气候过渡区典型森林生物固氮对林冠氮沉降的响应及机理

Biological nitrogen fixation (BNF) is the largest natural source of “new” nitrogen (N) to forest ecosystems. Knowledge of BNF rate and their regulating factors is crucial for understanding the processes of N cycling in forests and their response to elevated atmospheric N deposition. However, there is still huge uncertainty in terms of BNF rates and their controls in forests. Currently, there is no study on N deposition regulating BNF rate of typical forests in China. Using traditional manipulation of N addition approaches which added N directly onto understory and/or forest floors, many important processes of N cycle occurring in the forest canopies were neglected. The effects of simulated N addition on understory and soil processes, including BNF in soil and/or litterfall layers, might be overestimated. Two novel simulated N deposition studies were conducted in central and southern China, which located in Jigongshan Natural Reserve (a climate-transitional zone from northern subtropical climate to warm temperate climate) and Shimentai Natural Reserve (a climate-transitional zone from southern subtropical climate to middle subtropical climate), respectively. In this study, we will investigate the changes in both symbiotic N fixation and free-living N fixation (asymbiotic N fixation) rates and their regulating factors in different compartments of two typical forests after N fertilizers (Canopy vs. Understory N addition). In order to find out the major factors of regulating forest BNF, soil properties, soil moisture and temperature, abundance and diversity of N-fixers (based on nifH functional genes) were measured simultaneously. Our goal is to compare different responses of forest BNF rates to the different manipulations of N deposition (canopy addition of N vs. understory addition of N). Results from the study will theoretically provide new understanding of the responses of BNF and N cycling in forests to elevated N deposition in the future, and practically contribute useful information for sustainable management of forest ecosystems in China.

生物固氮(BNF)是森林外源氮输入的重要途径，也是氮循环研究的最薄弱环节。大气氮沉降增加背景下，研究BNF速率变化及驱动机理对认识森林氮循环具有重要意义，然而目前国内外相关报道十分有限。本项目依托在河南鸡公山自然保护区（北亚热带-暖温带）和广东石门台自然保护区（南亚热带-中亚热带）已建成的“林冠模拟氮沉降”平台，系统测定森林不同组分（土壤、凋落物层、根瘤、苔藓/地衣、冠层叶片及附生植物）的BNF速率，比较气候过渡区典型森林BNF速率及其对不同氮沉降模拟方式（林冠vs.林下模拟施氮）的响应差异，分析固氮微生物群落组成和丰度地域差异及氮处理间响应不同，结合森林不同结构层次的理化性质变化，阐明森林BNF速率的生物与非生物调控机制。本项目可为研究森林BNF提供代表性数据及解析BNF调控因子，对理解区域森林氮循环、构建生态模型及预测氮沉降背景下我国森林结构和功能的响应及适应性调控等均具有重要意义。

已圆满完成了项目书中研究计划的全部内容，实现了任务书预期研究目标。依托已建成的“林冠模拟氮沉降”控制实验平台和“长期氮沉降模拟实验”野外样地，测定了温带和南亚热带代表性森林不同组分的生物固氮速率、微生物及酶活性等的变化，对比了“林冠”与“林下”模拟施氮对我国温带和南亚热带森林自由固氮、养分循环及生物酶活性的影响差异性。结果显示：1）“林冠”与“林下”不同模拟氮沉降方法对南亚热带森林生物固氮速率的影响差异性不明显，而“林下”施氮显著高估了氮输入对温带（鸡公山）森林自由固氮速率的影响程度（约高估了3.8-4.7倍）；鸡公山森林自由固氮速率（2.57±0.19 mg N m-2d-1）高于石门台（南亚热带）森林（1.34±0.09 mg N m-2d-1）；2）林冠可截留氮沉降，鸡公山森林低氮和高氮样方中林冠对氮添加的截留率分别达52%和44%；冠层穿透雨可能是森林土壤碳、氮循环有关的生物酶的潜在来源之一；3）对南亚热带人工林施用磷肥时需慎重考虑其对系统中其它元素（如硫）循环的“负面影响”，因为磷添加可降低人工林土壤硫的可利用性，改变土壤硫酸酯酶活性；4）长期氮添加显著降低鼎湖山森林土壤硫酸脂酶活性，而对碳、氮获取有关的酶活性没有影响；5）南亚热带大叶相思（固氮树种）人工林土壤溶解性有机质（DOM）富含微生物来源的分子，表明固氮树种人工林的DOM可能有更高的微生物周转贡献。本项目研究结果共发表学术论文7篇（其中SCI论文6篇），为全球变化背景下理解森林氮循环及其关键过程对大气氮沉降的响应及适应性改变提供了科学依据。项目实施期间培养硕士毕业生1名（已转博，目前在丹麦的哥本哈根大学联合培养博士学位）、协助培养博士毕业生1名。