The soil emissions of NO and N2O were measured continously at high frequency throughout more than a year at 15 European forest sites as part of the EU-funded project NOFRETETE. The sites represent different forest types (coniferous/deciduous) and different nitrogen loads. Geographically they range from Finland in the north to Italy in the south and from Hungary in the east to Scotland in the west. The highest NO emissions were observed from coniferous forests, whereas the lowest NO emissions were observed from deciduous forests. The NO emission from coniferous forests were highly correlated with N-deposition. Very low (~ 0) NO emission was observed from non-spruce forests when the N-deposition was lower than 1 g N m-2 y-1. The site with the highest average annual emission (82 μg NO-N m-2 h-1) was a spruce site in Höglwald receiving an annual N-deposition of 2.9 g m-2. No systematic differences in N2O emission were observed between deciduous and coniferous forests, and the correlation between N2O emission and N-deposition was weaker than for NO. The highest annual N2O emission (137 μg N2O-N m-2 h-1) was found in a Douglas fir site in Speulderbos (The Netherlands) receiving an annual N-deposition 3.9 g m-2. A possible explanation for the difference in N-oxide emission between coniferous and deciduous forests could be that NO is mainly derived from nitrification whereas N2O is mainly derived from denitrification. Soil moisture is lower at coniferous sites (at least during spring time) and the litter layer of coniferous forests is thick and well aerated favouring nitrification and thus release of NO. Conversely, the higher rates of denitrification in deciduous forests due to a compact and moist litter layer lead to N2O production and NO consumption in the soil.
