中金杨幼苗叶功能性状与碳氮分配对氮添加的响应

Responses of leaf functional traits and carbon and nitrogen allocation in hybrid poplar (Populus × euramericana ‘Zhongjin2’) seedlings under two-year nitrogen addition

氮沉降对森林生态系统产生了重要影响,但树木个体对氮沉降的生理生态响应过程和机制有待进一步明确。为探究杨树幼苗功能叶经济性状、植株生长以及生物量和碳氮分配对氮添加的响应,本文以欧美杂交杨中金杨2号(Populus×euramericana‘Zhongjin2’)幼苗为材料,研究了连续2年氮添加梯度处理(0、3、6和9 g NH_(4)NO_(3)·a^(-1)·plant^(-1))对其叶片光合速率、叶经济性状、生长以及器官间碳氮分配的影响。结果表明:氮添加第一年,随氮添加浓度增加,中金杨叶片变大变薄,叶肉组织厚度显著降低,单位叶面积的净光合速率和气孔导度则随氮添加浓度增加而先升后降,而蒸腾速率、瞬时水分利用效率和光合氮利用效率不变;单株生物量和碳氮元素积累量显著增加,茎器官的碳投资显著增加而根的碳投资显著降低;氮添加第二年,功能叶光合固碳速率随氮添加量增加更快达到饱和,且高剂量氮添加处理表现出氮毒害现象;仅低量氮添加处理的单株生物量积累显著增加,并且进一步增加了对茎器官的碳投资,而中高剂量氮添加的根系生长明显受到抑制。氮添加下中金杨叶片耗水增加导致的茎输水需求增加可能是驱动向茎碳投资的原因之一,而持续降低的根冠比可能会降低其对干旱等逆境的抵抗力。

Atmospheric nitrogen deposition affects forest ecosystems. The eco-physiological process and mechanism of trees in response to nitrogen deposition need further clarification. To explore the changes of photosynthetic traits, biomass accumulation, and carbon and nitrogen allocation in poplar under nitrogen addition, hybrid poplar(Populus × euramericana ‘Zhongjin2’) seedlings were grown in pots under a rain-out shelter in a semiarid area of China. Seedlings were treated with different nitrogen addition levels(0, 3, 6 and 9 g NH_(4)NO_(3)·a^(-1)·plant^(-1)) for two years. Leaf photosynthetic rate, leaf functional traits, organ biomass, and carbon and nitrogen allocation were investigated. In the first year, with the increases of nitrogen addition levels, leaf area significantly increased but leaf became thinner, with decreased mesophyll tissue thickness. Leaf area-based net photosynthetic rate and stomatal conductance increased firstly and then decreased with increasing nitrogen addition level, while transpiration rate, instantaneous water use efficiency and photosynthetic nitrogen use efficiency did not change. Whole-plant biomass and carbon and nitrogen accumulation significantly increased. The carbon investment in stem increased significantly while that in root decreased significantly. In the second year, photosynthetic carbon fixation rate of functional leaves reached saturation more quickly with increasing nitrogen addition level, with toxicity of high-dose addition. The biomass accumulation of individual plant increased significantly and carbon investment to stem was increased in low nitrogen addition, while root growth was inhibited in high nitrogen addition. The enhanced leaf water consumption in response to nitrogen addition may be one of the driving factors of carbon investment to stem, while the decreasing root-shoot ratio may reduce the resistance of poplar seedlings to frequent drought in semiarid area.