Sources of nitrous oxide emitted from European forest soils

Abstract. Forest ecosystems may provide strong sources of nitrous oxide (N₂O), which is important for atmospheric chemical and radiative properties. Nonetheless, our understanding of controls on forest N₂O emissions is insufficient to narrow current flux estimates, which still are associated with great uncertainties. In this study, we have investigated the quantitative and qualitative relationships between N-cycling and N₂O production in European forests in order to evaluate the importance of nitrification and denitrification for N₂O production. Soil samples were collected in 11 different sites characterized by variable climatic regimes and forest types. Soil N-cycling and associated production of N₂O was assessed following application of ¹⁵N-labeled nitrogen. The N₂O emission varied significantly among the different forest soils, and was inversely correlated to the soil C:N ratio. The N₂O emissions were significantly higher from the deciduous soils (13 ng N₂O-N cm^-3 d^-1) than from the coniferous soils (4 ng N₂O-N cm^-3 d^-1). Nitrate (NO₃^-) was the dominant substrate for N₂O with an average contribution of 62% and exceeding 50% at least once for all sites. The average contribution of ammonium (NH₄⁺) to N₂O averaged 34%. The N₂O emissions were correlated with gross nitrification activities, and as for N₂O, gross nitrification was also higher in deciduous soils (3.4 µg N cm^-3 d^-1) than in coniferous soils (1.1 µg N cm^-3 d^-1). The ratio between N₂O production and gross nitrification averaged 0.67% (deciduous) and 0.44% (coniferous). Our study suggests that changes in forest composition in response to land use activities and global change may have implications for regional budgets of greenhouse gases. From the study it also became clear that N₂O emissions were driven by the nitrification activity, although the N₂O was produced per se mainly from denitrification. Increased nitrification in response to accelerated N inputs predicted for forest ecosystems in Europe may thus lead to increased greenhouse gas emissions from forest ecosystems.