中文摘要：

黄土高原特别是干草原地区植被演替的研究比较薄弱。当前植物生态化学计量学的研究主要集中在植物叶片方面,对根系的研究较少。选取宁夏云雾山草原植被不同封育年限的土壤和植物样品,以生态化学计量学原理为基础,测定并分析了土壤与根系的碳（C）、氮（N）、磷（P）及其生态化学计量比与相互关系。结果表明：（1）随着封育年限的增加,土壤容重逐渐减小,土壤有机碳和全氮变异性较大,全磷变异性较小,且封育初期土壤有机碳和全氮含量先降后升,至封育20、30年,保持相对平稳。0~20 cm土层土壤的碳氮比（C∶N）、碳磷比（C∶P）、氮磷比（N∶P）分别为9.04~9.63、19.62~32.27、2.14~3.37,20~40 cm土层土壤的分别为8.68~9.22、15.74~26.32、1.80~3.03。土壤有机碳与全氮、全磷之间存在极显著的正相关。（2）植物根系C、N、P含量变化范围分别为357.6~381.4 g kg-1、7.35~8.18 g kg-1、0.54~0.70 g kg-1;根系中的C元素含量随封育年限的增加逐渐升高,N、P元素含量均小于全球平均值。根系C∶N随着封育年限的增加变异性较大,C∶P、N∶P随着封育年限的增加变异性较小。（3）植物根系的C∶N∶P化学计量特征受土壤的影响调控大于其自身,且土壤磷含量对植物根系C∶N∶P生态化学计量特征影响的显著性（p〈0.01）大于土壤氮含量（p〈0.05）。此外,该地区封禁后,草地生产力易受到土壤N含量的限制。

英文摘要：

【Objective】 The knowledge about vegetation succession,is essential to vegetation restoration,especially in steppe regions on the Loess Plateau. Recently stoichiometry has been effectively used to illustrate plant community succession and vegetation restoration. Balance of carbon（C）,nitrogen（N）,and phosphorus（P）in plant has been a hotspot of the research on biogeochemical cycle and plant ecology. However,most of the studies have focused on nutrients in plant leaves,and few did on the elements in plant roots,let alone,on ecological stoichiometrical characteristics of the soils and plant roots in steppe regions under enclosure on the Loess Plateau,relative to enclosure history. 【Method】This paper explored ecological stoichiometrical characteristics of the soils and plant roots in grasslands different in enclosure history（1 a,12 a,20 a and 30 a）on the Yunwu mountain of Ningxia Province. The steppe region under study is located between 106°26′E and 106°30′E and between 35°59′N and 36°02′N. The Yunwu Mountain lies across the center of the Loess Plateau,with an altitude ranging from 1 000 to 1 800 m above sea level. The study area has a semi-arid climate,characterized by heavy seasonal rainfalls causing recurrent flooding and drought,with mean annual temperature being 5 ℃ and mean annual precipitation being 445 m. The typical natural vegetation of this region is grassland. Samples of the soils and plant roots in the region were collected and analyzed for organic carbon（SOC）,total nitrogen（TN）,and total phosphorus（TP）,and their stoichiometrical ratio and interrelationships,using the time-space substitution method,based on the principles of stoichiometry. 【Result】Result show that with the enclosure going on,soil bulk density decreased gradually,SOC and TN varied more significantly,while TP did less. In the early years of enclosure,SOC and TN declined first and then rose till the enclosure reached 20 or 30 years old,when they leveled off. Soil organic carbon,total nitro