中文摘要：

对内蒙古贝加尔针茅草原群落开展氮素添加实验，设置6个氮素添加水平，氮素类型为硝酸铵（NH4 NO3）：对照（N0），1．5 g N／m^2（N15）、3．0 g N／m^2（N30）、5．0 g N／m^2（N50）、10．0 g N／m^2（N100）和15．0 g N／m^2（N150）。利用凋落物网袋分解法，研究贝加尔针茅、羊草和冷蒿3种植物不同器官凋落物分解对氮素添加的响应。结果表明：1）3种植物不同器官凋落物C残留率的季节变化与植物残留率的变化趋势相同，呈释放模式。2）贝加尔针茅各器官凋落物分解95％所需时间为3．79～5．75年。羊草各器官凋落物分解95％所需时间为3．12～6．34年。冷蒿各器官凋落物分解95％所需时间为2．69～6．82年。3）不同植物、不同器官凋落物分解速率对氮素添加响应不同，初始化学组成不是控制凋落物分解的主要影响因子。4）凋落物分解速率与土壤铵态氮、硝态氮和土壤含水量无显著相关性，冷蒿凋落物与土壤微生物量碳氮显著相关。5）本试验进行期间，氮素添加对各器官凋落物分解无一致影响，仅个别施氮水平对不同器官凋落物分解速率及养分释放有促进或抑制作用。在氮素添加背景下，凋落物分解受到时间、分解底物类型及氮素输入水平的共同影响。

英文摘要：

In grassland sites dominated by Stipabaicalensis grassland,we imposed six nitrogen levels：N0 , N15 ,N30 ,N50 ,N100 and N150 ,respectively.At the various levels of nitrogen deposition,we placed litterbag with leaf,stalk and root litter ofS.baicalensis（Sb）,Leymuschinensis（Lc）,Artemisiafrigida（Af）on the experiment plots.Plots were sprayed NH4 NO3 in the middle of June and July.The dynamics of C and N during litter decomposition were different.Different decomposition and nitrogen exchange patterns were observed a-mong the three dominant plants studied.The current paradigm suggests close relationships between decompo-sition rates,C and N content and ,C/N ratio.The results contrasted with that.Decomposition time for 95%degradation of litter was 3.79-5.75 years for Sb,3.12-6.34 years for Lc,and 2.69-6.82 years for Af.The initial nutrient element concentration of litter determined the nutrient release pattern and the amount of the fi-nal net element release ratio.Through correlation analysis of litter decomposition and soil physical and chemi-cal properties,we found：litter decomposition to be weakly correlated with NH4 ＋-N ,NO3 --N and soil mois-ture content.The carbon and nitrogen content of soil microbial biomass was significantly correlated with litter decomposition rate of Af.Thus,microorganisms are the key factor controlling litter decomposition rate of Af. Therefore,in the short term,nitrogen deposition had no significant effect on litter decomposition,only individ-ual level of nitrogen promoting or inhibiting effect on different litter decomposition rate and nutrient releasing. It implied that the effects of N addition on litter decomposition depended on time,organs,and also N addition levels.