Abstract: Leaf chlorophyll is a critical substance for plant photosynthesis,while nitrogen is an important component of chlorophyll. Therefore,soil nitrogen may be an important factor to affect leaf chlorophyll content. However,little attention has been paid to whether leaf chlorophyll content exhibits a nonlinear response to a soil nitrogen gradient and the possible mechanisms behind it. In this study,dominant plants in alpine meadows on the Qinghai-Tibet Plateau were used as the research object,and an eight-year nitrogen addition gradient experiment was conducted with nitrogen addition levels of 0,2,4,8,16,and 32 g·m^-2·a^-1. We found that leaf chlorophyll content showed a first increasing and then decreasing pattern in response to nitrogen addition gradient,and the peak value mostly appeared at 8 g·m^-2·a^-1. At low nitrogen levels,leaf chlorophyll content was positively correlated with soil inorganic nitrogen but negatively with soil pH and plant height. At high nitrogen levels,leaf chlorophyll was not correlated with soil inorganic nitrogen or pH,but exhibited a nonlinear relationship with plant height. In addition,we found that regardless of low or high nitrogen additions,there was a parabolic relationship between leaf chlorophyll content and specific leaf area. This study demonstrated that the leaf chlorophyll of alpine plants exhibited a threshold in response to nitrogen additions gradient,and the underlying mechanisms below or above that threshold are different. Overall,this study suggests that in the context of nitrogen enrichment,the relationship between alpine plant chlorophyll and soil nitrogen availability is complex,which may further affect carbon sequestration in alpine grassland ecosystems.

Key words: Chlorophyll content, Nitrogen addition gradient, Soil physicochemical properties, Plant functional traits, Alpine meadow

摘要： 叶绿素是植物进行光合作用的关键物质，氮素是叶绿素的重要组成元素。因此，土壤氮可能是影响叶绿素含量变化的重要因素。然而，目前研究较少关注氮梯度下叶绿素含量的变化及背后原因。本研究以青藏高原高寒草甸优势植物为研究对象，利用8年的氮添加梯度实验，氮添加水平为0,2,4,8,16,32 g·m^-2 ·a^-1。研究发现叶绿素含量沿氮添加梯度呈抛物线响应，且峰值多在8 g·m^-2 ·a^-1左右。低氮水平下，叶绿素含量与土壤无机氮显著正相关，与土壤pH值和植物高度负相关。高氮水平下，叶绿素与土壤无机氮和pH值均不相关，与植物高度呈非线性关系。此外，不管低氮或高氮水平添加叶绿素含量和比叶面积均呈抛物线关系。本研究表明高寒植物叶绿素对氮添加梯度表现出非线性响应且存在阈值，并且低于或高于氮阈值的影响因子是不同的。总之，本研究表明在氮富集的背景下高寒植物叶绿素与土壤氮可利用性存在复杂关系，可能会进一步作用于高寒草地生态系统碳固持。

关键词: 叶绿素含量, 氮添加梯度, 土壤理化性质, 植物功能性状, 高寒草甸