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DIVISION S-7—FOREST & RANGE SOILS

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

Univ. of Göttingen, Institute of Soil Science and Forest Nutrition, Büsgenweg 2, D-37077 Göttingen, Germany

^* Corresponding author (rbrumme{at}gwdg.de).

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.

Abbreviations: AFF, acid forest floor • Am, ammonium • AM, acid mineral soil • KIM, Kinetic Isotope Model • LAM, less acid mineral soil • Nat, nitrate • Nit, nitrite • WFPS, water-filled pore space • WHC, water holding capacity