中文摘要：

采用野外条件下人工外源施加氮素模拟氮沉降的方式，设置了0、6、12和24g·m^-2 4个纯氮素投加水平，在试验第一年（2007年）将不同水平的氮素随机投加到样方中，随后两年（2008～2009年）不再继续投加，研究氮沉降对荒漠化草原草本植物物种多样性和群落物种组成的影响。结果表明：植物物种丰富度和多度在年内均随着氮素投加水平的增大而降低，且物种多度比丰富度的降低程度更大；植物物种丰富度和多度年际间的变化则表现为低氮水平下差异显著（P〈0.05），高氮水平下差异不显著的规律（P〉0.05）。氮沉降改变了草本层片植物群落的物种组成，相对于多年生禾本科植物，多年生非禾本科植物在氮素处理下消失的概率更大。可见，氮沉降会降低荒漠化草原草本植物的物种多样性，改变草本植物群落的物种组成，且对荒漠化草原草本植物群落的影响是一个长期的过程。

英文摘要：

In order to investigate the impacts of nitrogen （N） deposition on species diversity and community composition of herbaceous vegetation in desert steppe,we conducted a simulative N deposition experiment at ambient environment by adding N fertilizer into the plots in Shapotou area,northern China. We set four levels of N addition,0,6,12 and 24 g · m ^-2 ,the different levels of N were singly added in the first year and then none for the following two years. The results indicated that both plant species diversity and communi- ty abundance decreased with the increase of N addition levels at a given year, especially,community abun- dance showed the greatest decline compared to the species diversity. The interannual variations of both spe- cies diversity and community abundance showed a similar phenomenon that the difference among the levels of N addition was significant under the low levels of N addition,but nor for the high levels of N addition. N deposition changed plant community composition of herbaceous vegetation layer in desert steppe, and the perennial forbs were more sensitive to be lost under N enrichment in contrast to perennial grasses,and fi- nally the transformed community was dominated by fewer dominate species. It can be concluded that N deposition would reduce species diversity, change community composition of herbaceous vegetation, and the impacts of N deposition on desert ecosystems would not be eliminated immediately when N enrichment ceased.