Sources of nitrous oxide emitted from European forest soils
Forest ecosystems may provide strong sources of nitrous oxide (N 2O), which is important for atmospheric chemical and radiative properties. Nonetheless, our understanding of controls on forest N 2O 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 N 2O production in European forests in order to evaluate the importance of nitrification and denitrification for N 2O production. Soil samples were collected in 11 different sites characterized by variable climatic regimes and forest types. Soil N-cycling and associated production of N 2O was assessed following application of 15N-labeled nitrogen. The N 2O emission varied significantly among the different forest soils, and was inversely correlated to the soil C:N ratio. The N 2O emissions were significantly higher from the deciduous soils (13 ng N 2O-N cm -3 d -1) than from the coniferous soils (4 ng N 2O-N cm -3 d -1). Nitrate (NO 3-) was the dominant substrate for N 2O with an average contribution of 62% and exceeding 50% at least once for all sites. The average contribution of ammonium (NH 4+) to N 2O averaged 34%. The N 2O emissions were correlated with gross nitrification activities, and as for N 2O, 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 N 2O 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 N 2O emissions were driven by the nitrification activity, although the N 2O 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.