The contribution of trees to tropical wetland methane emissions

树木对热带湿地甲烷排放的贡献

• 批准号: NE/J01057X/1
• 负责人: Susan Page
• 金额: $ 2.09万
• 依托单位: University of Leicester
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2012
• 资助国家: 英国
• 起止时间: 2012 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FJ01057X%2F1
• 关键词: contribution; trees; tropical; wetland; methane

Methane (CH4) is an important greenhouse gas that is ~25 times more powerful than CO2 at trapping the Sun's energy. There is therefore considerable interest in the processes involved in CH4 production, principally in waterlogged soils in wetlands, and the processes that lead to its emission to the atmosphere. This study is concerned with processes that enhance the amount of CH4 emitted to the atmosphere, in particular, a novel mechanism for transferring CH4 from soil to the atmosphere. It is generally thought that CH4 produced in waterlogged soils is emitted by a combination of three processes: 1) by diffusion through water-filled pores, 2) by abrupt release of bubbles, and 3) through internal spaces in the stems of grass-like plants which are adapted to live in waterlogged soils. We propose that the stems of wetland trees also provide an important conduit for the transfer of CH4 from wet soils to the atmosphere, a possibility that to date has been almost entirely overlooked. This project builds on published data gathered by this team which showed that mature temperate wetland alder trees indeed emit CH4 via their trunks, a finding that is corroborated by one other recent study of ash trees in Japan. This is an important finding because wetlands are the largest single source of CH4 emissions to the atmosphere and 60% of these ecosystems are forested. We now have additional unpublished data that was collected in the spring of 2011 (10 weeks before the call deadline) which show that tropical peat swamp forest trees in Borneo emit 65% off all ecosystem methane emissions and twice as much as emissions currently quantified from the peatland surface. At present, researchers working in forested wetlands typically measure only CH4 emitted from the soil surface and thus we assert that the total amount of CH4 being released from these ecosystems is being grossly underestimated. This oversight in the past may also explain why different ways of estimating CH4 emissions for a region rarely agree. Estimates of CH4 emission obtained from satellite or atmospheric measurements are often greater than estimates based on observations made at ground level. This is particularly evident in forested tropical areas. Our finding that trees enhance venting of CH4 from soil is a possible explanation to account for the discrepancy, in part, because soils in many of the forested areas are flooded either seasonally and in many cases permanently, which means an abundance of CH4 should be present in soils. We suggest that there are two ways by which CH4 produced in wet soils may be transported and emitted through trees: i) as a gas through air-filled tissue in trees that has formed as an adaptation to enable transfer of oxygen from the atmosphere to the tree's roots which are growing in oxygen-poor waterlogged soil, and ii) dissolved in sap and then liberated to the atmosphere when tree water is lost by transpiration through pores in tree stems and leaves. In the proposed study we will measure CH4 emissions from tropical wetlands, principally in Borneo but also in Panama using techniques to help distinguish the tree emission routes and establish their contribution to ecosystem methane flux as measured using larger scale micro-meteorlogical methods. We will also measure the ratio of two naturally occurring 'versions' (isotopes) of carbon: the relatively rare heavy isotope carbon-13 and the lighter more common carbon-12. The ratio of these isotopes of carbon in CH4 in the soil and in tree emissions provides valuable information about how CH4 is produced and how it moves through the tree. Ours will be the first multi-year study of tropical wetland tree emissions which should, for the first time, establish the true contribution of these ecosystems to the atmospheric methane concentration.

甲烷(CH4)是一种重要的温室气体，其捕获太阳能量的能力约为二氧化碳的25倍。因此，人们对甲烷生产所涉及的过程非常感兴趣，主要是在湿地的淹水土壤中，以及导致其排放到大气中的过程。这项研究涉及增加向大气排放CH4的过程，特别是一种将CH4从土壤转移到大气的新机制。通常认为，在淹水土壤中产生的CH4是通过三个过程的组合释放出来的：1)通过充满水的孔隙扩散，2)气泡突然释放，3)适应于淹水土壤的草类植物茎的内部空间。我们认为，湿地树木的茎也为甲烷从湿润的土壤转移到大气中提供了一个重要的管道，这一可能性到目前为止几乎完全被忽视了。这个项目建立在这个团队收集的公开数据的基础上，这些数据表明，成熟的温带湿地赤杨树确实通过树干排放CH4，这一发现得到了日本最近对白蜡树的另一项研究的证实。这是一个重要的发现，因为湿地是向大气排放甲烷的最大单一来源，其中60%的生态系统被森林覆盖。我们现在有更多的未公布的数据，这些数据是在2011年春季(电话截止日期前10周)收集的，显示婆罗洲的热带泥炭沼泽森林树木排放的甲烷占所有生态系统甲烷排放量的65%，是目前泥炭地表面量化排放量的两倍。目前，在森林湿地工作的研究人员通常只测量土壤表面排放的CH4，因此我们断言，从这些生态系统释放的CH4总量被严重低估。过去的这种疏忽也可能解释了为什么一个地区估算CH4排放量的不同方法很少能达成一致。从卫星或大气测量获得的甲烷排放量估计值往往大于根据地面观测得出的估计值。这在森林覆盖的热带地区尤为明显。我们的发现是树木促进了甲烷从土壤中的排放，这可能是解释这种差异的一个原因，部分原因是许多林区的土壤要么季节性地被淹，要么在许多情况下是永久性的，这意味着土壤中应该存在丰富的甲烷。我们认为，在潮湿土壤中产生的甲烷可以通过两种方式通过树木运输和排放：i)作为一种气体，通过树木中充满空气的组织，作为一种适应，使氧气从大气转移到生长在缺乏氧气的湿润土壤中的树木的根部；ii)溶解在树液中，然后通过树木茎叶中的气孔蒸腾失去水分，释放到大气中。在拟议的研究中，我们将测量热带湿地的CH4排放，主要是在婆罗洲，但也在巴拿马，使用技术来帮助区分树木的排放路线，并确定它们对生态系统甲烷通量的贡献，这是使用更大规模的微型气象学方法测量的。我们还将测量碳的两种自然“版本”(同位素)的比率：相对罕见的重同位素碳-13和较轻的更常见的碳-12。土壤和树木排放的甲烷中碳的这些同位素的比率提供了关于甲烷是如何产生的以及它如何在树木中移动的有价值的信息。我们将首次对热带湿地树木排放进行多年研究，这将首次确定这些生态系统对大气甲烷浓度的真正贡献。