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The carbon and nitrogen biogeochemistry of a montane Norway spruce (Picea abies (L.) Karst.) forest: a synthesis of long-term research

Background: Long-term ecological studies provide reference data for evaluating the response of ecosystems to external factors over time. Aims: At the Mühleggerköpfl forest research site, Austria air pollution, bioindication, hydrology, and biogeochemistry have been studied over 20 years. This paper reports the biogeochemical budget for nitrogen and carbon. Methods: The site is a Norway spruce (Picea abies) stand on limestone that intercepts air pollutants from long-range transport. As the over-mature trees have a low ability to assimilate surplus nitrogen (N) the forest is vulnerable to N saturation and the forest overall may release N into the groundwater. Results: Nitrate concentrations in the soil solution of the upper mineral soil indicate a high nitrification rate. Nevertheless, the nitrogen content in spruce needles barely exceeds the threshold of deficiency. Conclusions: Potential N-leaching from the soil is reduced by the slowly increasing N demand of the regenerating European beech (Fagus sylvatica) forest that is establishing under the spruce stand. The release of NOx is low, yet significant for the greenhouse gas balance. With respect to CO2 emissions, the forest currently is neutral, but as warming stimulates the decomposition of soil organic matter it can turn the forest into a carbon (C) source. In addition, disturbances, such as bark beetle (Ips typographus) attacks that cause tree mortality strongly affect ecosystem dynamics and the biogeochemistry of C and N.