Effects Of Simulated Nitrogen Deposition On Soil Microbial Diversity

Soil microbes are important components of soil ecosystem, and they are also important indicators for soil fertility. Nitrogen (N) deposition can influence the growth and proliferation of microorganisms, which can change the community structure and function, and then affect the material circulation and energy flow of soil ecosystem.Different N application methods, namely soil surface spraying and leaf surface spraying, and different N deposition levels (5.6 g N m-2a-1、15.6 g N m-2a-1、20.6 g N m-2a-1) was simulated in laboratory with four different tree species, namely Schima superba, Pinus massoniana, Acacia mangium, Ormosia pinnata. The effects of N deposition on the soil microbes were examined with phospholipid fatty acids (PLFAs) and polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) techniques. The main results were as follows:1) N deposition affected the soil properties. Soil pH reduced and soil organic matter increased under elevated N deposition, but the effects of the foliar spraying on soil properties were not significant.2) The soil microbial biomass was influenced by N application methods, N levels, tree species and sampling times. The influence of soil surface spraying method were higher than that of leaf surface spraying method, but no significant differences for soil biomass were found between N application methods. With the increasing of N levels, soil microbial biomass reduced for soil surface spraying, especially for bacteria and actinomycetes, but no significance for the foliar spraying. For the soil surface spraying method, soil microbial biomass were different among the tree species, with higher values for N fixing plants(Acacia mangium and Ormosia pinnata) than those for other species. Soil microbial biomass in October were higher than those in April for the same tree species and the same N levels. The ratios of fungus to bacteria (F/B) were affected significantly by both tree species and sampling times, but not significantly by N levels and N application methods.3) The microbial community structure was also influenced by N application methods, N levels, tree species and sampling times. The responses of soil microbial community to elevated N were greater for the soil surface spraying than those for foliar spraying. Gram-negative bacterium (G-) (represented as 18:1w9c,16:1w5c, 18:lw7c,16:lw7c) occurred mainly in control sites, and Gram-positive bacterium (G+) (represented as a 17:0, i17:0, i16:0) and G-(represented as 15:0, i16:0) appeared mainly high N level sites. The microbial community structure was different among the tree species, particularly for the soil surface spraying. It showed that the relative abundance of G-was high for Acacia mangium, fungi for Acacia mangium and Ormosia pinnata, and actinomycetes for Pinus massoniana Sampling times had significant effects on microbial community structure for the soil surface spraying, but less for foliar spraying.4) Elevated N deposition could chang the diversity of soil bacteria community by increasing the diversity after high N addition. The diversity of soil bacteria community were also effected by the N application methods. With the elevated N deposition the diversity of soil bacteria community of Pinus massoniana soil reduced for low N deposition and increased for high N deposition. The dominant bacteria groups of Pinus massoniana soil were Acidobacteria_Gpl, Gammaproteobacteria and Alphaproteobacteria. The diversity of soil bacteria community of Schima superba increased with high N treatment. The dominant bacteria groups of Schima superba were Acidobacteria_Gpl. The diversity of soil bacteria community of Pinus massoniana and Schima superba were influenced by N application methods, with the diversity of soil bacteria community a little higher for foliar spraying than that for the soil surface spraying.5) The diversity of soil bacteria community were influenced by sampling times and tree species. The diversity of soil bacteria community of Pinus massoniana was higher in October than that in April, and the soil microbial communities were more abundant for Schima superba soil than that for Pinus massoniana soil.6) The change of microbial community had significant correlations with soil moisture, pH and NO3--N content. The bacteria communities of Pinus massoniana were mainly influenced by soil pH, while those of Schima superba were mainly influenced by soil pH, total N and NH4+-N content.