[Effects of nitrogen addition on the structure of rhizosphere microbial community in Pinus tabuliformis plantations on Loess Plateau, China].

Atmospheric nitrogen deposition in China displays a pronounced southeast-northwest gradient, with a range of ～35 kg·hm·a in southern and central China and ～7.55 kg·hm·a in the northwest. With the Grain for Green project, the environment of Loess Plateau has been greatly improved in the past decades. Little is known about how nitrogen deposition impacts the soil microbial communities in plantations in this region. In order to investigate the responses of diversity and structure of soil microbial community in Pinus tabuliformis plantations to nitrogen addition on Loess Plateau, China, we used high-throughput sequencing of 16S ribosomal RNA genes of bacteria and ITS genes of fungi to investigate the effects of nitrogen addition (200 kg N·hm·a) on the bacterial and fungal community over a 40-year chronosequence of coniferous P. tabuliformis plantation forests. After nitrogen addition, the Shannon diversity of both bacteria and fungi significantly increased in the 25-year-old stand, while the abundance-based coverage estimator of bacteria increased significantly in the 40-year-old stand. Nitrogen addition enhanced the relative abundance of Bacteroidetes in the 40-year-old stand and that of Acidobacteria/Zygomycota in the 25-year-old stand. In contrast, the relative abundance of Thaumarchaeota was reduced by nitrogen addition in the 40-year old stand. The effects of nitrogen addition were stronger on bacterial than on fungal community structure as indicated by the results from non-metric multidimensional scaling analysis. Moreover, the effects of nitrogen addition were stronger in the 25-year-old stand than in the 40-year-old stand. These results indicated that the effects of nitrogen addition on microbial community structure diminished over time in temperate forests on Loess Plateau, China. The stronger effects of nitrogen addition was found on the community structure of bacteria than that of fungi, and on the microbial community structure in the 25-year-old stand than in the 40-year-old stand. Our findings suggested that forest ecosystem at a certain stage (～40-year old) would be less sensitive to disturbance such as nitrogen addition than young forests.