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

Abstract

A 15N tracer study was conducted to determine N2 and N2O fluxes and the processes responsible for the formation of N2O 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 15N. The evolved gases, N2O and N2, and the ammonium and nitrate concentrations in the soils were measured together with the 15N abundances over a period of 8 d. Nitrous Oxide was produced in all soils by denitrification. Nitrate was reduced to N2 at higher soil pH (LAM) and in the AFF. The end product of denitrification at the lower soil pH (AM) was N2O. The N2O and N2 emission calculated on an areal basis decreased from LAM, AFF, to AM. The N2O/(N2O + N2) 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 N2O. On prolonged incubation the N2O/(N2O + N2) 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 N2 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 N2O emissions and our N2/N2O ratios. The in situ N2O emissions of the Göttinger Wald and Solling are 0.17 and 3 kg ha−1 yr−1 and the N2 emissions increased the annual denitrification losses to about 0.88 and 3.51 kg ha−1 yr−1 The approach for estimating in situ N2 emissions has to be improved in future studies.