Global warming effects on forest soil processes

Current forests serve as important atmospheric carbon (C) sink and mitigate the anthropogenic CO 2 release. A huge amount of organic C is stored in forest soils in various labile (e.g. fresh litter) and recalcitrant (up to 1000s of years old humus) pools. Decomposing soil microorganisms use this organic soil matter as energy source for maintenance and growth. The activity of decomposing microorganisms generally increases with increasing soil temperature. Therefore, there is concern that global warming increases the decomposition of soil organic matter and the resulting CO2 release from forest soil. As the global soil CO2 efflux is approximately 10 times higher than the global anthropogenic CO2 emission, even minor warming effects on soil organic matter decomposition could have important implications on future atmospheric CO2 concentrations and climate. Artificial soil warming in the field is a way to study the effects of warming on soil C and nutrient dynamics and greenhouse gas (GHG) fluxes. Due to the highly dynamic nature of soils and the complex interactions between plants and soil microorganisms, field soil warming experiments provide highly relevant information, as they investigate the effect of a single factor (temperature) by keeping the whole natural soil system intact. Most importantly, field soil warming provides the opportunity to study long-term effects on soil processes and GHG fluxes. With ongoing soil warming, the availability of soil organic matter and nutrients such as nitrogen (N) and phosphorous (P) can change and affect C and nutrient cycling. Hence, the short-term warming response does not necessarily reflect the long- term warming effects on soil processes. In the Achenkirch soil warming experiment, we have increased the soil temperature by 4C during the growing seasons since 2004. The goal is to prolong the warming experiment for further 3 years as we observed a strong sustained increase of soil CO2 (~ 40%) and N2O efflux (~50%). Such a strong and persisting soil CO2 efflux has not been observed in forest ecosystems so far and may be attributed to the large soil C reservoir at the site which is typical for Alpine forests. Our experiment is one of a mere handful of studies world-wide where the effects of soil warming could be studied for more than 10 years. We will continue soil warming and asses how long-term warming affects soil CO2 and N2O fluxes. The long-term dataset will allow quantifying the warming induced soil C loss and the effects on N and P cycling. A special focus will lie on warming effects on plant/soil interface processes. For that, tree fine root turnover will be assessed as well as the labile C release from fine roots in form of root exudates and its effect on soil C and nutrient cycling and fungal colonization. We will apply a new generation of models to assess the potential effects of warming on different soil C pools and fluxes. Together, continuing the Achenkirch warming experiment enables a meaningful forecast of soil C and nutrient stocks and cycling in Alpine forests on a warmer earth.