Direct contribution of nitrogen deposition to nitrous oxide emissions in a temperate beech and spruce forest – a ¹⁵ N tracer study

The impact of atmospheric nitrogen (N) deposition on nitrous oxide (N2O) emissions in forest ecosystems is still unclear. The objective of our study was to investigate the direct contribution of N deposition to N2O emissions in temperate forests exposed to chronic high N deposition using a 15N labelling technique. In a Norway spruce stand (Picea abies) and in a beech stand (Fagus sylvatica) in the Solling, Germany, we added a low concentrated 15N-labelled ammoniumnitrate solution to simulate N deposition. Nitrous oxide fluxes and 15N isotope abundances in N2O were measured using the closed chamber method combined with 15N isotope analyses. Emissions of N2O were higher in the beech stand (2.6 ± 0.6 kg N ha−1 yr−1) than in the spruce stand (0.3 ± 0.1 kg N ha−1 yr−1). We observed a direct effect of N input on 15N2O emissions, which lasted less than three weeks and was mainly caused by denitrification. No progressive increase in 15N enrichment of N2O occurred over a one-year experiment, which we explained by immobilisation of deposited N. The annual emission factor for N2O from deposited N was 0.1% for the spruce stand and 0.6% for the beech stand. Standard methods used in the literature applied to the same stands grossly overestimated emission factors with values of up to 25%. Only 6–13% of the total N2O emissions were derived from direct N deposition. Whether the remaining emissions resulted from accumulated anthropogenic N deposition or native N, can not be distinguish with the applied methods. The 15N tracer technique represents a precise tool, which may improve estimates of the current contribution of N deposition on N2O emissions.