人类活动和自然过程引起的大气氮沉降的加剧对全球氮循环过程产生了极大影响。随着全球氮沉降水平的持续增加，植物的生理生态过程也发生巨大变化，进而可能对陆地生态系统的结构和功能产生极大影响。青藏高原高寒草地生态系统主要受到氮素限制，目前关于氮沉降对高寒草地植物的生理生态过程的影响依然不明确。基于此，本研究主要以青藏高原高寒草甸四种不同功能群优势植物赖草（禾本科）、线叶嵩草（莎草科）、多裂委陵菜（杂类草）、扁蓿豆（豆科）为研究对象，在原生生境中采用野外原位控制实验，设置不同的氮添加梯度以模拟不同程度的氮沉降水平（0、8、24、40、56、72kgNha^-1yr^-1）。通过连续三年的持续定点观测和数据采集，重点研究不同功能群优势植物对氮沉降加剧的响应策略，基于植物形态和生理特征解析不同功能群优势植物对氮沉降加剧的差异性响应特点，并结合土壤因子和气候因素，探明高寒草甸不同功能群植物对氮沉降差异性响应的生理生态机制。本研究主要研究结果和创新性结论如下：

（1）土壤理化性质的变化表现出一定的年际差异。土壤硝态氮和铵态氮含量随着氮添加梯度的增加均有明显上升趋势，土壤速效磷表现出先升高后降低的变化趋势。土壤主要营养元素含量均随着氮添加梯度的增加出现不同程度的波动。土壤pH随着氮添加梯度的增加呈现下降趋势，随着施氮年份的增加下，高氮添加下下降趋势明显。氮添加梯度对高寒草甸土壤硝态氮、速效钾含量、硫含量和土壤pH均有显著影响。施氮年份对土壤速效钾和钾含量无显著交互作用。氮添加与年份对土壤速效磷、总碳和土壤pH有显著交互作用。在氮添加对整个土壤理化性质的影响变量中，土壤pH，施氮年限、土壤铵态氮、硝态氮和磷、硫、钾含量在所有变量中具有较高的解释权重。土壤硝态氮含量与赖草的高度和生物量呈正相关，而对其他功能群植物的高度和生物量有不同程度的负效应。土壤硝态氮与赖草光合速率呈正相关关系，与其他三种功能群植物光合速率均呈负相关关系。氮添加下，土壤pH的变化对不同功能群植物的生理过程影响较大。整体而言，氮添加和气候因素更倾向于直接影响植物的生理生态过程。

（2）氮添加显著改变了青藏高原高寒草甸的植物群落组成，增加了禾本科植物的相对多度，降低了莎草科、杂类草和豆科植物相对多度。氮添加对高寒草甸植物群落组成的影响存在一定的年际差异。禾本科植物优势度的增加可能对其他功能群植物生理生态过程产生不同程度的负面影响。其高度和生长优势可能会导致遮蔽效应和资源竞争，进而削弱其他功能群植物的光合竞争力，并导致生长受到抑制。

（3）不同功能群植物的高度排列顺序为赖草>线叶嵩草>多裂委陵菜>扁蓿豆。高氮添加下四种不同功能群植物地上生物量的大小排列顺序为赖草>线叶嵩草>多裂委陵菜>扁蓿豆。氮添加和年份均对四种植物的地上生物量和高度有不同程度的影响。氮添加显著促进了赖草的生长，而高氮添加（72kgNha^-1yr^-1）则显著抑制了豆科植物扁蓿豆的生长。

（4）赖草的净光合速率随着氮添加梯度的增加而增加，中高氮添加水平对赖草净光合速率（Pn）的促进作用最明显。线叶嵩草、多裂委陵菜、扁蓿豆的Pn在高氮条件（72kgNha^-1yr^-1）下均受到明显抑制，同线叶嵩草相比，多裂委陵菜和扁蓿豆的Pn对氮添加梯度的增加更为敏感。赖草的气孔导度（Gs）随着氮添加梯度的增加称此案明显的上升趋势，而其他三种植物Gs的变化随氮添加梯度的增加出现一定的变异性，但均在高氮添加水平下（72kgNha^-1yr^-1）显著降低。氮添加下，四种不同功能群植物叶片胞间二氧化碳浓度（Ci）变化波动较小，整体无明显一致性的变化规律。赖草的蒸腾速率（Tr）的变化同Pn、Gs的变化趋势相近，都随着氮添加梯度的增加显著增加。而氮添加处理下线叶嵩草、多裂委陵菜和扁蓿豆叶片Tr的变异性较大，无明显一致性的变化规律，可能受环境影响较大。赖草、多裂委陵菜和扁蓿豆的水分利用效率（WUE）在高氮添加下显著降低，而线叶嵩草的WUE变异性较大，无明显规律，且年际间差异较大。

