Climate change effects on forest soil respiration

A prediction of the carbon balance of forest ecosystems under changing climate is a complex issue. It requires understanding soil organic matter (SOM) decomposition such as temperature and moisture sensitivity and substrate quality for soil microorganisms. For a meaningful forecast of future C storage in forest ecosystems the long-term response of the pool of SOM to climate change is relevant. Increasing soil temperature enables soil microbes to faster decompose SOM. This induces an increase in the CO 2 flux from the soil to the atmosphere (soil respiration). After an initial increase of soil respiration, a limitation of the availability of easily decomposable SOM can weaken the temperature effect in the long run. In that regard, the role of the soil microbial population is only partially understood. A shift of functional groups within the microbial community directly influences the rate of the decomposition process. A shift from bacteria towards fungi may induce the decomposition of previously not accessible, more recalcitrant SOM, thereby increasing the C release from soils. On the other hand physiological adaptation of soil microbes may offset temperature effects by lowering soil respiration rates. Further uncertainties are introduced by insufficient knowledge of the soil microbial activity during the dormant season. In mountain ecosystems with low productivity, CO 2 losses during the winter do significantly add to the annual C release from soils. Other climate parameters such as precipitation interact with temperature. Changing precipitation can promote or offset temperature effects depending on the type of the ecosystem and temporal precipitation patterns. Climate simulations for the experimental site predict drought periods in summer and an extended duration of the annual snow cover. These effects may partly offset the stimulating effect of higher temperatures on the turnover rate of SOM. We will utilize an already existing soil warming experiment in order to quantify the annual CO 2 emissions at different rainfall/snow scenarios of artificially warmed and control plots. Soil temperature of the warmed plots will be kept 3C above ambient throughout the growing season. Summer drought will be induced by means of a roof system that temporarily keeps off the rain; a prolonged snow cover is created by adding snow in late spring. The existing instrumental setup allows distinguishing between autotrophic and heterotrophic soil respiration. The organisms responsible for the heterotrophic soil respiration will be identified with molecular methods of microbiology. The results of the experiment will show whether mountain forest soils in a warmer climate are a potential source or a sink of C. Such results are the basis for the accounting of terrestrial sinks of greenhouse gases within climate change agreements such as the Kyoto Protocol.

Climate change will have a considerable impact on forest ecosystems all over the world. The rising atmospheric CO 2 concentrations increase the air temperatures via the greenhouse gas effect. The future precipitation pattern is less understood and is expected to be variable on the regional and local scale. In forest ecosystems, changing temperature and precipitation will affect tree productivity, species composition of herbaceous plants, and soil processes. The current study focussed on the effect of artificial soil warming and modified precipitation regimes on the decomposition of soil organic matter. For the studied area, climate models predict decreased summer rainfall and increased air temperatures. To simulate these conditions in a field experiment, we warmed the soil by 4 C during the growing season and created summer-drought conditions by means of temporarily erected roofs. Soil carbon consists of organic compounds in various stages of decomposition ranging from dead leaves to thousands of years old humic substances. These compounds are decomposed by soil microbes, thereby emitting carbon dioxide into the atmosphere. As the production of CO 2 is temperature dependent, there is concern that increasing temperatures would increase CO 2 efflux from soils. This hypothesis was corroborated by our soil warming experiment: the soil CO 2 emissions increased by 33 - 40% or 2.5 t ha -1 per year. Forest soil at the studied site will lose high amounts of carbon to the atmosphere if soil temperatures increase. So far, no acclimation to warming has been observed. The situation, however, changed when increased temperatures coincided with reduced summertime precipitation. Especially in warmed soil, moisture levels decreased and caused a sharp decline in CO 2 production. Although the artificial drought was moderate, the decrease in CO 2 efflux was so strong that the warming effect was offset. Hence, a combination of warmer conditions with less precipitation during summer, as predicted by models, likely does not alter the CO 2 efflux from the forest soil as increases by warming are cancelled out by drought effects. A special focal point of the study was set on the structure and the activity of the soil microbial community and on how soil microbes have been affected by warming. Microbial community structure varied with season, but surprisingly, there were no changes in the amount of microbial biomass or in the microbial community composition after five years of consecutive soil warming. Warming enhanced the metabolic activity of the microbes. So far, no acclimation of the microbial community structure to warming was observed. This, however, may change in the coming years. Therefore, and as only long-term experiments produce viable data when dealing with potential changes in soil carbon, the experiment will be continued for another four years.