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[交流] [优秀文章推荐]中科院揭示附生苔藓对氮沉降的响应模式已有2人参与

哀牢山山地森林中发达的附生苔藓

　　苔藓植物因独特的形态结构及生理特性而对大气N沉降非常敏感，被作为大气N沉降的指示生物。目前，关于森林附生苔藓对大气N沉降的响应及其机制，附生苔藓对N污染的临界载荷仍缺少系统、量化的研究，尤其是对亚热带山地森林生态系统而言，尚无相关的研究报道。

附生苔藓组织中C、N、P、K、Mg和15N丰度对不同浓度N添加的响应

　　中国科学院西双版纳热带植物园恢复生态研究组博士研究生石贤萌在副研究员宋亮和研究员刘文耀的指导下，在云南哀牢山中山湿性常绿阔叶林中进行了为期2年的原位N沉降模拟实验，从多个尺度研究了树干附生苔藓（群落、生理及组织养分含量等指标）对不同N添加水平的响应曲线、生理机制，并估算了附生苔藓植物对N污染的临界载荷，其研究结果表明：1）超过一定浓度的N沉降（> 7.4 ha–1 yr–1）造成了附生苔藓群落盖度和物种丰富度的显著下降；2）过量N供给导致K, Mg等离子流失和叶绿素降解，引起附生苔藓植物碳（C）代谢的失衡，抑制苔藓植物的光合速率和生长，并对附生苔藓造成直接的毒害，从而影响到山地森林生态系统物种组成、多样性与分布；3）通过分析与比较，构建了亚热带山地森林附生苔藓植物响应N沉降的生理生态机制模型，并估算出该区森林附生苔藓植物对N沉降的临界载荷为18 kg ha–1 yr–1；4）过多N添加造成森林附生苔藓植物的盖度及生物量下降，在一定程度上暗示了山地森林冠层附生植物群落中C、N等元素向林下地表转移的趋向。

N添加影响附生苔藓多样性丧失的结构方程模型示意图

　　本研究首次系统阐明了N沉降对亚热带山地森林附生苔藓的影响，该研究结果为深入研究山地森林附生苔藓植物对大气N沉降的响应及其机理，探讨应用苔藓植物作为大气N沉降指示生物的方法和技术，以及亚热带山地森林生态系统生物多样性的保护与管理提供了科学依据。

Study site and the experimental design.

　　Increasing trends of atmospheric nitrogen (N) deposition due to pollution and land-use changes are dramatically altering global biogeochemical cycles. Bryophytes, which are extremely vulnerable to N deposition, often play essential roles in these cycles by contributing to large nutrient pools in boreal and montane forest ecosystems. To interpret the sensitivity of epiphytic bryophytes for N deposition and to determine their critical load (CL) in a subtropical montane cloud forest, community-level, physiological and chemical responses of epiphytic bryophytes were tested in a 2-year field experiment of N additions. The results showed a significant decrease in the cover of the bryophyte communities at an N addition level of 7.4 kg ha−1 yr−1, which is consistent with declines in the biomass production, vitality, and net photosynthetic rate responses of two dominant bryophyte species. Given the background N deposition rate of 10.5 kg ha−1yr−1 for the study site, a CL of N deposition is therefore estimated as ca. 18 kg N ha−1 yr−1. A disordered cellular carbon (C) metabolism, including photosynthesis inhibition and ensuing chlorophyll degradation, due to the leakage of magnesium and potassium and corresponding downstream effects, along with direct toxic effects of excessive N additions is suggested as the main mechanism driving the decline of epiphytic bryophytes. Our results confirmed the process of C metabolism and the chemical stability of epiphytic bryophytes are strongly influenced by N addition levels; when coupled to the strong correlations found with the loss of bryophytes, this study provides important and timely evidence on the response mechanisms of bryophytes in an increasingly N-polluted world. In addition, this study underlines a general decline in community heterogeneity and biomass production of epiphytic bryophytes induced by increasing N deposition.

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