模拟大气氮沉降对中国森林生态系统影响的研究进展

Effects of Simulated Atmospheric Nitrogen Deposition on Forest Ecosystems in China: An Overview

摘要:

人类活动加剧了活性氮的生产和排放，并导致氮沉降日益增加并全球化。目前，人类活动对全球氮循环的干扰已经超出了地球系统安全运行的界限。中国已成为全球氮沉降的高发区域，高氮沉降已经威胁到生态系统的健康和安全，并成为生态文明建设过程中亟待理清和解决的热点问题。对国际上和中国森林生态系统模拟氮沉降研究的概况进行了综述，并从生物学和非生物学两大过程重点阐述模拟氮沉降增加对中国主要森林生态系统影响的研究进展。中国自2000年以后才开始重视大气氮沉降产生的生态环境问题，中国科学院华南植物园在国内森林生态系统模拟氮沉降试验研究上做出了开创性的贡献。模拟氮沉降研究表明，持续高氮输入将会显著改变森林生态系统的结构和功能，并威胁生态系统的健康发展，特别是处于氮沉降热点区域的中国中南部。森林生态系统的氮沉降效应依赖于系统的氮状态、土地利用历史、气候特征、林型和林龄等。最后，对未来的研究提出了一些建议，包括加强长期跟踪研究和不同气候带站点之间的联网研究，特别是在森林生态系统对长期氮沉降响应与适应的过程机制、地下碳氮吸存潜力研究、以及与其他全球变化因子的耦合研究等方面，以期为森林生态系统的可持续发展提供理论基础和管理依据。

Abstract:

Human activities, such as combustion of fossil fuel, production and application of nitrogenous fertilizer, and intensive livestock production, have been accelerating the production and emission of reactive nitrogen (e.g., NH₄⁺, NO₃^-), leading to elevated nitrogen (N) deposition at regional and global scales. Human interference with N cycle has gone beyond the safe operating space for humanity. China is one of the three regions with the highest N deposition in the world. High N deposition has threatened the health and safety of terrestrial ecosystems, which should be addressed urgently during the process of ecological civilization construction. The research history on simulated N deposition in China and world was reviewed, focused on how simulated N deposition affects forest ecosystems in China, including soil acidification, plant element chemistry, plant growth and diversity, soil microbial community and enzyme activities, soil fauna, greenhouse gas emissions, ecosystem N and phosphorus cycles, soil N transformation, ecosystem N fixation, litter decomposition, and ecosystem carbon sequestration. The atmospheric N deposition has been concerned since 2000s. In 2002, the first long-term forest ecosystem N manipulative experiments were established by South China Botanical Garden (SCBG) of the Chinese Academy of Sciences, which is playing a leading role in the field of nitrogen deposition and forest ecosystems in China. In 2013, SCBG, for the first time, designed a novel experiment with canopy addition of N (CAN) vs. understory addition of N (UAN) in China. Results from N manipulative experiments across China showed that continuing high N deposition greatly altered forest structure and functioning, threatening ecosystem health, especially in the south-central China. The main results are as follows:(1) There is a fertilization effect of N deposition in temperate and boreal forests, but there seem no positive effects on plant growth in N-rich tropical forests because of N saturation. (2) Excess N deposition can lead to soil acidification and nutrient imbalance. (3) Elevated N deposition has accelerated N cycling rate and its transformation process, but depressed ecosystem N fixation rate, and altered ecosystem P availability and cycling, litter decomposition process and greenhouse gas emissions. (4) High N deposition reduced understory plants diversity and changed the structure of soil microbial community. (5) Nitrogen deposition generally simulates aboveground vegetation C sequestration across China, but there remains uncertain on belowground soil C sequestration. (6) Tropical and subtropical forest ecosystems are non-ignorable N sinks, depending on the forms and fates of added N. (7) The effects of N deposition on forest ecosystems are variable, depending on ecosystem N status, land-use history, climate, and forest types and ages. Considering that there remain uncertainties on the long-term effects of N deposition in China, it is suggested that it is necessary to continue the present studies in a longer term, and to expand a network research among field sites along climate gradients. It would be further highlighted to explore how forest ecosystems respond and acclimate to long-term N inputs, to quantify belowground C and N sequestration, and to jointly consider multiple global change factors (e.g., climate warming, CO₂ enrichment, changes in precipitation patterns), all of which are important for forest management and sustainable development in the future.