中文摘要：

大气氮沉降已经成为全球性的环境问题,为更好地了解氮沉降增加对草地土壤质量的影响,2013年7月,以白羊草（Bothrichoa ishaemum）群落为研究对象,通过人工构建白羊草种群小区,在中国科学院水土保持研究所人工干旱大厅进行模拟氮沉降的控制试验。本试验设置了裸地（LD）、对照（N0）和3个氮（尿素）添加处理,依次为2.5g·m（-2）·year（-1）（N2.5）,5.0g·m（-2）·year（-1）（N5.0）和10g·m（-2）·year（-1）（N10.0）,探讨了氮沉降对白羊草群落土壤总有机碳（TOC）和易氧化有机碳（OC）各组分的影响。结果表明,低浓度（N2.5）氮添加提高了白羊草土壤TOC含量,高浓度（N10.0）氮添加降低了土壤TOC含量;氮添加降低了白羊草土壤C_（active）（活性碳组分）/TOC比例,增加了C_（passive）（惰性碳组分）/TOC比例;在N5.0和N10.0氮添加水平,C_（passive）/TOC趋向于达到50%;土壤活性系数随氮添加浓度的增加而降低。综上所述,随着氮添加浓度增加土壤C_（active）与C_（passive）含量趋于平衡,土壤碳库更加趋于稳定。

英文摘要：

Atmospheric N deposition has become a global environmental problem, in order to examine the effects of N deposition on soil organic carbon fractions in grassland, a stimulated N deposition experiment was conducted in the Institute of Soil and Water Conservation in July 2013 by building Bothriochloa ischae- mum community plots. The four treatments selected for present experiment were： 0 g m-2 year-1 （N0）,2.5 g m-2 year 1（N2.5）,5.0 g m-2 year-1（N5.0） and 10 g m-2 year-l（N10.0）. The re- sults showed that low-level（N2.5） N addition increased total soil organic carbon （TOC） contents signifi- cantly while high-level（N10.0） N addition decreased soil TOC contents. N addition decreased the first （C1） fraction of OC, C1/TOC, Csctive/TOC and lability index, and increased the third （C3） fraction of OC and Cpassive/TOC. Moreover, at N addition level of N5.0 and N10.0, Cpassive/TOC tended to reach 50%. In conclusion, the soil Cactive and Cpassive tended to reach a balance with N addition, and soil carbon pool tended to be more stable.