Shi Xian-Meng, Song Liang, Liu Wen-Yao, Lu Hua-Zheng, Qi Jin-Hua, Li Su, Chen Xi, Wu Jia-Fu, Liu Shuai, Wu Chuan-Sheng
Key Laboratory of Tropical Forest Ecology, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Kunming, Yunnan 650223, PR China; University of Chinese Academy of Sciences, Beijing 100049, PR China.
Key Laboratory of Tropical Forest Ecology, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Kunming, Yunnan 650223, PR China.
Environ Pollut. 2017 Oct;229:932-941. doi: 10.1016/j.envpol.2017.07.077. Epub 2017 Aug 4.
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 yr, 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 hayr for the study site, a CL of N deposition is therefore estimated as ca. 18 kg N ha yr. 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.
由于污染和土地利用变化，大气氮（N）沉降呈上升趋势，正极大地改变全球生物地球化学循环。苔藓植物极易受到氮沉降的影响，它们通常通过为北方和山地森林生态系统中的大量养分库做出贡献，在这些循环中发挥重要作用。为了解附生苔藓植物对氮沉降的敏感性，并确定其在亚热带山地云雾森林中的临界负荷（CL），在一项为期两年的氮添加田间试验中，对附生苔藓植物的群落水平、生理和化学响应进行了测试。结果表明，在氮添加量为7.4千克·公顷·年的水平下，苔藓植物群落的覆盖度显著下降，这与两种优势苔藓植物物种的生物量生产、活力和净光合速率响应的下降一致。鉴于研究地点的背景氮沉降速率为10.5千克·公顷·年，因此估计氮沉降的临界负荷约为18千克·氮·公顷·年。镁和钾的泄漏以及相应的下游效应导致细胞碳（C）代谢紊乱，包括光合作用抑制和随之而来的叶绿素降解，以及过量氮添加的直接毒性作用，被认为是导致附生苔藓植物数量下降的主要机制。我们的结果证实，附生苔藓植物的碳代谢过程和化学稳定性受到氮添加水平的强烈影响；再加上与苔藓植物数量减少的强烈相关性，本研究为苔藓植物在日益受氮污染的世界中的响应机制提供了重要且及时的证据。此外，本研究强调了氮沉降增加导致附生苔藓植物群落异质性和生物量生产普遍下降。
