty years(20a), twenty-five year(25a). In surface soils: Little differences were observed in nitrogen-fixing bacterial community structure between 10 a and CK, however, the structures of 15 a and 20 a clearly differed from the CK, and the differences became weak in 25 a. With the increasing cultivation history, the abundance and diversity of the nifH gene decreased firstly and then increased, with the minimum values at year 15. Redundancy analysis showed that soil available phosphate, available potassium, soil nitrite nitrogen, and soil alkalytic nitrogen were closely related to the change of nitrogen-fixing bacterial community structure, which explained 31% of the total variations of soil nitrogen fixation bacterial community, but there was no significant difference. In the subsoils: Redundancy analysis(RDA) showed that the soil nitrate N, alkalytic nitrogen, organic matter and pH were main factors and significantly affecting nitrogen-fixing bacterial community in soils.Short-term intensive managed moso bamboo forest. In this study, three soil mixed samples included 0~20 cm layer and 20~40 cm layer were collected from three moso bamboo forest, these forests were applied fertilizer separately in 2012(1a), 2006(7a), and a soil with no fertilization as control. The experiment showed that soil under 1a management was richer in nitrogen-fixing bacterial diversity. The effects on soil nitrogen fixing bacteria in topsoils were bigger than subsoils. With the increasing history, the abundance of the nifH gene decreased firstly and then increased in topsoils,while the opposite result observed in subsoils.