Abstract: To explore the response of canopy structure and diversity characteristics to nitrogen (N) and phosphorus (P) addition,the typical grassland community dominated by Bothriochloa ischaemum was selected in the loess hilly-gully region. A split-plot experiment,N main-plot:0,25,50 and 100 kg·hm^-2·a^-1,P subplot:0,20,40 and 80 kg·hm^-2·a^-1,was conducted. Community height,leaf area index,fraction of photosynthetically active radiation,light asymmetry,aboveground biomass and Shannon-Wiener index were measured. The results showed that community height and leaf area index increased significantly after N addition;N and/or P addition significantly increased fraction of photosynthetically active radiation,light asymmetry and aboveground biomass,while decreased Shannon-Wiener index. N and P addition increased the biomass contribution of short perennial grasses and perennial legumes,respectively;50 and 100 kg·hm^-2·a^-1 combined with P addition increased the biomass contribution of tall perennial forbs. Structural equation model revealed that increased light asymmetry by N and P addition had no effect on Shannon-Wiener index. In summary,when taking measures of fertilization to promote degraded grassland restoration in loess hilly-gully region,the relationship between increasing aboveground biomass and decreasing species diversity should be considered.

Key words: Community height, Leaf area index, Light asymmetry, Shannon-Wiener index

摘要： 为探究氮磷添加对黄土丘陵区典型草地群落冠层结构和多样性特征的影响，本研究以白羊草(Bothriochloa ischaemum)群落为研究对象，采用裂区试验设计，主区设4个氮(N)处理(0，25，50，100kg·hm^-2·a^-1)，副区设4个磷(P₂O₅)处理(0，20，40，80kg·hm^-2·a^-1)，测定了群落高度、叶面积指数、光合有效辐射百分比、光不对称性、地上生物量和Shannon-Wiener指数。结果表明：氮添加显著增加了群落高度和叶面积指数；氮磷添加显著增加了光合有效辐射百分比、光不对称性和地上生物量，降低了Shannon-Wiener指数。单施氮和磷分别增加了矮多年生禾本科草和多年生豆科草生物量贡献比；50和100kg·hm^-2·a^-1的氮添加水平与磷配施增加了高多年生杂类草生物量贡献比。结构方程模型表明，氮磷添加下光不对称性的增加对物种多样性的影响不显著。综上所述，黄土丘陵区在采取施肥措施促进退化草地恢复过程中，要权衡地上生物量提高和物种多样性下降的关系。

关键词: 群落高度, 叶面积指数, 光不对称性, Shannon-Wiener指数