Unravelling the effects of tropospheric ozone on below-ground processes driving methane and carbon dioxide fluxes

在上个世纪，能源生产和消费、工业生产、运输、农业和土地使用变化等人类活动向大气层排放的气体急剧增加。这导致全球主要“温室”气体二氧化碳和甲烷的浓度增加，这与气候变化有关。它还导致臭氧等空气污染物的区域浓度增加，可直接影响人类健康、作物产量和生态系统功能。在过去的十年里，人们认识到这两个问题不能孤立地考虑。为了充分了解全球变化的根本原因并预测下个世纪大气排放对全球环境的影响，有必要了解区域空气污染物和温室气体影响产生的所有反馈。特别是，区域空气污染对陆地碳循环的影响可能会改变二氧化碳和甲烷的净通量，并改变这些主要温室气体全球浓度的增加速度。在这方面，臭氧是一种独特的气体，因为它既是一种重要的温室气体，也是对生态系统影响最大的区域气体空气污染物。整个北方的臭氧背景浓度正在增加，全球变化模型预测，在本世纪，臭氧浓度将进一步增加。因此，必须了解臭氧如何影响陆地生态系统中二氧化碳和甲烷的净通量。大多数全球变化模型都没有考虑这种特殊的反馈，而那些只考虑臭氧降低光合作用速率的影响的模型。实验表明，臭氧还影响地下的碳输入，并影响土壤和根系呼吸等关键过程的速率。然而，很少有研究试图综合测量臭氧对不同地下过程的影响，或审查对关键微生物群落的影响。此外，迄今为止进行的几乎所有先前的研究都集中在二氧化碳上，而不是甲烷。该项目的目的是填补这些重要的知识空白，方法是利用一系列创新性的新工具，增进对臭氧对影响二氧化碳和甲烷通量的地下过程的影响的了解。我们将使用稳定同位素13 C标记的二氧化碳来评估臭氧如何影响植物固定的碳的地下命运，并量化不同微生物组中碳的周转率。我们的工作将集中在草原和泥炭地，因为它们是北方碳预算的重要组成部分。我们将进行两个主要的实验。在第一个实验中，泥炭地的中型生态系统将暴露在四个臭氧水平（一个控制加上现在和未来可能的sceanrios）的开顶室中。在第二个实验中，我们将使用一个自由空气臭氧暴露系统，将一个高地草原群落暴露在一系列臭氧浓度下。在这两种情况下，我们将在为期三年的实验期间每隔两个月测量一次甲烷和二氧化碳通量，以评估臭氧的影响。我们还将在该项目的第二年和第三年进行13 C标记研究，以量化臭氧对关键地下碳通量的影响，并使用新的分子技术，我们将测试有关微生物群落结构变化在测量通量变化中的作用的假设。

Emissions of gases to the atmosphere from human activities such as energy production and consumption, industrial production, transport, agriculture and land use change have increased dramatically over the last century. This has led to global increases in the concentrations of the major 'greenhouse' gases carbon dioxide and methane, which are associated with climate change. It has also led to increased regional concentrations of air pollutants, such as ozone, which can directly affect human health, crop yields and ecosystem function. Over the last decade, it has been realised that these two issues cannot be considered in isolation. In order to fully understand the underlying causes of global change and to predict the global environmental consequences of atmospheric emissions over the next century, it is essential to understand all the feedbacks resulting from the impacts of both regional air pollutants and greenhouse gases. In particular, effects of regional air pollution on the terrestrial carbon cycle may alter the net fluxes of both carbon dioxide and methane, and change the rate of increase in the global concentrations of these major greenhouse gases. Ozone is a unique gas in this context because it is both an important greenhouse gas and it is also the most important regional gaseous air pollutant in terms of effects on ecosystems. Background concentrations of ozone are increasing across the northern hemisphere, and global change models predict that, over this century, ozone concentrations will increase further. Therefore it is important to understand how ozone affects the net fluxes of carbon dioxide and methane in terrestrial ecosystems. Most models of global change do not consider this particular feedback, and those that do only consider the effect of ozone in reducing rates of photosynthesis. Experiments show that ozone also influences carbon inputs below ground, and affects the rate of key processes such as soil and root respiration. However, few studies have tried to integrate measurements of ozone effects on different below-ground processes or examined effects on key microbial communities. Furthermore, virtually all previous studies conducted to date have focussed on carbon dioxide, and not on methane. The aim of this project is to address these important gaps in knowledge, by using a range of innovative new tools to increase understanding of the effects of ozone on the below-ground processes that influence fluxes of both carbon dioxide and methane. We will use carbon dioxide labelled with the stable isotope 13C to assess how ozone affects the below-ground fate of the carbon which is fixed by the plants and to quantify the turnover rates of carbon in different microbial groups. Our work will focus on grasslands and peatlands, because they are important components of the northern hemisphere carbon budget. We will carry out two major experiments. In the first, peatland mesocosms will be exposed in open-top chambers to four ozone levels (a control plus present-day and possible future sceanrios). In the second, we will use a free-air ozone exposure system to expose an upland grassland community to a range of ozone concentrations. In both cases, we will measure methane and carbon dioxide fluxes at two-monthly intervals during the three-year period of experimentation to assess the effects of ozone. We will also conduct 13C labelling studies in the second and third years of the project to quantify ozone effects on key below-ground carbon fluxes and, using novel molecular techniques, we will test hypotheses about the role of shifts in microbial community sructure in the measured changes in flux.