长期氮沉降影响南亚热带森林生态系统可溶性有机碳流失的过程与机制

The globalization of atmospheric nitrogen (N) deposition has significantly altered terrestrial ecosystem carbon cycle. Dissolved organic carbon (DOC) plays a key role in linking the bio-, hydro- and pedosphere, and also in soil-forming processes and carbon sequestration via DOC mobilization and transport for both temperate and tropical soils. However, the fates of DOC have received less attention under the elevated N deposition, especially in the tropical and subtropical ecosystems. In this project, combined with our previous results, we will investigate the responses of fluxes of DOC to long-term high N deposition in N-saturated and N-limited forest ecosystems of subtropical China. This study will be conducted in the first established long-term N experimental forest site in China. The forests are an old-growth forest (the monsoon evergreen broadleaf forest, N-saturated) and two N-limited forests (pine-broadleaf mixed forest and pine forest). We focus our objectives on the followings: (1) To clarity how elevated N deposition affects the patterns of DOC dynamics in three subtropical forests and the possible C sequestration induced by N deposition in these ecosystems. (2) To explore the differences between N-rich and N-limited ecosystems on the responses of DOC to N deposition. (3) To develop biogeochemistry models on the cycling of DOC in forest ecosystems with different N status. Results from this study will theoretically provide new understanding on the responses of carbon cycling to long-term N deposition in tropical or subtropical forests, and practically to provide data base to assess effects of N deposition on carbon cycling in regional forests and to contribute useful information to sustainable forest and environmental management.

氮沉降全球化已显著改变了陆地生态系统碳循环。可溶性有机碳（DOC）在连接生物圈-水圈-土壤圈，以及在有机碳的矿化、运输和吸存过程中均扮演着重要角色。然而氮沉降背景下DOC的去向通常被忽略，由此限制了我们对陆地生态系统碳循环的理解，特别是在热带亚热带地区。在前期研究基础上，本项目利用我国最早建立的森林生态系统长期氮沉降试验样地，来研究我国南亚热带代表性森林(马尾松林、混交林和季风常绿阔叶林) DOC动态对长期高氮沉降的响应，旨在：揭示长期氮沉降驱动南亚热带森林土壤DOC流失的过程机制，及其对碳吸存的贡献；探讨"富氮"生态系统和"氮限制"生态系统DOC动态对氮沉降响应的异同格局；发展"富氮"与"氮限制"生态系统DOC循环的生物地球化学模型。本项目实施，一方面可以丰富未来高氮沉降背景下热带亚热带区域碳循环理论，另一方面对评估和预测我国热带亚热带森林生态系统碳循环及其变化规律也具有重要意义。

人类活动导致的大气氮沉降日益增加并全球化，改变了生态系统原有的碳氮循环模式，威胁到了生态系统的健康和安全。中国南方是大气氮沉降的高发区，但相关的研究仍然十分薄弱。本项目依托鼎湖山森林生态系统长期氮沉降研究样地，探讨了我国南亚热带代表性森林(季风常绿阔叶林/成熟林、针叶阔叶混交林和马尾松针叶林)可溶性有机碳（DOC）淋失动态对长期高氮沉降响应的过程及其机制。主要研究成果如下：（1）发现长期高氮输入促进了南亚热带森林氮素的迁移和流失（以硝态氮为主），加剧了森林土壤酸化，并提出了长期氮沉降下成熟林比次生林和人工林更加脆弱的观点；（2）在成熟林和针叶林，长期高氮沉降降低了表层土壤或根际区土壤DOC的淋溶输出，有利于促进土壤碳吸存，但是混交林DOC的输出动态没有显著响应；（3）在淋溶过程中，DOC与可溶性无机氮（DIN）和可溶性有机氮（DON）解耦联，但是受溶液酸度的影响；（4）发现“氮饱和”的成熟林仍然是不可忽视的“氮汇”，丰富了热带森林生态系统氮素循环理论；（5）长期高氮输入可降低土壤微生物量、以及微生物（真菌）残留物对土壤有机碳的贡献；（6）发展了典型“富氮”和“氮限制”森林生态系统DOC生物地球化学循环对氮沉降响应的框架模型，并提出物理化学机制和生物学机制分别主导两类系统的DOC循环。综上，在氮沉降全球化背景下，本项目丰富了热带亚热带区域生态系统碳循环理论，研究结果对评估和预测我国热带亚热带森林生态系统碳氮循环规律具有重要的理论和现实意义。