A 3-year continuous record of nitrogen trace gas fluxes from untreated andlimed soil of a N-saturated spruce and beech forest ecosystem in Germany -2. NO and NO2 fluxes

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 Hoglwald Forest, Bavaria, Germany, with high temporal resolution in order to gain detailed information about (1) the impacts of forest type, li ming, 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 an d emission. In addition to identification of seasonal variations of flux ra tes 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 mu g NOx N m(-2) h(-1) for the spruce control site, 17.3 mu g NOx N m(-2) h(-1) for the limed spruce s ite, and 4.0 mu g NOx N m(-2) h(-1) for the beech site. These extremely hig h soil NOx emissions from a temperate forest most likely reflect the status of N saturation of the Hoglwald Forest as a consequence of yearlong heavy atmospheric N input. Multiple regression analyses revealed the following se quence of importance of environmental factors on NO flux: soil temperature to water-filled pore space to soil NO3- concentrations to soil NH4+ concent rations. Nitrification was the dominating biotic modulator of NO emission a t all sites: >60% of the variation of NO emission rates were associated wit h variations of net nitrification rates. There was a strong positive correl ation 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 l ost 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 d ue to an increase in NO consumption within the limed soil. In contrast to N O flux, NO2 flux was modulated by physico-chemical rather than biological f actors. Using the data of this study, we estimate that the contribution of N-affected temperate coniferous and deciduous forests to the global NOx rel ease is 0.3 Tg NOx N yr(-1).