Abstract: [Objective] Soil microbial community is a sensitive indicator of changes in soil quality and soil ecosystem. The changes in soil microbe community structure in response to simulated nitrogen deposition were investigated by phospholipids fatty acids (PLFA) biomarkers. This study can be accurate to understand the effects of short-term nitrogen deposition on soil ecosystem, and hence predict the change of soil properties and plant growth. This study aims at providing microbial parameters and indicators for the sustainable management of artificial forest under nitrogen saturation condition, and would have guiding significance for the real-time control and governance of nitrogen deposition. [Method] In May 2013, we established 30 plots of 1 m × 1 m in a hectare of young Chinese fir forest, at the Shanxia forest farm in Fenyi, Jiangxi Province. Nitrogen loadings were designed at 5 levels such as N0, N1, N2, N3 and N4 at the doses of 0, 20, 40, 60 and 80 kg N·hm^-2 a^-1, respectively, with 2 nitrogen forms of NH₄⁺-N (I) and NO₃^--N (II). After one year treatment, soil samples were collected with a soil drill. The fatty acid phospholipid was extracted with potassium hydroxide-methyl methanol solution, and assayed by Agilent 6850N with nineteen alkyl acid as internal standard. PLFA pattern was analyzed with the Sherlock MIS4.5 system, before the fatty acid content was converted into the number of nmol·g^-1 dry soil. [Result] A total of 72 PLFAs were detected, and among them the characteristic fatty acids were 36 kinds. The analysis on type and content of the characteristic fatty acids indicated that prokaryotic microorganism was the predominant group in the plot soil treated with all nitrogen treatments. In different nitrogen treatment plots, the range of soil microbial biomass with characteristics of total PLFAs content was 20-44 nmol·g^-1. In deposition of ammonium nitrogen, the amount of total PLFAs, Gram-positive bacterial PLFAs and Gram-negative bacterial PLFAs in the soil were higher compared with control group. The bacterial PLFAs, fungal PLFAs, actinomycete PLFAs and protozoa PLFAs had a same trend under the treatments of ammonium nitrogen. The NH₄⁺-N N4 was the optimal concentration for the growth of microorganisms. In the NH₄⁺-N N2 treatment, the PLFAs of soil microorganisms had the highest quantity and diversity. However, along with the increased rates of nitrate nitrogen deposition, the amount of total PLFAs, Gram-positive bacterial PLFAs and Gram-negative bacterial PLFAs showed a trend that they first increased and then decreased, with the peaks occurred at the NO₃^--N N2 treatment. Bacterial PLFAs had a same trend as actinomycete PLFAs. Moreover, the PLFAs in the NO₃^--N N4 treatment had the highest diversity, while the microbial fatty acids in NO₃^--N N2 treatment had the highest content. According to the Canonical Correlations, ammonium nitrogen in soil was positively correlated with the bacteria and actinomycetes, while nitrate nitrogen and soil moisture were most significantly correlated with bacterial. [Conclusion] When the nitrogen deposition amount was less than 80 kg N·hm^-2 a^-1, both ammonium nitrogen and nitrate nitrogen treatments could promote the growth of microorganisms, but the growth rate was different. The nitrogen treatments of the highest ammonium nitrogen concentration and the medium nitrate nitrogen were more conducive to the total soil microorganism growth, whereas treatments with the medium ammonium nitrogen and the highest nitrate nitrogen were more conducive to increase soil microbial diversity. Ammonium nitrogen in soil was correlated with the bacteria and actinomycetes, while the nitrate nitrogen and soil moisture is most significantly correlated with bacterial.

Key words: nitrogen deposition, phospholipid fatty acids, soil microbe, microbial diversity