中文摘要：

以黄土丘陵沟壑区延河流域3个植被带（森林带、森林草原带、草原带）不同植物群落的细根（直径d≤2 mm）为研究对象,对植物群落0~20 cm土层细根根长密度、根系生物量、比根长进行了研究。结果表明,3个植被带不同植物群落直径d≤1 mm的根系根长密度占总细根的90%以上,根系生物量占53%以上。有较大比例的直径d≤1 mm的根系不仅在植物的生长活动中贡献很大,且在改善土壤结构方面发挥重要作用;植物群落细根d≤2 mm生物量从南部森林带到北部草原带呈递减趋势,比根长从南部森林带到北部草原带呈递增趋势,草原带的根长密度显著低于其他植被带（P〈0.05）。根长密度与根系生物量变化趋势一致,乔、灌群落高于草本群落,但比根长表现为草本群落显著高于乔、灌群落（P〈0.05）;乔、灌群落和以达乌里胡枝子为次优种或优势种的群落根长密度和根系生物量较高,植物根系生长能力强,根系发达。以狼牙刺、杠柳、白羊草为优势种的群落比根长较高,具有较高的水分、养分潜在吸收率和生长速率;以狼牙刺、杠柳、铁杆蒿、白羊草为优势种的群落,根系参数（根长密度、根系生物量、比根长）在森林草原带显著高于其他植被带（P〈0.05）,这些群落在此环境下,更能有效吸收土壤水分、养分,在提高土壤抗侵蚀能力方面发挥着重要作用。

英文摘要：

The objective of this study was to clarify the characteristics of fine roots（ diameter d≤2 mm）parameters（ root biomass,root length density and specific root length） in 0 ~ 20 cm soil layer under different plant communities in forest zone,forest steppe zone and steppe zone of the Yan river basin in hill-gully region of Loess Plateau. Soil drill method was used to collect root samples. Vernier caliper was used to measure root diameter in two grades of 0 ~ 1 mm and 1 ~ 2 mm. Root length of two diameter grades was measured,then root samples were dried to constant weight at 85℃ and weighed. The main results were as follows：（ 1） In the three vegetation zones,the proportion of root length density（ d≤1 mm） and root biomass of different communities accounted for 90% and 53% of the total fine root,respectively. The root（ d≤1 mm）,which accounted for a large proportion of the total fine root,not only contributed greatly for the plant growth,but also played an important role in improving soil structure.（ 2） Root biomass（ d≤2 mm） of different plant communities showed a decreasing trend from southern forest zone to northern steppe zone,while specific root length showed a increasing trend from southern forest zone to northern steppe zone,root length density in steppe zone was significantly lower than those in other two vegetation zones（ P〈0. 05）. The variation tendency of root length density and root biomass was the same,and root length density and root biomass of tree community and shrub community were higher than those of herb community. But specific root length of herb community was significantly higher than those of tree community and shrub community（ P〈0. 05）.（ 3） Root length density and root biomass of tree community,shrub community and Lespedeza davurica dominated community were higher than those of other communities,the root growth of these communities was vigorous,and root system of these communities developed well. The specific root lengths of Sophora viciifolia,P