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

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.