Abstract: As decomposers, soil microbes play great roles in material circulation and energy flow in ecosystems. Elevated nitrogen deposition caused by human activities can directly or indirectly change the structure and function of soil microbial community, and then influence the material circulation and energy flow in ecosystems. From May 2012, nitrogen deposition was simulated in laboratory with ammonium nitrate at three levels monthly (5.6, 15.6, 20.6 g N·m^-2·a^-1) at the beginning of every month, and the effects of nitrogen deposition on soil microbial community structure were investigated by phospholipid fatty acids (PLFAs) method in the middle of April (rainy season) and October (dry season) 2013 with one pine species and three broad leaf species, namely Pinus massoniana, Ormosia pinnata, Schima superba and Acacia mangium. The results showed that with the increase of nitrogen level for all tree species, total soil microbial PLFAs decreased, but bacteria were still the dominant microorganisms. The soil microbial PLFAs in dry season were higher than those in rainy season at all the tested nitrogen levels for all the tree species. Soil microorganisms related significantly to soil water content, soil pH and total nitrogen. In rainy season, soil microbes were mainly influenced by the soil pH and soil water content, but by soil nutrient in dry season. Under the stress of nitrogen deposition, soil microbial PLFAs of nitrogen fixation plants (O. pinnata and A. mangium) were higher than those of the other two species. Sampling time, nitrogen levels and tree species all had significant influences on soil microbial (total soil microorganisms, bacteria, fungi and actinomycetes) PLFAs, but had no impacts on the ratio of fungal to bacterial PLFAs (F/B). The timing effect, the nitrogen deposition level and the effect of tree species should be taken into integrated consideration when studying the effects of nitrogen deposition on soil microorganisms, to provide a scientific basis for the environment protection and the sustainable development of ecological system.

Key words: irrigation district, winter wheat, ridge and furrow rain harvesting, supplemental irrigation, photosynthetic characteristics, yield, water use efficiency.