长期氮沉降对南亚热带不同土壤氮梯度森林自由固氮的影响和机理

Free-living nitrogen (N) fixation (FNF) represents one of the important sources of N in forest ecosystems and plays a critical role in nutrient cycling and net primary productivity of forests. Global changes, such as N deposition, have altered terrestrial N cycling and exerted potential effects on forest FNF. There are three knowledge gaps in regard to current studies of forest FNF under N deposition scenario. First, our current knowledge about how N deposition affects forest FNF is mainly derived from studies conducted in Europe and America, whereas it remains poorly understood how N deposition regulates forest FNF in Asian areas. Second, traditional theories indicate that N fixers down-regulate N fixation rates under soil N richness, which contradicts recent observations that N-fixing microbes remain active in N-rich tropical forests. Third, previous studies of forest FNF in response to N deposition applied single or short-term N-addition approaches, which cannot reflect the real responses of forest FNF to chronic N deposition. In this research project, we aim to address how chronic N deposition affects FNF in forests with different gradients of soil N and to explore potential mechanisms. We will conduct experiments of FNF in Dinghushan (there are three natural forests) and Heshan (there are two planted forests) sites, which are located in Guangdong Province of southern China. The five forests have differences in soil total N (ranging from 1.1 to 2.5 mg g-1) and are following chronic (lasting 16 and 9 years for Dinghushan and Heshan forests, respectively) N-addition treatments. Results from this research will make important contributions to the estimation of N fixation capacity of subtropical forests in China, offering novel knowledge to the theories and experiments of ecosystem N cycling and providing academic references for the modeling and prediction of forest N budgets under chronic N deposition scenario.

自由固氮是森林生态系统重要的氮素来源之一，在维持森林养分循环和增加森林净初级生产力方面发挥重要作用。全球变化（如，氮沉降）已经改变了陆地生态系统氮循环，同时对森林自由固氮产生潜在影响。目前有关氮沉降和森林固氮关系的研究存在以下局限性：1）相关研究集中在欧美地区，而在亚洲地区的情况仍不清楚；2）氮富集抑制生物固氮的传统理论与现实观测存在矛盾；3）许多研究采用单一或短期模拟氮沉降试验，无法准确反映长期氮沉降对森林自由固氮的真实影响。针对这些问题，申请人将在我国广东省鼎湖山（3个自然林）和鹤山（2个人工林）共5个不同土壤氮梯度（1.1‒2.5 mg g-1）森林开展模拟长期（鼎湖山16年、鹤山9年）氮沉降对森林自由固氮影响的研究。研究结果可以为我国南亚热带森林固氮潜力的估算做出原创性的贡献，为生态系统氮循环理论和试验研究提供新的知识，为长期氮沉降背景下森林氮循环收支的模拟和预测提供新的理论依据。

自由固氮是森林生态系统重要的氮素来源之一，也是森林养分循环和植被生产力的重要驱动力。全球变化（氮沉降）已经改变了陆地生态系统氮循环，同时对森林自由固氮产生潜在影响。目前有关氮沉降和森林固氮关系的研究存在以下局限性：1）相关研究集中在欧美地区，而在亚洲地区的情况仍不清楚；2）氮富集抑制生物固氮的传统理论与现实观测存在矛盾；3）许多研究采用单一或短期模拟氮沉降试验，无法准确反映长期氮沉降对森林自由固氮的真实影响。项目主持人在我国广东省鼎湖山（3个自然林）和鹤山（2个人工林）共5个不同土壤氮梯度森林开展模拟长期氮沉降对森林自由固氮影响的研究。通过本项目研究，发现森林演替过程驱动生物固氮变化及其化学计量驱动机制，而长期氮富集环境下，磷元素成为限制森林固氮的重要因素。通过氮同位素示踪方法，发现氮沉降背景下豆科固氮树种仍然具有较高的截留外源氮素的能力。进一步利用大数据分析方法，阐明了生态系统固氮的环境调控机制。为了深入了解其机制，本研究还探索了氮沉降对森林碳循环过程以及土壤氮磷循环酶的影响。这些研究结果有助于理解“富氮缺磷”热带地区中豆科树种广泛分布这一“悖论”现象，以及帮助模拟和预测陆地生态系统氮收支、净初级生产力和生态系统反馈。在本项目的资助下，共发表学术论文6篇，培养研究生3名，参加学术会议4次并作报告。