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 Soiling and a less acid mineral soil (LAM) (pH=5.2) from the Gottinger 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 he 0.71 and 0.51 kg N ha(-1) yr(-1) for the Gottinger 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 Gottinger 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.