Nitrogen availability in Norway spruce forest floor - the effect of forest defoliation induced by bark beetle infestation

The objective was to evaluate whether lower nitrogen (N) immobilization by spruce trees (Picea abies) or higher microbial N mineralization in the soil is the main mechanism changing the soil N balance after forest defoliation caused by bark beetle. We measured in situ mineral N availability using ion exchangers, net N mineralization (N-miner, ammonification and nitrification) and N content in microbial biomass (N-microb) in the forest floor of infested and control plots in an unmanaged area of The Bohemian Forest National Park. In situ N availability already increased before the defoliation culminated, which affirms the primary effect of reduced N immobilization by vegetation. N mineralization was enhanced after maximum forest defoliation (2 vs. 30 mu g N g(-1) d(-1)). The contribution of N-miner to in situ N availability was supported by the correlations found between in situ mineral N availability and N-miner/N-microb and N-miner/C-miner ratios. The influence of litter input with low C/N ratio on N mineralization is discussed.