中文摘要：

土壤微生物是草原土壤生态系统的重要组成部分。为研究氮沉降增加对草原土壤微生物群落结构的影响，以内蒙古贝加尔针茅草原为研究对象，开展连续6年（2010—2015年）模拟氮沉降试验，以N计算，设置：N0（0kg·hm-2）、N50（50kg·hm-2）、N100（100kg·hm-2）、N150（150kg·hm-2）和N300（300kg·hm-2）5个处理，采用磷脂脂肪酸（PLFA）技术测定0~10cm土壤特征微生物PLFA生物标记数量并探讨土壤微生物群落结构对氮沉降的响应。结果表明：随氮添加量增大，土壤微生物总PLFAs、细菌PLFAs、革兰氏阳性细菌PLFAs、革兰氏阴性细菌PLFAs和放线菌PLFAs含量呈先升高后降低的趋势，均以N100（100kg·hm-2）处理最高。土壤微生物群落PLFA标记的主成分分析显示，不同氮添加下土壤微生物PLFA标记有显著差异。相关分析表明，土壤革兰氏阳性菌、放线菌PLFA含量、G+/G-与土壤pH值呈显著负相关，土壤微生物总PLFAs、土壤细菌PLFAs、革兰氏阳性菌PLFAs、革兰氏阴性菌PLFAs、放线菌PLFAs和饱和脂肪酸PLFAs含量均与土壤速效磷含量呈显著正相关。综合研究表明，连续6年氮添加改变了贝加尔针茅草原土壤微生物群落结构，土壤pH值和土壤速效磷含量是驱动这种变化的主要因素。

英文摘要：

Nitrogen deposition increases the productivity of grassland.However,little is known about the relationship betweennitrogen deposition and soil microbial community structure in temperate meadow steppe in Inner Mongolia.The objective of thisstudy was to reveal the effects of increasing nitrogen deposition on soil microbial community structure and the relationship amongsoil microbial community structure,soil properties and the concentration of nitrogen addition in Stipa Baicalensis steppe in InnerMongolia,China.This study was conducted from2010to2015,five treatments including N0(0kg·hm-2,by N,the same below),N50(50kg·hm-2),N100(100kg·hm-2),N150(150kg·hm-2),and N300(300kg·hm-2)were set up.Soil microbial communitystructure under different nitrogen treatments were measured by phospholipid fatty acids(PLFA)method.Soil samples were collectedfrom the0~10cm soil layer.The results showed that nitrogen addition significantly increased the content of the microbial PLFAs.Soil total PLFAs,bacterial PLFAs,gram-positive bacterial PLFAs,gram-negative bacterial PLFAs and actinomycetes PLFAs contenthad the highest values at the N100(100kg·hm-2)nitrogen addition treatment.Soil microbial communities showed obvious differenceunder various nitrogen additions.Correlation analysis showed that soil gram-positive bacteria PLFAs,actinomycetes PLFAs and soilG+/G-ratio were significantly correlated to soil pH(P<0.05);Soil microbial total PLFAs,soil bacterial PLFAs,gram-positive bacteriaPLFAs,gram-negative bacteria PLFAs and actinomycetes PLFAs content and soil saturated fatty acid contents were positivelycorrelated with soil available phosphorus(P<0.05).In conclusion,continuous six-year nitrogen addition shifted soil microbialcommunity structure in Stipa Baicalensis Steppe,soil pH and soil available phosphorus were two important factors affecting soilcommunity structure under different nitrogen addition treatments.