湿地土壤有机碳与铁的交互作用及其对干旱的响应

As an important carbon sink globally, wetlands are vulnerable to drought induced by climate change and human activities. However, the response of wetland soil organic carbon (SOC) to drought and its underlying mechanisms are highly uncertain. Interactions between SOC and iron play an important role in governing the fate of SOC during wetland drought, but studies of their interaction mechanisms are constrained by the lack of sensitive analytical methods. This project proposes to employ highly sensitive and complementary techniques at the molecular level and aims to explore three key research questions: How does iron regulate the distribution and molecular composition of SOC in natural wetlands experiencing drought? Does iron oxidation contribute to the preservation of SOC (in particular, lignin)? How does iron transformation mediate the response of wetland SOC to drought? To address these questions, we propose to use biomarkers, nuclear magnetic resonance (NMR), and Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) to examine the composition and origin of iron-protected SOC in natural wetlands witnessing various degrees of drought. We also propose to utilize water-table decline experiments in the field and redox incubation experiments in the laboratory. With in-tandem measurements of soil iron, enzyme activities, iron reduction/oxidation rates and genome sequencing, this project seeks to characterize chemical, microbial, redox and sorptive properties of typical wetland soils, to investigate the dynamics and coupling of wetland SOC components and iron during simulated soil redox changes, and to further elucidate the mechanisms of SOC-enzymes-iron interactions during wetland drought. Results from this project will provide molecular evidence on SOC-iron interactions and shed novel insights on the processes controlling SOC responses to drought in wetlands.

干旱正严重威胁着湿地的碳汇功能；而其土壤有机碳（SOC）对干旱的响应及机制极不明确。SOC与铁的交互作用直接影响湿地碳库对干旱的响应；但因缺乏灵敏的分析方法，其作用机制的研究尚不深入。本项目拟利用灵敏而互补的分子地球化学方法，回答三个科学问题：湿地铁氧化物与SOC的分子组成有何耦合关系？铁的氧化是否影响土壤碳（特别是木质素）的保存？铁转化如何调控湿地SOC对干旱的响应？针对这些问题，本研究拟利用生物标志物、核磁共振、傅立叶变换-离子回旋共振质谱、高通量测序等现代分析技术，揭示典型湿地干旱序列中被铁保护的SOC的分子组成及来源；利用野外水位控制及室内吸附/培养实验，结合微生物类群、酶活、铁的氧化/还原速率的测定，阐明SOC关键组分与铁转化的动态关系，厘清“SOC−酶−铁”在湿地干旱过程中的交互机理。结果有望为碳−铁耦合机制提供分子水平的解释，并为阐明湿地SOC对干旱的响应提供新颖的理论基础。

干旱和人为排水正严重威胁着湿地的碳汇功能；但是湿地土壤有机碳（SOC）对干旱的响应及其机制仍不明确。最近研究发现，干旱可通过诱导亚铁氧化加强碳-铁交互作用并抑制酚氧化酶的活性，进而促进湿地SOC保存（即“铁门封碳”机制；Wang et al., 2017, Nature Communications），但其作用在不同湿地中的普遍性和效果亟待探究。基于此，本项目对若尔盖、海北、大九湖、三江平原等我国典型淡水湿地进行了大尺度样带调查和配对采样，利用生物标志物、高分辨率质谱等分子地球化学方法，结合铁形态、铁的氧化还原转化、胞外酶和微生物特性分析，剖析了金属（铁）保护有机碳在干旱梯度上的变异与调控因素，研究了湿地SOC组成和来源对干旱的响应；并利用干旱模拟实验，解析了不同土壤中铁的氧化还原转化对SOC组成、释放与固存的影响。主要研究发现包括：1、金属（包括铁）保护有机碳对湿地和旱地的SOC有同等的贡献，且钙离子发挥重要的调控作用；2、湿地干旱和干旱区变湿均促进了微生物残体对SOC的贡献，且湿地干旱增加了矿物保护有机碳的比重；3、土壤铁还原强度与SOC淋溶流失潜力正相关，且厌氧发酵大大促进了铁还原、有机碳释放和酚类氧化；4、与之相反，人为增加亚铁氧化提高了不同淡水湿地土壤中与铁结合有机碳的含量，表明“铁门封碳”机制普遍存在；但是，在铁还原菌相对丰富的泥炭藓湿地中，强烈的铁还原促进了CO2排放；5、同时，泥炭藓酸可显著促进土壤铁氧化物的还原溶解，诱导活性铁（氢）氧化物生成，并通过淬灭自由基、抑制土壤酶活等三条途径共同增加土壤中铁保护有机碳的含量。以上研究揭示了“铁门封碳”机制在不同湿地中的作用范围和途径，厘清了“SOC−酶−铁”在湿地干旱过程中的交互机理，为阐明湿地SOC对干旱的响应提供新的理论依据。