为探究添加外源氮对黄土高原不同恢复年限林下草地生态系统恢复的影响，对黄土丘陵区不同氮添加量下恢复10、20和40 a刺槐（Robinia pseudoacacia Linn.）林林下草地的物种组成、α-多样性指数和功能多样性指标进行了比较。结果表明：不同恢复年限林下草地的物种组成和生活型结构差异较大。比较而言，恢复初期（10 a）物种最少，恢复中期（20 a）物种最多；生活型结构则随着恢复年限增加变得越来越复杂。不同氮添加量下林下草地的物种组成和生活型结构也存在较大差异，物种数量在恢复初期低氮（氮添加量为4 g·m^-2）下最多，在恢复中期和后期（40 a)中氮（氮添加量为8 g·m^-2）下最多；并且，氮添加量与恢复中期一年生和二年生草本重要值呈显著（P＜0.05）负相关，与恢复后期灌木和半灌木重要值呈显著正相关。总体来看，恢复中期的物种丰富度和Shannon-Wiener指数最高；氮添加量对α-多样性指数的影响效应因恢复年限而异，低氮利于恢复初期、中期和后期林下草地物种多样性提高，中氮利于恢复中期和后期林下草地物种多样性提高。供试所有林下草地的功能多样性对氮添加量的响应极显著（P＜0.01），但各个恢复年限林下草地的功能多样性对氮添加量的响应却不显著。综合分析认为，恢复中期和中氮水平下黄土丘陵区刺槐林林下草地的物种多样性最高、功能多样性最佳。

To investigate the effects of exogenous nitrogen addition on the restoration of understory grassland ecosystems with different restoration years in the Loess Plateau of China, the species composition, αdiversity indexes, and functional diversity indexes of understory grasslands of Robinia pseudoacacia Linn. plantation in the Loess Hilly Region with 10, 20, and 40 a of restoration under different nitrogen addition dosages were compared. The results show that there are relatively large differences in species composition and life form. structure among understory grasslands with different restoration years. Comparatively, the early restoration stage (10 a) has the fewest species, while the mid-restoration stage (20 a) has the most species; the life form. structure becomes increasingly complex with increasing restoration years. There are also relatively large differences in species composition and life form. structure among understory grasslands under different nitrogen addition dosages, the species number is the highest under low nitrogen (nitrogen addition dosage of 4 g·m^-2) in the early restoration stage, and under medium nitrogen (nitrogen addition dosage of 8 g·m^-2) in both the mid-and late (40 a) restoration stages; additionally, nitrogen addition dosage shows a significant (P<0.05) negative correlation with the importance value of annual and biennial herbs in the mid-restoration stage, and a significant positive correlation with the importance value of shrub and semi-shrub in the late restoration stage. Overall, species richness and Shannon-Wiener index are the highest in the mid-restoration stage; the effects of nitrogen addition dosage on α-diversity indexes vary with restoration years, low nitrogen is beneficial to the enhancement of species diversity in understory grasslands in early, mid-, and late restoration stages, while medium nitrogen is beneficial to species diversity in mid- and late restoration stages. The responses of functional diversity of all tested understory grasslands to nitrogen addition dosage are extremely significant (P<0.01), but the responses of functional diversity of understory grasslands with different restoration years to nitrogen addition dosage are not significant. The comprehensive analysis suggests that in the mid-restoration stage and medium nitrogen levels, the species diversity is the highest in understory grasslands of R. pseudoacacia plantation in the Loess Hilly Region, and the functional diversity is the best.