Environmental drivers of CO2 fluxes from 'historical' soil organic matter and the implications for European carbon stocks

Predictions about how Earth's climate will change depend on good information about how climate influences, and is influenced by, the amount of CO2 in the atmosphere. The atmosphere's CO2 content is the balance between CO2 production (mainly from respiration and from biomass and fossil fuel burning) and CO2 consumption (mainly by photosynthesis). Photosynthesis and respiration rates vary with climate: they slow down when it becomes too dry or wet, or too warm or cold. But models must be more explicit than this. They require robust mechanistic relations between CO2 fluxes and the main climatic factors - temperature and moisture - that govern them. Models also need to simulate how these relations vary among ecosystems. The problem is that some of the most important CO2 fluxes are difficult to measure; arguably, the most difficult of all is that resulting from the decomposition of soil organic matter (SOM). This flux is particularly important because it, along with photosynthesis, determines if an ecosystem is a net source or sink for CO2. If ecosystems are net sources, the CO2 produced contributes to the rise in atmospheric CO2 concentration that has been occurring for the past 150 years. But if an ecosystem is a net sink, it partly moderates atmospheric CO2 increases. It is important to know the mechanisms that cause an ecosystem to become a source or sink to predict the contributions of different ecosystems to future climate forcing. SOM contains most of the carbon (C) in land ecosystems. Most of the C in SOM is strongly resistant to decomposition. It takes centuries or millennia for this C to fully decompose (hence its description as 'historical' C). Yet it is the CO2 from 'historical' C that, with photosynthesis, determines an ecosystem's source-sink balance. To measure this flux, it must be distinguished from the CO2 arising the respiration of living roots, fungal hyphae and microbes, as all contribute to the soil flux. Conventional methods involve killing roots and hyphae, removing litter from the soil surface, or introducing SOM whose C is distinct from that of native soil C. All of these invasive procedures can, worryingly, produce estimates of CO2 fluxes that do not necessarily reflect those of undisturbed systems. Knowing how the 'historical' CO2 flux varies between different ecosystems and in relation to temperature and moisture would be a major step towards improving climate models, most of which use information about CO2 fluxes. Our solution to this long-standing problem has been to develop a method that measures 'historical' CO2 fluxes directly, but with minimal disturbance. We do this by accurately measuring the natural isotopic (13C) composition of CO2 derived from the decomposition of SOM and from the respiration of living roots. These sources of CO2 usually differ in their 13C content by a small, but measurable, amount. By measuring the CO2 flux at the soil surface and its 13C content, we can estimate directly how much of it derives from 'historical' C. We will use this technique to measure the 'historical' soil CO2 flux in three similar forests across a European climatic gradient at three times of year in two consecutive years. This is to obtain a wide a range of temperatures and soil moistures to which we can relate the 'historical' soil CO2 flux. The C balances of the sites that we will use (in Italy, Germany and Finland) have been measured for many years, allowing us to compare our measurements with other ecosystem C fluxes. The relationships we obtain between 'historical' soil CO2 flux, temperature and moisture will be used to improve the description of temperature and moisture sensitivity of SOM decomposition in existing models that predict interactions between climate and the C cycle, and which forecast resulting changes in the C stocks of ecosystems. We will then use these models to assess with greater reliability the effects that future climate is likely to have on the sustainability of Europe's soil C.