中文摘要：

为探究氮沉降对白羊草种群及土壤属性的影响,以黄土高原典型地带性植被白羊草种群为研究对象,通过人工添加氮素,设置不同施氮处理（0、2.5、5、10 g/（m~2·a））,系统研究土壤有效氮升高对白羊草种群特征及土壤养分的影响。结果表明：1）氮添加量在一定范围内,可显著改善白羊草种群特征,超过该范围,则会影响白羊草种群正常生长。在2.5 g/（m~2·a）施氮量下,白羊草花序数量和株高最大,当施氮量大于该值时,白羊草花序数量和株高呈减小趋势,地上生物量随施氮量的增加显著增加（P〈0.05）;在5 g/（m~2·a）施氮量下,白羊草根系生物量密度最大,超过该值后根系生物量密度减小。2）随施氮量的增加,土壤中有机质和硝态氮质量分数增加,速效磷质量分数减小,全氮和铵态氮无显著变化,表明土壤中有效氮质量分数增加,可影响自身及其他营养元素的吸收转化过程,从而对白羊草种群的生长产生一定影响。研究结果可为黄土丘陵区草地植被建设与管理,提供基础数据和科学依据。

英文摘要：

[Background] Grassland ecosystem plays a great role in improving regional environment condition and maintaining soil and water.With the nitrogen deposition increasing,vegetation characteristics and soil properties of grassland ecosystem would be changed,which in turn influence their effects on reducing soil erosion.Bothriochloa ischaemum community is considered as typical vegetation on the Loess Plateau.Besides,the nitrogen content of soil in this region is relatively low.Hence,researching nitrogen addition effect on the characteristics of B.ischaemum community has a great significance in vegetation construction and soil and water conservation.[Methods] B.ischaemum was seeded in the soil bin（2 m in length and 1m in width;soil bulk density was 1.2 g/cm~3） with 20 cm rowspacing for each plant.Totally,50 plants of B.ischaemum were seeded for each soil bin.Watered regularly during the first year and added nitrogen in the next year.Ammonium nitrate was used as nitrogen source with four nitrogen levels（0,2.5,5 and 10 g N/（m~2·a））,which was added to the soil once every two weeks from June to August（six times in total）.Moreover,the bare loess soil was selected as control.For each treatment,there were three repetitions and 15 soil bins totally.Inflorescence number and plant height of B.ischaemum were measured before each nitrogen addition,also the soil was sampled（0-5,5-10,10-20,and 20-30 cm） at the same time to determine the soil organic matter,total nitrogen and phosphorus,available nitrogen and phosphorus,p H and particle size distribution.At the end of growth stage,vegetation coverage,biomass of aboveground and roots（65 ℃,48 h） were measured.In addition to this,root characteristics such as total surface area were measured with WinRHIZO software.The differences of vegetation characteristics and soil properties between all treatments were analyzed by multiple comparison method.[Results] 1） The height and inflorescence number of B.ischaemum reached the maximum at the nitrogen addition level o