（5）高氮添加显著提高了高寒草甸禾本科植物赖草叶片的总叶绿素含量（Chl）、叶绿素a含量（Chla），Chla/b值和Rubisco酶活性，降低了赖草叶片的Chlb含量，也在一定程度上提高了赖草叶片的类胡萝卜素含量（Car）。氮添加对莎草科植物线叶嵩草叶片光合色素和Rubisco酶活性的影响有明显的年际差异，受外界条件的影响较大。不同年份、不同氮添加处理下豆科植物扁蓿豆，杂类草植物多裂委陵菜的光合色素的响应特征较稳定。高氮添加显著抑制了多裂委陵菜和扁蓿豆的光合酶活性，但是对其叶绿素含量并无显著影响。

（6）赖草叶片的非结构性碳水化合物（NSC）含量表现出随着氮添加梯度的增加而降低的变化趋势。线叶嵩草、多裂委陵菜、扁蓿豆叶片的NSC含量均在高氮添加水平下显著降低。赖草叶片的可溶性糖（SS）含量表现出随着氮添加梯度的增加而降低的变化趋势，线叶嵩草、多裂委陵菜、扁蓿豆叶片的SS含量在不同氮添加梯度下均无显著变化。赖草叶片的淀粉（ST）含量表现出随着氮添加梯度的增加而降低的变化趋势，线叶嵩草、多裂委陵菜、扁蓿豆叶片的ST含量在高氮添加处理下均显著下降。

（7）氮添加对不同功能群植物叶片含碳量无显著影响。氮添加显著提高了赖草和线叶嵩草叶片含氮量，而对杂类草多裂委陵菜和豆科植物扁蓿豆无明显影响。氮添加显著降低了赖草和线叶嵩草叶片C/N值，而对多裂委陵菜和扁蓿豆叶片C/N无显著影响。

（8）氮添加对不同功能群类型植物叶片丙二醛（MDA）含量影响不同，且有一定的年际差异。高氮添加（72kgNha^-1yr^-1）显著降低赖草叶片的MDA含量，而对线叶嵩草和多裂委陵菜叶片MDA含量无显著影响，但是显著提高了扁蓿豆叶片的MDA含量。氮添加对不同功能群类型植物叶片超氧化物歧化酶（SOD）、过氧化物酶（POD）和过氧化氢酶（CAT）活性的影响不同，且有一定的年际差异。不同氮添加处理均不同程度的提高了赖草叶片的SOD活性，线叶嵩草叶片的SOD活性对氮添加不敏感，高氮添加显著提高了多裂委陵菜和扁蓿豆叶片的SOD活性。氮添加对赖草叶片POD活性提升作用明显，高氮添加（72kgNha^-1yr^-1）显著提高了扁蓿豆叶片的POD活性，对线叶嵩草和多裂委陵菜叶片POD活性的影响具有一定的变异性。赖草叶片的CAT活性有随着氮添加梯度的增加而上升的趋势，线叶嵩草叶片的CAT活性在不同年份间出现了一定的变异性。N4、N5水平氮添加均对多裂委陵菜和扁蓿豆叶片的CAT活性有不同程度的提升作用。

（9）在形态可塑性方面：赖草通过其快速生长策略提高地上生物量累积和地上部高度以获取更多的地上空间和光照资源；在光合生理特性方面，赖草具有较高的氮利用效率，通过吸收可利用氮素，促进其叶绿素的合成，特别是Chla的合成，并提高了光合相关酶Rubisco的活性，气孔导度也随着氮添加梯度的增加而增加，提高了其光合能力；在抗逆性方面，氮添加提高了赖草的主要抗氧化酶的活性，降低了体内MDA的累积，抗氧化能力明显提高。而其他三种功能群类型植物在表型和生理上受氮添加的消极影响较大，特别是豆科植物扁蓿豆，高氮添加（72kgNha^-1yr^-1）会对其产生一定的胁迫作用。

