三峡消落带植物群落逆向演替对氮源汇变化的影响机制研究

With the large interception reservoirs are applied, the riverine nitrogen cycle has changed, and the nitrogen sink-source transition has also been altered due to the submergence and destruction of the original ecological ecosystem in the water level fluctuation zone. This has received widespread attention across various scientific communities. Our proposal will take the water level fluctuation zone of the Three Gorges Reservoir as our study area.The purpose of this study is to test the hypothesis that the retrogressive vegetational succession in the water level fluctuation zone may form unexpected active nitrogen source to the water body, and further increase in nitrogen loads and exert a significant influence on nitrogen cycling in watersheds. We will use multidisciplinary theory and methods including ecology, plant physiology, biogeochemistry and molecular biology in this study. The objectives of our proposal are to identify the characteristics of resistant plants in retrogressive vegetational succession, reveal the enhancement mechanism of nitrogen fixation by a gramineae plant and its symbiotic bacterial community, determine the nitrogen turnover during the lifetime of vegetation under water level fluctuations, and estimate the nitrogen balance between water level fluctuation zone and water body. Finally, we aim to clarify the key scientific issue of effects of retrogressive vegetational succession on the regulation of nitrogen cycling in the water level fluctuation zone, explore the “missing nitrogen sources” in watersheds, and form a new scientific understanding of the nitrogen cycling in watersheds. The proposed research will provide effective support for the water quality and safety of reservoirs, and for the sustainability of ecological civilization construction in watersheds of our country.

大型水库因拦截作用改变了河流氮循环，也因淹没破坏原有消落带生态系统而导致氮源-氮汇格局的重大变化，受到科学界广泛关注。项目以三峡典型库区消落带为研究区，以验证“消落带抗逆植物群落演替可能导致预料之外的氮源形成，进而增加水库水体氮负荷并对流域氮循环产生重要影响”的科学假说为目标，利用生态学、植物生理学、生物地球化学和分子生物学的交叉理论和方法，以消落带植物群落抗逆演替特征、抗逆演替中禾本科植物及附生菌群的固氮增强机制、交替涨落水文情势下植物生命期内的氮周转、消落带-水体氮平衡为研究内容，阐明消落带抗逆植物群落演替对水库氮循环的调控机制及影响的科学问题，探索流域未知的“迷失氮源”，形成关于流域氮循环新的科学认知，为我国水库水质安全及流域生态文明建设的可持续发展提供有效支撑。

近年来长江流域三峡库区的氮素污染问题越来越受到关注，消落带作为连接陆域和水域生态系统的交互地带，对流域生态系统物质能量循环起着至关重要的作用。同时，库区消落带在三峡大坝建成后在不断的演变当中，形成了以禾本科植物为优势物种的新植物群落结构。在很多人工系统中的禾本科植物中发现了丰富的具有固氮酶活性的内生固氮菌。因此本研究针对三峡水库消落带生境变化对湿地生态和环境的影响，通过多学科交叉，基于环境调查与微生物学实验的分析方法，比较不同位置、不同类型消落带土壤环境理化指标的差异；探索自然生态系统中狗牙根植物地上部分不同构件（茎、叶）中内生固氮菌的定植规律以及生存能力；同时通过微生物培养实验评估分离所得内生固氮菌的固氮酶活性。得到结果如下：.（1）三峡消落带根际土壤与非根际土壤之间有较大的差异性，一般来说，根际土壤氮与磷营养元素的含量普遍高于远根际土壤。亚硝态氮（NO2⁻）最大差异可达93.44%。而黄壤和紫色土之间同样表现较大的差异性，不同的理化性质之间，黄壤的含量高于紫色土。证明植物的生命活动会对土壤养分的聚集与变化产生一定影响。.（2）成功从不同的狗牙根植物组织内分离出54株内生微生物，其中25株具有固氮酶活性。后将25株分离得到的内生固氮菌聚为8类，观察每一类菌种的形态特征。说明植物可以通过定植于体内的固氮菌来吸收利用大气中的氮素。.（3）分别挑选8株固氮酶活性高的不同类菌株测序后，得到的8种菌株分别为Microbacterium arborescens、Exiguobacterium indicum、Microbacterium zeae、Bacillus aryabhattai、Curtobacterium citreum、Microbacterium testaceum、Curtobacterium albidum、Microbacterium hydrothermale。菌属丰富，其中微杆菌属最多，不同菌属间固氮酶活性差异较大，最大可达到5.17 ± 0.31 n mol C2H4/ ml，最小仅为0.15 ± 0.02 n mol C2H4/ ml。.本研究探索了三峡消落带氮循环过程，从微观到宏观，从植物内生固氮菌到氮磷元素在三峡消落带的分布，解释了消落带植物群落逆向演替的内在逻辑。同时本研究的结论对后续三峡库区水域生态治理提供了有力的支持。