高寒沼泽湿地水位变化对固氮生物群落结构及其固氮能力的影响

Drainage is one of the main anthropogenic factors causing marsh degradation. The area of marsh ecosystems has been declining rapidly as a consequence of drainage and lowering water table, which have led to profound changes in ecosystem processes and functions. Biological nitrogen fixation (BNF) is one of the main pathways through which N enters ecosystems, and plays a critical role in nitrogen cycling in alpine marshes. The BNF is influenced by many environmental factors. However, it is unclear how water table change may have impact on the community structure of nitrogen fixers and their N-fixing abilities. The study proposed here will carry out both field investigations and manipulative mesocosm experiments in the Zoige alpine wetland to examine how variations in water table may have impact on BNF. Firstly, field campaigns will be conducted to investigate the abundance and richness of symbiotic plants, and molecular techniques will be used to analyze the composition of dizotrophs (i.e. symbiotic and free-living bacteria that can fix atmospheric nitrogen). Secondly, stable isotope techniques will be used to examine the proportion of nitrogen fixed by the symbiotic plants, the proportion of nitrogen transferred from symbiotic plant species to non-symbiotic ones, and the potential nitrogen fixing rate of free-living diazotrophs. Thirdly, the changes in vegetation and soil physicochemical factors will also be investigated to help explain the changes in BNF with water table. This study will reveal the responses of BNF processes to variations in water table, and contribute to a better understanding of the mechanisms underlying the response of nitrogen cycling to human activities. In addition, results of this study will have implications for the conservation and adaptive management of wetland ecosystems.

排水疏干是人类扰动导致沼泽湿地退化的主要原因之一，水位降低使湿地快速萎缩，生态系统过程和功能发生显著变化。生物固氮是氮元素进入生态系统的主要途径之一，对高寒沼泽湿地氮循环起重要作用。然而，水位变化对其生物群落结构、固氮能力的影响规律和机制仍不清晰。本项目拟以若尔盖高寒沼泽湿地为研究对象，结合野外调查和受控实验，运用分子生态学技术分析不同水位条件下共生与非共生固氮生物的优势度与多样性；运用稳定同位素技术测定共生固氮植物的固氮量、其向非固氮植物转运的氮量、以及自由固氮微生物的固氮速率，从而揭示水位对固氮生物群落结构及其固氮能力的影响。预期结果将揭示生态系统生物固氮过程随水位变化的规律（以植物共生固氮为主，还是自由微生物固氮为主？），加深理解湿地生态系统氮循环对人类活动的响应机制，为湿地生态系统保护和适应性管理提供科学依据。

高寒沼泽是青藏高原的重要生态系统之一，正面临不同程度的退化，生态系统过程也随之改变。水位下降是高寒沼泽退化的主要驱动因素之一，生物固氮作为高寒沼泽的主要氮输入途径，对水位变化的响应格局仍不清楚。本项目以若尔盖高寒沼泽湿地为研究对象，研究不同水位环境中生物固氮的格局及其影响因素。..我们在若尔盖高寒湿地生态系统定位观测研究站搭建中宇宙实验系统，研发自动控制水位的原状土生态模拟箱，施以不同的水位和施氮处理，研究土壤非共生固氮过程的变化。研究发现土壤固氮速率随着土壤含水量增加而提高，而施氮对土壤固氮速率没有显著效应。另外，我们发现固氮速率在施氮处理中对水位变化更敏感，说明施氮可能通过提高土壤有机碳间接促进非共生固氮。..为了探明若尔盖沼泽水位降低对土壤自由固氮微生物多样性与活性的影响，我们根据水位不同选择原始沼泽地到沙化草甸的10个样点。研究表明若尔盖地区的土壤生物固氮速率为0.018–3.00 μmol N g d.w.-1 day-1。随着沼泽地干化土壤生物固氮速率呈下降趋势。土壤含水量通过影响植物群落、土壤特性和固氮微生物群落组成间接影响土壤固氮速率。我们发现在原始沼泽和中度退化草甸中固氮微生物主要由异养菌构成，而在沙化草甸中固氮微生物主要由混合营养菌和自养菌构成。..通过比较不同退化程度高寒湿地的氮库和氮稳定同位素特征，我们发现不同氮库随着退化程度的敏感性不同，为：土壤>根系>叶。生物固氮对生态系统氮库的贡献率随着退化而降低，其中生物固氮对土壤氮库的贡献率约25%-50%。在氮元素从土壤向植物转运过程中，15N的分馏效应随着退化加剧而更加显著，因此生物固定的氮在退化地中对植物贡献更大。..本研究揭示了若尔盖湿地退化过程中氮输入过程的变化格局及机理，能为若尔盖高寒湿地的水位管理提供科学依据。基于本项目资助，共发表学术论文6篇，获得实用新型专利1项。