（10）通过构建结构方程模型（SEM）分析发现，氮添加能够直接提高赖草的光合能力、抗逆性和生长优势。氮添加下，土壤pH比土壤养分状况对赖草生理生态过程的影响更为显著，赖草的生理生态过程对土壤pH的变化更为敏感。气候因素（温度和降水）更倾向于通过直接和间接作用对赖草生理生态过程产生影响，而氮添加对赖草的直接作用则更明显。线叶嵩草生理生态过程更易受到气候因素的影响。线叶嵩草对土壤养分的变化不敏感，氮添加和气候因子都倾向于直接作用于线叶嵩草的生理生态过程。氮添加和气候因素更倾向于直接作用于多裂委陵菜的生理生态过程。多裂委陵菜的生理生态过程可能对土壤养分变化并不敏感。扁蓿豆的生理和生长过程可能对氮添加和土壤酸化更为敏感。扁蓿豆的光合过程同样对气候因素较敏感，温度和降水的增加也并不利于扁蓿豆的生长。氮添加下，土壤pH的下降均不利于四种不同功能群植物的光合和生长，且对扁蓿豆的生长和光合的抑制作用最强。

综上，氮添加和气候因素改变了土壤养分状态和pH。与土壤养分相比，土壤pH对植物生理生态过程的调控作用更大。氮添加对禾本科植物赖草生理生态过程的正效益要高于其他功能群植物。气候因素对氮添加的影响效果也起到重要的调控作用。整体而言，氮添加下植物群落向禾本科植物为优势群落演变很可能是因为氮添加提高了禾本科植物在表型特征、光合生理及抗逆能力方面的优势，进而提高其在群落中的优势地位，而其他功能群则可能是因为氮添加和气候变化下土壤养分失衡和土壤酸化、以及种间竞争等多重压力的影响导致生长和生理生态过程均受到不同程度的抑制。

The increase in atmospheric nitrogen deposition caused by human activities and natural processes has a great impact on the global nitrogen cycle. As the level of global nitrogen deposition continues to increase, the physiological and ecological processes of plants may also undergo tremendous changes, which in turn will have a great impact on the structure and function of terrestrial ecosystems. The alpine grassland ecosystem on the Qinghai-Tibet Plateau is mainly limited by nitrogen. At present, the impact of nitrogen deposition on the physiological and ecological processes of alpine grassland plants is still unclear. Therefore, this study took four dominant species of plants within different functional groups in the alpine meadow of the Qinghai-Tibet Plateau as the research objects including Leymus secalinus (Grass), Kobresia capillifolia (Sedge), Potentilla multifidi (Forb) and Medicago ruthenica (Legume). In the native habitat, field in-situ control experiments were used, and different nitrogen addition gradients were set to simulate different levels of nitrogen deposition (0, 8, 24, 40, 56, 72kgNha^-1yr^-1). Through fixed-point observation and data collection for three consecutive years, we focused on the response strategies of dominant species of different functional groups to increased nitrogen deposition. Differential response mechanism of dominant species of different functional groups were analysed and explored based on plant eco-physiological traits, soil properties and climatic factors. The main research results and innovative conclusions of this study are as follows:

(1) The changes in soil physical and chemical properties showed certain inter-annual differences. The content of soil nitrate nitrogen and ammonium nitrogen had an obvious upward trend with the increase of nitrogen addition gradients, and soil available phosphorus shows a trend of first increasing and then decreasing. The content of the main nutrient elements in the soil fluctuates to varying degrees with the increase of nitrogen addition gradients. The soil pH showed a downward trend with the increase of nitrogen addition gradients, and with the increase of nitrogen application years, the downward trend of high nitrogen addition was much more significant. Nitrogen addition had a significant effect on grassland nitrate nitrogen, available potassium content, sulfur content and soil pH. The year of nitrogen application had no significant interaction effect on soil available potassium and potassium content. Nitrogen addition and year have a significant interaction effect on soil available phosphorus, carbon and soil pH. Among the influencing variables, soil pH, nitrogen application years, soil ammonium nitrogen, nitrate nitrogen and phosphorus, sulfur, and potassium content had higher weights. The soil nitrate nitrogen content is positively correlated with the height and biomass of Leymus secalinus, but has negative effects on the height and biomass of other functional groups of plants to varying degrees. Soil nitrate nitrogen is positively correlated with the photosynthetic rate of Leymus secalinus, yet negatively correlated with the photosynthetic rate of dominant species in other three functional groups. Under nitrogen addition, changes in soil pH have a greater impact on the physiological processes of species of different functional groups. On the whole, nitrogen addition and climatic factors tend to directly affect the physiological and ecological processes of plants.

