Dinitrogen and Nitrous Oxide Formation in Beech Forest Floor and Mineral Soils

A 15 N tracer study was conducted to determine N 2 and N 2 O fluxes and the processes responsible for the formation of N 2 O in two beech ( Fagus silvatica L.) forest soils: an acid mineral soil (AM) (pH = 3.8) and the overlying acid forest floor (AFF) (pH = 3.8) from the Solling and a less acid mineral soil (LAM) (pH = 5.2) from the Göttinger Wald. Ammonium and nitrate of undisturbed soil cores were labeled by injecting 15 N. The evolved gases, N 2 O and N 2 , and the ammonium and nitrate concentrations in the soils were measured together with the 15 N abundances over a period of 8 d. Nitrous Oxide was produced in all soils by denitrification. Nitrate was reduced to N 2 at higher soil pH (LAM) and in the AFF. The end product of denitrification at the lower soil pH (AM) was N 2 O. The N 2 O and N 2 emission calculated on an areal basis decreased from LAM, AFF, to AM. The N 2 O/(N 2 O + N 2 ) ratios decreased from AM (1.0), AFF (0.97), to LAM (0.80) during the initial period indicating that the main product of denitrification was N 2 O. On prolonged incubation the N 2 O/(N 2 O + N 2 ) ratios decreased for AFF and LAM to 0.78 and 0.32, respectively, and was attributed to a gradual decrease in nitrate concentration. We estimate the in situ N 2 emissions to be 0.71 and 0.51 kg N ha −1 yr −1 for the Göttinger Wald and the Solling, using published annual in situ N 2 O emissions and our N 2 /N 2 O ratios. The in situ N 2 O emissions of the Göttinger Wald and Solling are 0.17 and 3 kg ha −1 yr −1 and the N 2 emissions increased the annual denitrification losses to about 0.88 and 3.51 kg ha −1 yr −1 The approach for estimating in situ N 2 emissions has to be improved in future studies.