Collaborative Research: Halocarbon Biogeochemistry in Coastal Wetland Ecosystems - Exploring the Transition from Forested Wetland to Salt Marsh

相对平坦的美国东南部沿海平原，从北卡罗来纳州到德克萨斯州，特别容易受到海平面上升的影响。随着海平面的上升，低洼的沿海淡水森林和高沼泽之间的边界向上移动。高产的森林湿地依次被退化的湿地所取代，最终被沿海盐沼所取代。盐水入侵不仅改变了植被组成，高卤化物水平（称为卤素）可以通过卤化过程与大量土壤有机质相互作用，但人们对卤化过程知之甚少。这些卤化过程很重要，因为它们产生挥发性卤化碳，在大气中充当臭氧消耗化合物。有机质的卤化也会影响有机质的分解速率。这项跨学科的研究将提高我们对氯和溴生物地球化学的理解，并证明卤素在碳循环中的重要性。这项合作研究代表了来自美国东北部、东南部和西部大学不同专业的四名研究人员之间的新合作。研究生和本科生将有机会在一个正在进行的地球观察项目中与公民科学家互动，学习如何向公众传播科学知识。这项研究还将提高人们对海平面上升对美国东南部低洼沿海地区影响的认识。卤素历来被视为自然腐殖化过程中的惰性元素。然而，最近的许多研究表明，氯和溴是C循环中的活性成分。本研究的总体目标是评估高氯化物和溴化物水平的森林湿地中陆生有机质的新分解过程路线。该研究项目包括实地调查和对照实验，以确定海平面上升对南卡罗来纳州温耶湾沿盐度梯度的C和卤素生物地球化学循环的影响。将沿含盐量样带和控制样地对土壤、凋落物和水中消耗臭氧的卤化碳化合物和温室气体的通量以及有机氯和有机溴的浓度进行量化。研究小组认为，测定土壤和腐质层中卤化有机质的x射线吸收近边结构（XANES）的新方法，加上使用傅里叶变换离子回旋共振质谱（FT-ICR MS）测量水中卤化碳的发射和组成，将有助于了解卤素在C循环中的作用。确定来自淡水森林湿地、盐退化湿地和盐沼地的空气、土壤和水中卤化碳的季节变化，代表了海平面上升下沿海湿地的过渡，将有助于发展对海平面上升如何影响沿海湿地陆生有机质分解、腐殖化和卤化过程的机制和景观理解。使用不同浓度氯化物和溴化物的对照田间试验将说明它们在腐殖化和分解过程中的作用。

The relatively flat southeastern US coastal plain, from North Carolina to Texas, is particularly susceptible to sea level rise. As sea level rises, the boundary between the low-lying coastal freshwater forest and high marsh moves upslope. Highly productive forested wetlands are replaced successively by degraded wetlands and eventually by coastal salt marsh. Not only does saltwater intrusion change vegetation composition, high halide levels (called halogens) can interact with the large pool of soil organic matter through halogenation processes that are still poorly understood. These halogenation processes are important to elucidate as they produce volatile halocarbons that act as ozone-depleting compounds in the atmosphere. Halogenation of organic matter may also affect the decomposition rates of organic matter. This interdisciplinary research will improve our understanding of chlorine and bromine biogeochemistry and demonstrate the importance of halogens in carbon cycling. This collaborative research represents a new collaboration between four investigators with different specialties from universities in the northeast, southeast and western United States. Graduate and undergraduate students will have opportunities to interact with citizen scientists in an on-going EarthWatch project and learn how to disseminate the scientific knowledge to the general public. This study will also raise the awareness of the impacts of sea level rise on low-lying coastal areas in the Southeastern US.Halogens have historically been treated as inert elements in natural humification processes. However, numerous recent studies have demonstrated that chlorine and bromine are active components in C cycles. The overall goal of this research is to assess novel decomposition process routes of terrestrial organic matter in forested wetlands with high levels of chloride and bromide. The research project includes both field investigations and controlled experiments to determine the impacts of sea level rise on C and halogen biogeochemical cycles along salinity gradients in Winyah Bay, South Carolina. Fluxes of halogenated ozone depleting C compounds and greenhouse gases, as well as concentrations of organochlorine and organobromine in soil and litter and water will be quantified along salinity transects and within controlled plots. The research team contends that the novel method of determining X-ray absorption near edge structures (XANES) of halogenated organic matter in soil and detritus layers, coupled with measurements of halocarbon emission and composition in water using Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS), will be useful in understanding the roles of halogens in C cycling. Determining the seasonal variation of halocarbon in air, soil, and water from freshwater forested wetland, salt-degraded wetland, and salt marsh sites, representing the transition of coastal wetland under sea level rise, will be useful in developing a mechanistic and landscape understanding of how sea level rise affects decomposition, humification, and halogenation processes of terrestrial organic matter in coastal wetlands. The controlled field experiments using different concentrations of chloride and bromide waters would illustrate their roles in humification and decomposition processes.