潮汐作用下干湿交替对盐沼湿地碳交换过程及其碳汇形成机制的影响

Tidal salt marshes have high carbon sequestration capacity and carbon accumulation rate, therefore they are one of the most dense carbon sinks in the world. The "Blue carbon" in tidal salt marshes plays an important role in global carbon cycle and mitigation of climate change. The salt marsh is periodically flooded and exposed by tides, which leads to the alternation of salt accumulation and leaching. Therefore, the drying and wetting cycles induced by tides in the salt marsh have a profound impact on carbon biogeochemical cycle and carbon balance. However, till now it is not very clear that how the carbon exchange and carbon sequestration in the salt marsh response to drying and wetting cycles. In this project, using the eddy covariance method and artificial in-situ sampling, we try to measure the ecosystem exchange of CO2 and CH4 and explore their response to the drying and wetting cycles in a salt marsh in the Yellow River Estuary. The objectives are (1) to analyze the effect of drought and salt stress on the processes of carbon exchange of the salt marsh; (2) to explore the Birch effect of gas efflux after rewetting of previously dry soils induced by tides; (3) to analyze the response of the ecosystem carbon exchange to tidal flooding and saturated soil after tides; (4) to determine the relationship between the ecosystem exchange of CO2 and CH4 and the drying and wetting cycles in the salt marsh; (5) to explore the effect of the drying and wetting cycles on the size, direction, and correlation of each component of the carbon cycle (NEE, GPP, Reco, and CH4 emission), and to clarify their impacts on the mechanisms of carbon sequestration in the salt marsh. This project would improve knowledge of effect of drying and wetting cycles on the key processes of carbon exchange and the mechanisms of carbon sequestration in a salt marsh, and aid in the development of a dynamic model of carbon cycling in the tidal salt marsh and assessment of its carbon sequestration responses to climate and sea-level change.

潮汐盐沼湿地具有很高的固碳能力和碳积累速率，是地球上最密集的碳汇之一。盐沼湿地最显著的环境特征是在潮汐作用下出现周期性淹没与暴露的干湿交替过程，可能是控制盐沼湿地碳收支平衡的关键因素。然而，目前盐沼湿地碳交换过程及其碳汇功能对干湿交替的响应机制尚不明确。本项目将以此为切入点，系统研究盐沼湿地生态系统CO2和CH4交换过程对干湿交替不同阶段的响应机制，重点分析干旱阶段尤其是土壤盐分表聚对碳交换的限制作用，涨潮时再湿润过程对碳交换的Birch效应，涨潮后淹水阶段以及落潮后的湿润阶段对盐沼湿地碳交换过程的控制机理；揭示周期性干湿交替与盐沼湿地生态系统CO2和CH4交换之间的相关关系。在此基础上，阐明干湿交替对碳循环各分量大小、方向以及之间转化的影响，揭示干湿交替在盐沼湿地碳汇形成中的作用，为解答盐沼湿地蓝色碳汇的形成机制，发展盐沼湿地碳循环动态模型和评估盐沼湿地碳汇功能提供理论依据和数据支持。

潮汐盐沼湿地具有很高的固碳能力和碳积累速率，是地球上最密集的碳汇之一。盐沼湿地最显著的环境特征是在潮汐作用下出现周期性淹没与暴露的干湿交替过程，可能是控制盐沼湿地碳收支平衡的关键因素。然而，目前盐沼湿地碳交换过程及其碳汇功能对干湿交替的响应机制尚不明确。本项目以此为切入点，基于涡度相关法获取的生态系统碳通量观测资料，以及相应的环境因子、生物因子和潮汐水文过程等观测资料，结果发现（1）利用小波分析技术，发现潮汐作用在多日尺度和季节尺度上控制着盐沼湿地的净生态系统CO2交换（NEE）。此外，潮汐作用还改变了盐沼湿地NEE对光、温条件的响应，并且对夜间的生态系统CO2排放具有显著的抑制作用。（2）潮汐作用下干湿交替频率的改变也会对盐沼湿地生态系统CO2和CH4交换过程产生显著影响。盐沼湿地CO2排放会随干湿交替频率增加而降低，CH4排放则与之相反；干湿频率增加也加剧了盐沼湿地土壤可溶性有机碳的流失。（3）淹水梯度显著增加了生态系统CO2和CH4交换，这与芦苇植株密度、叶面积指数、高度和生物量的增加密切相关。生态系统CO2和CH4交换与淹水梯度呈现显著的抛物线关系，这主要是由于植物性状对淹水梯度的响应呈现抛物线所致。另外，淹水梯度降低了全球增暖潜势（GWP）。（4）发现了降雨量季节分配对盐沼湿地净生态系统CO2交换具有双重影响，阐明了降雨脉冲降低土壤呼吸的作用机制。（5）揭示了潮汐作用和干湿交替下氮输入对黄河三角洲滨海湿地土壤有机碳分解与流失的影响机制。（6）通过项目实施，在国内外核心期刊发表论文19篇，其中SCI论文12篇（6篇TOP期刊文章）；出版专著3部。项目研究成果对于阐明盐沼湿地蓝色碳汇的形成机制，建立和发展潮汐盐沼湿地碳交换模型，评估潮汐盐沼湿地对区域及全球碳收支的贡献，制定应对气候变化的湿地管理政策和履行国际义务具有十分重要的作用和意义。