中文摘要：

生物量的地上-地下分配受到植物种类、年龄、气候等诸多因素的影响,并且生物量地上-地下分配会影响植物向土壤碳的输入,进而影响陆地生态系统碳循环。然而,由于根系采样的困难使得作为陆地生态系统碳循环模型中重要参数的地下-地上生物量比（根冠比,R/S）还存在很大不确定性。因此,本研究通过2007,2008和2009年连续3年对黄土高原围封草地和放牧草地的地上、地下生物量进行测定,并结合降水和气温资料,试图揭示围封与放牧草地根冠比的季节动态及其与环境因素的关系,为地下生物量的预测和陆地生态系统碳循环模型提供参考。结果表明,围封与放牧样地根冠比的季节变化呈现反抛物线型,最低值都出现在9月下旬;3年放牧样地的根冠比在不同生长季都显著高于围封样地（P〈0.05）,其平均值放牧样地分别为3.13,3.36和3.77,围封样地为2.55,2.56和3.13。相对于围封草地,放牧草地根冠比的变异更大;2个样地的根冠比年际间差异都不显著;根冠比与上月降水量、当月气温之间均呈显著负相关（P〈0.05）。利用降水量和气温建立的多元回归方程,可以很好地预测根冠比。

英文摘要：

The allocation between above-ground and under-ground biomass is influenced by many factors such as plant species,plant age and climate conditions.It can influence carbon input from plant to soil and the carbon cycle in terrestrial ecosystems.Because it is difficult to measure the root biomass precisely,there are still some uncertainties in the root-shoot ratio（R/S）,which is considered an important parameter in carbon cycle models of terrestrial ecosystems.The seasonal dynamics of R/S were investigated in grazed and ungrazed grasslands of the Loess Plateau in 2007,2008,and 2009,and the relationships between R/S,precipitation,and air temperature were analyzed to provide useful information for prediction of under-ground biomass and carbon cycle models in terrestrial ecosystems.The seasonal dynamics of R/S in both grasslands showed an anti-parabolic pattern,with the minimum values occurring in September.The mean R/S values in the three years in the grazed grassland（3.13,3.36 and 3.77 respectively） were significantly higher than those in the un-grazed grassland（2.55,2.56 and 3.13 respectively）（P0.05）;At a seasonal scale,there were significant negative correlations between R/S with precipitation and mean temperature in the last two months（P0.05）.Multiple regression equations were built to better predict R/S.