(2) Nitrogen addition significantly changed the composition of plant communities in alpine meadows on the Qinghai-Tibet Plateau. The relative abundance of grasses was significantly increased, and other functional groups showed certain decrease under high nitrogen addition. Also, The effects of nitrogen addition on the composition of plant communities in alpine meadows have certain interannual differences. The dominance increase of grass may have negative effects on the physiological and ecological processes of other functional groups of plants. Its absolute advantages in height and growth may lead to sheltering effects and resource competition, which in turn weakens the photosynthetic competitiveness of plants in other functional groups and leads to growth inhibition.

(3) The order of height of plants of different functional group types is Leymus secalinus> Kobresia capillifolia> Potentilla multifida> Medicago ruthenica. The aboveground biomass of four different functional group types of plants under high nitrogen addition is arranged in the order of Leymus secalinus> Kobresia capillifolia> Potentilla multifida> Medicago ruthenica. Both nitrogen addition and year have certain influences on the aboveground biomass and height of the four functional type plants. The addition of nitrogen significantly promoted the growth of Leymus secalinus. The addition of high nitrogen significantly inhibited the growth of Medicago ruthenica.

(4) The net photosynthetic rate of Leymus secalinus increased with the increase of nitrogen addition gradients, and the effect of medium and high nitrogen addition levels on the net photosynthetic rate of Leymus secalinus was most obvious. The net photosynthetic rate of Kobresia capillifolia, Potentilla multifida, and Medicago ruthenica were significantly inhibited under high nitrogen conditions (72kgNha^-1yr^-1). Compared with Kobresia capillifolia, Potentilla multifida and Medicago ruthenica were significantly inhibited under high nitrogen conditions (72kgNha^-1yr^-1). The net photosynthetic rate of Medicago ruthenica was more sensitive to high nitrogen addition gradients. The stomatal conductance of Leymus secalinus increased with the nitrogen addition gradient showing an obvious upward trend in this case, while the changes in the stomatal conductance of the other three functional type plants showed certain variability with the increase of the nitrogen addition gradients, but their stomatal conductance had certain decrease at high nitrogen addition level (72kgNha^-1yr^-1). The intercellular carbon dioxide concentration of the four functional type plants fluctuated slightly under the influence of nitrogen addition, and there was no obvious consistent changes overall. The variation trend of the transpiration rate of Leymus secalinus was similar to that of net photosynthetic rate and stomatal conductance, and both increase significantly with the increase of nitrogen addition gradients. However, the transpiration rate of of Kobresia capillifolia, Potentilla multifida and Medicago ruthenica under the treatment of nitrogen addition had greater variability, and there was no obvious consistent variation trend. The water use efficiency of Leymus secalinus, Potentilla multifida, and Medicago ruthenica decreased significantly under high nitrogen addition, while the water use efficiency of Kobresia capillifolia had large variability, no obvious regularity was observed, and there were large inter-annual differences.

(5) The Chl content, Chla content, Chla/b value and Rubisco activity of Leymus secalinus in alpine meadow were significantly increased by high nitrogen addition, while the chlorophyll b content in was decreased, and the carotenoid content of Leymus secalinus was also increased to a certain extent. The effects of nitrogen addition on photosynthetic pigment and Rubisco enzyme activity of Kobresia capillifolia were significantly different from year to year, greatly affected by external conditions. Under different years and different nitrogen addition treatments, the photosynthetic pigments response of Medicago ruthenica and Potentilla multifida were much more stable. High nitrogen loads significantly inhibited Rubisco activity of Medicago ruthenica and Potentilla multifida, but had no significant effect on their chlorophyll contents.

(6) The non-structural carbohydrate content of Leymus secalinus showed a trend of decreasing with the increase of nitrogen addition gradients. The NSC content of Kobresia capillifolia, Potentilla multifida, and Medicago ruthenica were all significantly decreased under high nitrogen addition levels. The soluble sugar content of Leymus secalinus showed a trend of decreasing with the increase of nitrogen addition gradients, and the soluble sugar content of Kobresia capillifolia, Potentilla multifida, and Medicago ruthenica were not significant under different nitrogen addition gradients. The starch content of Leymus secalinus showed a decreasing trend with the increase of nitrogen addition gradients. The starch content of Kobresia capillifolia, Potentilla multifida, and Medicago ruthenica significantly decreased under the treatment of high nitrogen addition.

(7) Nitrogen addition has no significant effect on the carbon content of four different functional type plants. Nitrogen addition significantly increased the leaf nitrogen content of Leymus secalinus and Kobresia capillifolia, but had no significant effect on Potentilla multifida, and Medicago ruthenica. Nitrogen addition significantly reduced the C/N value of Leymus secalinus and Kobresia capillifolia, while had no significant effect on the that of Potentilla multifida, and Medicago ruthenica.

