中文摘要：

2008年和2009年（均为枯水年）,在半干旱区内蒙古太仆寺旗农田-草地生态系统国家野外站开展观测实验,通过观测蒸散发（波文比系统）、土壤水分（烘干称重法）、降水量,以及植被土壤特征调查,基于水量平衡理论,对比研究了3块天然草地、3块不同退耕时间草地共6个样地的水分收支,旨在定量地评估退耕草地的水分收支,为采取科学措施促进退耕草地尽快向天然草地过渡提供依据。结果表明：1）随着退耕时间增加,植被盖度逐渐增加,但是群落中科、属、种的数量趋于减少,且优势种从一年生的中旱生草本植物逐渐转变成多年生的旱生草本植物；2）植被蒸腾是草原植被主要的耗水途径,随着退耕时间增加,退耕草地的蒸散发量呈增加趋势,其最大值在4.5-5.8 mm·d-1之间；3）退耕草地土壤含水量平均值为0.09 m3·m-3,其水分剧烈变化主要发生在距地表60 cm内,且随退耕时间增加土壤含水量减少,而天然草地土壤含水量平均值为0.06 m3·m-3,其水分剧烈变化发生在距地表20 cm内；4）随退耕时间增加,退耕草地与天然草地的土壤水分与蒸散发在数值上差距逐渐缩小；5）退耕草地水分收支基本平衡,但在极枯年份（降水量174 mm）的生长季,降水不能满足蒸散发需求,呈现水分亏损。退耕草地逐步向天然草地过渡,但是退耕草地的土壤水分在逐渐减少,呈现“生境干旱化现象”。今后应加强对草地的封育与监测,促进植物群落向水分利用效率更高、更适应半干旱环境的方向演替。

英文摘要：

Aims Water is the most constraining factor for implementing the“Grain for Green”project in arid and semiarid steppe regions of China. Our objective was to determine how vegetation and water budget would change following restoration in semiarid grasslands. Methods Field experiments were conducted in the Farmland and Grassland Ecosystem Observation Station of Beijing Normal University during the growing seasons between 2008 and 2009 in Taibus Banner, located in a typical agro-pastoral zone in Inner Mongolia, northern China. Vegetation composition was studied in six plots representing the natural grasslands （consisting of three separate plots at three sites） and the restored grasslands with two, four and eight years of history, respectively. Three Bowen ratio towers were set up to collect the local meteorological data and to estimate evapotranspiration in each plot. Soil water content was measured using the gravimetric method and soil evaporation was determined by micro-lysimeters with a volume of 200 cm-3 each. Important findings In the restored grassland plots, plant community coverage increased but the number of spe-cies decreased with time of restoration, and the dominant species were gradually transformed from the annual xeric herbs into perennial xerophytic herbs. Plant transpiration, with maximum values of 4.5 to 5.8 mm·d-1, con-sumed most of the water in the restored grasslands, and longer the restoration higher the rate of evapotranspira-tion. Soil water in the restored grassland plots was 0.09 m3·m-3 in average, compared with 0.06 m3·m-3 in the natural grassland plots. The soil water content was found to be greatest at 20 cm depth in the natural grassland plots;whereas it was found to be greatest at 60 cm depth, and decreased with time of restoration, in the restored grassland plots. The water budget was balanced at an annual scale in the restored grassland plots because the＆amp;nbsp;evapotranspiration was lower than precipitation, while in the growing season of an extreme dry year when annual