For 3 years we followed the complete annual cycles of NO and NO2 flux rates from soil of a spruce control site, a limed spruce site, and a beech site at the Höglwald Forest, Bavaria, Germany, with high temporal resolution in order to gain detailed information about (1) the impacts of forest type, liming, and atmospheric N input by wet deposition on the magnitude of NO and NO2 flux rates and (2) the microbial processes involved in NO production and emission. In addition to identification of seasonal variations of flux rates the huge database allowed calculation of annual mean NO and NO2 fluxes with high accuracy and identification of interannual variations of fluxes. The long-term annual mean NOx emission was 61.7 μg NOx N m−2 h−1 for the spruce control site, 17.3 μg NOx N m−2 h−1 for the limed spruce site, and 4.0 μg NOx N m−2 h−1 for the beech site. These extremely high soil NOx emissions from a temperate forest most likely reflect the status of N saturation of the Höglwald Forest as a consequence of year-long heavy atmospheric N input. Multiple regression analyses revealed the following sequence of importance of environmental factors on NO flux: soil temperature to water-filled pore space to soil NO3− concentrations to soil NH4+ concentrations. Nitrification was the dominating biotic modulator of NO emission at all sites: >60% of the variation of NO emission rates were associated with variations of net nitrification rates. There was a strong positive correlation between amount of in situ N input by wet deposition and magnitude of in situ NO flux rates. Approximately 15% and 7% of the actual N input was lost as NO from the soil stocked with spruce and beech, respectively. Liming resulted in 49% reduction of NO emissions as compared to an unlimed spruce control site. The results indicate that the reduction in NO emission was due to an increase in NO consumption within the limed soil. In contrast to NO flux, NO2 flux was modulated by physico-chemical rather than biological factors. Using the data of this study, we estimate that the contribution of N-affected temperate coniferous and deciduous forests to the global NOx release is 0.3 Tg NOx N yr−1.
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