(8) Nitrogen addition had different effects on leaf MDA content of the four different functional type plants, and there were certain inter-annual differences. High nitrogen addition (72kgNha^-1yr^-1) can significantly reduce the MDA content of Leymus secalinus, but had no significant effect on that of Kobresia capillifolia and Potentilla multifida. The high nitrogen addition significantly increased MDA content of Medicago ruthenica. Nitrogen addition had different effects on the activities of SOD, POD and CAT in the leaves of the four different functional groups plants, and there were certain inter-annual differences. Different nitrogen treatments all increased the SOD activity of Leymus secalinus to a certain degree. The SOD activity of Kobresia capillifolia was not sensitive to nitrogen addition. High nitrogen addition significantly increased the SOD activity of Potentilla multifida and Medicago ruthenica. Nitrogen addition had a significant effect on improving the POD activity of Leymus secalinus. High-nitrogen addition (72kgNha^-1yr^-1) significantly increased the POD activity of Medicago ruthenica, and has no regular effect on the POD activity of Kobresia capillifolia and Potentilla multifida. The CAT activity of Leymus secalinus had a tendency to increase with the increase of nitrogen addition gradients. The CAT activity of Kobresia capillifolia had a certain variability in different years. The addition of N4 and N5 levels of nitrogen both improved the CAT activity of Potentilla multifida and Medicago ruthenica to varying degrees.

(9) In terms of morphological plasticity, Leymus secalinus uses its rapid growth strategy to increase aboveground biomass accumulation and height to obtain more aboveground spaces and light resources. In terms of photosynthetic physiological characteristics, Leymus secalinus has a higher nitrogen utilization efficiency. With the increase of nitrogen addition gradients, Leymus secalinus  absorbs more available nitrogen and promotes chlorophyll synthesis, especially the synthesis of chlorophyll a, and improves the activity of photosynthetic-related enzyme Rubisco, as well as the stomatal conductance, thus to improve its photosynthetic capacity. In terms of stress resistance, nitrogen addition can increase the activity of major antioxidant enzymes of Leymus secalinus and decrease the accumulation of MDA in vivo, which can finally enhance its antioxidant capacity. Whereas, phenotypes and eco-physiology of the other three functional groups were negatively affected by nitrogen addition to some certain degrees, especially Medicago ruthenica. High nitrogen addition (72kgNha^-1yr^-1) would induce certain stress on Medicago ruthenica.

(10) Through structural equation model (SEM) analysis, it is found that nitrogen addition can directly improve the photosynthetic capacity, stress resistance and growth advantage of Leymus secalinus. Under nitrogen addition, soil pH has a more significant impact on the eco-physiological processes of Leymus secalinus than soil nutrient conditions. The eco-physiological processes of Leymus secalinus are more sensitive to changes in soil PH. Climatic factors are more inclined to influence the eco-physiological processes of Leymus secalinus through direct and indirect paths, while the direct effect of nitrogen addition on Leymus secalinus is more obvious. The eco-physiological processes of Kobresia capillifolia are more susceptible to climatic factors (temperature and precipitation). Kobresia capillifolia is not sensitive to changes in soil nutrients. Nitrogen addition and climatic factors tend to directly function on the eco-physiological processes of Kobresia capillifolia. Nitrogen addition and climatic factors (temperature and precipitation) are more likely to directly function on the eco-physiological processes of Potentilla multifida. The physiological and ecological processes of Potentilla multilobata may not be sensitive to changes in soil nutrients. The physiology and growth process of Medicago ruthenica may be more sensitive to nitrogen addition and soil acidification. The photosynthetic process of Medicago ruthenica is also sensitive to climatic factors. High temperature and precipitation may not be conducive to the growth of Medicago ruthenica. Under nitrogen addition, the decrease of soil pH is not conducive to the photosynthesis and growth of the four species of different functional groups, and the inhibitory effect on the growth and photosynthesis of Medicago ruthenica is the strongest.

To sum up, nitrogen addition caused the evolution of plant community shift to grasses- dominant, probably because nitrogen addition increased the advantages of grasses in terms of phenotypic characteristics, photosynthetic physiology, and stress resistance, thereby increasing their presence in the community. However, growth and eco-physiological processes of species in other functional groups may be inhibited to varying degrees due to the multiple pressure of nutrient imbalance, soil acidification and inter-species competition etc. induced by high nitrogen addition and climate change.