Efflux of methane (CH4) to the atmosphere from northern peatlands via ebullition: the role of plants and peat structure.

北半球的大片陆地上覆盖着泥炭土。泥炭是在淹水的条件下形成的。当泥炭地植物死亡并开始腐烂时，它们形成泥炭。几千年来，泥炭堆积起来，厚度可能超过5-10米。人们普遍认为，植物材料在受涝条件下不会发生腐烂。然而，衰变确实发生在地下水位以下，并产生一种名为甲烷的气体。甲烷是一种重要的温室气体--也就是说，它导致了温室效应--而北部泥炭地是这种气体的最大全球来源之一。科学家们感兴趣的是预测有多少甲烷进入大气，以便他们能够更好地预测气候变化。作为这项工作的一部分，他们编写了计算机模型，模拟泥炭土壤中甲烷的产生和这种气体向大气中的逃逸。在计算机模型中，假设甲烷可以通过三种主要方式从泥炭逃逸到大气中：(I)通过泥炭纤维之间的空间缓慢扩散，(Ii)通过扩散，有时通过莎草等维管湿地植物的质量流动，以及(Iii)通过泥炭上升的气泡，这一过程被称为沸腾。应用这些计算机模型的一个问题是，我们对有多少甲烷通过气泡逃逸以及气泡损失所涉及的因素知之甚少，因此无法准确地模拟沸腾过程。最近的一些研究表明，兴奋可能比之前认为的要重要得多。事实上，一些研究人员提出：(I)沸腾作用比其他两种途径(扩散和植物介导的运输)加在一起更能解释甲烷向大气的损失，以及(Ii)之前对北部泥炭地甲烷损失的测量严重低估了甲烷的损失。然而，泥炭地甲烷向大气输送的主要途径是沸腾，目前尚处于假说状态，北部泥炭地气泡的形成和损失特征亟待进一步研究。我们研究的目的是为了更好地了解泥炭/沼泽这一重要类别的这两个过程。我们将把泥炭样本(包括沼泽不断生长的表面)带回实验室，并将它们保存在最先进的环境橱柜中，在那里可以将光线、温度和湿度设置为现实的值。我们的目标是回答三个关键的研究问题：1.在沼泽中，甲烷外流的大小和这种外流机制(即扩散、植物介导和沸腾)的相对重要性如何随泥炭类型的不同而变化？2.泥炭结构如何影响气泡的建立和释放？3.维管植物的存在，特别是常见类型的莎草，如何影响泥炭的气泡建立和损失？有了实验室中的泥炭，就有可能对气泡动力学进行复杂的测量，而这在现场是不可能的。我们将测量在春季/夏季条件下(大多数甲烷产生的时候)，气泡是如何在泥炭中积累的，以及它们是如何从泥炭中释放出来的。涉及测量泥炭电学性质的新技术将使我们能够绘制出最多气泡形成的位置，以及气泡积累的体积如何随着更多甲烷的产生和泥炭表面气泡的损失而变化。实验结束后，我们将使用X射线扫描仪分析泥炭的结构。使用x射线，我们将能够重建泥炭的“骨架”，并将能够识别组成泥炭的植物残骸，如泥炭苔藓的茎和莎草的根。根据我们在样品中积累的关于气泡的知识，我们将能够确定泥炭中的哪些结构在捕获气泡方面最有效。

Large areas of the northern hemisphere's land mass are covered with peat soils. Peats form in waterlogged conditions. When peatland plants die and start to decay they form peat. Over many thousands of years, peat deposits have built up and may exceed 5-10 m in thickness. It is commonly thought that the decay of plant material cannot take place in waterlogged conditions. However, decay does occur below the water table and produces a gas called methane. Methane is an important greenhouse gas - that is, it contributes to the greenhouse effect - and northern peatlands are one of the largest global sources of this gas. Scientists are interested in predicting how much methane enters the atmosphere so that they are better able to predict climate change. As part of this effort, they have written computer models that simulate the production of methane in peat soils and the escape of this gas to the atmosphere. In the computer models it is assumed that methane can escape from peats to the atmosphere in three main ways: (i) by slow diffusion through the spaces between peat fibres, (ii) by diffusion and sometimes mass flow through vascular wetland plants like sedges, and (iii) as bubbles rising through the peat, a process called ebullition. A problem with applying these computer models is that we have very little understanding of how much methane escapes via bubbles and the factors involved in bubble loss, so it has not been possible to simulate accurately the process of ebullition. Some recent studies have shown that ebullition may be much more important than previously thought. Indeed, some researchers have suggested (i) that ebullition can account for more loss of methane to the atmosphere than the other two pathways combined (diffusion and plant-mediated transport) and (ii) that previous measurements of methane losses from northern peatlands are gross underestimates. However, that ebullition is the dominant pathway for transport of methane to the atmosphere in peatlands currently has the status of hypothesis and more work is urgently needed on characterising bubble build up and losses in northern peatlands. The purpose of our study is to gain a better understanding of both processes in one important class of peatland / bogs. We will take samples of peat (including the growing surface of the bog) back to the laboratory and keep them in state-of-the-art environmental cabinets where the light, temperature and humidity can be set to realistic values. We aim to answer three key research questions: 1. In bogs, how do the magnitude of the methane efflux and the relative importance of the mechanisms of that efflux (i.e. diffusion, plant-mediated, and ebullition) vary according to peat type? 2. How is bubble buildup and release affected by peat structure? 3. How does the presence of vascular plants, especially common types of sedge, affect bubble build up and loss from bog peats? Having the peat in the laboratory makes it possible to take sophisticated measurements of gas bubble dynamics that are not possible in the field. We will measure how gas bubbles accumulate in the peat during the onset of spring/summer conditions (when most methane is produced) and also how they are released from the peat. New technologies involving measuring the electrical properties of the peat will allow us to map where most bubbles form and how the volume of bubble accumulations changes in response to more methane being produced and the loss of bubbles to the surface of the peat. After the experiments, we will analyse the structure of the peat using an x-ray scanner. Using the x-rays we will be able to reconstruct the 'skeleton' of the peat and will be able to identify the plant remains that make up the peat, like stems of Sphagnum mosses and roots of sedges. With our knowledge of bubble build up in our samples, we will be able to identify which structures within the peat are most effective at trapping bubbles.