Key Laboratory of Vegetation Restoration and Management of Degraded Ecosystems, Guangdong Provincial Key Laboratory of Applied Botany, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou 510650, China.
Key Laboratory of Vegetation Restoration and Management of Degraded Ecosystems, Guangdong Provincial Key Laboratory of Applied Botany, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou 510650, China.
Sci Total Environ. 2018 Jan 1;610-611:555-562. doi: 10.1016/j.scitotenv.2017.08.087. Epub 2017 Aug 17.
Anthropogenic increase of nitrogen (N) deposition has threatened forest ecosystem health at both regional and global scales. In N-limited ecosystems, atmospheric N input is regarded as an important nutrient source for plant growth. However, it remains an open question on how elevated N deposition affects plant growth in N-rich forest ecosystems. To address this question, we used a simulated N deposition experiment in an N-rich mature tropical forest of southern China, with N addition levels as 0kgNhayr (Control), 50kgNhayr (Low-N), 100kgNhayr (Middle-N) and 150kgNhayr (High-N), respectively. We measured foliar nutrient element status (e.g., N, P, K, Ca and Mg), N metabolism and photosynthesis capacity of three dominant understory plant species (Cryptocarya concinna and Cryptocarya chinensis as medium-light species; and Randia canthioides as shade tolerant species) in this forest. Results showed that two years of N addition greatly increased foliar N content, but decreased the content of nutrient cations (e.g., K, Ca and Mg). Nitrogen addition also increased N accumulation as organic forms as soluble protein and/or free amino acid (FAA), but not as chlorophyll in all three species. We further found that the photosynthesis capacity (Pmax) of C. concinna and C. chinensis decreased significantly with elevated N addition, with no effects on R. canthioides. However, photosynthetic nitrogen use efficiency (PNUE) significantly declined with N addition for all three species, with significantly negative relationships between PNUE/Pmax and foliar N content. These findings suggest that excess N inputs can accelerate nutrient imbalance, and inhibit photosynthetic capacity of understory plant species, indicating continuous high N deposition can threat understory plant growth in N-rich tropical forests in the future. Meanwhile, PNUE can be used as a sensitive indicator to assess ecosystem N status under chronic N deposition.
人为增加的氮（N）沉积已经威胁到了区域和全球尺度的森林生态系统健康。在 N 限制型生态系统中，大气 N 输入被认为是植物生长的重要养分来源。然而，在 N 丰富的森林生态系统中，升高的 N 沉积如何影响植物生长仍然是一个悬而未决的问题。为了解决这个问题，我们在中国南方一个 N 丰富的成熟热带森林中进行了一个模拟 N 沉积实验，N 添加水平分别为 0kgNhayr（对照）、50kgNhayr（低 N）、100kgNhayr（中 N）和 150kgNhayr（高 N）。我们测量了三种主要林下植物物种（中光种的青冈和黧蒴；耐阴种的岗柃）的叶片养分元素状况（如 N、P、K、Ca 和 Mg）、N 代谢和光合作用能力。结果表明，两年的 N 添加大大增加了叶片 N 含量，但降低了养分阳离子（如 K、Ca 和 Mg）的含量。N 添加还增加了可溶性蛋白质和/或游离氨基酸（FAA）等有机形式的 N 积累，但对三种植物的叶绿素没有影响。我们进一步发现，青冈和黧蒴的光合作用能力（Pmax）随着 N 添加量的增加而显著下降，而岗柃则没有影响。然而，所有三种植物的氮素利用效率（PNUE）随着 N 添加而显著下降，PNUE/Pmax 与叶片 N 含量呈显著负相关。这些发现表明，过量的 N 输入会加速养分失衡，并抑制林下植物物种的光合作用能力，这表明在未来，持续高 N 沉积可能会威胁到 N 丰富的热带森林中的林下植物生长。同时，PNUE 可以作为评估慢性 N 沉积下生态系统 N 状况的敏感指标。
