Trace gas fluxes of CO₂, CH₄ and N₂O in a permanent grassland soil exposed to elevated CO₂ in the Giessen FACE study

Abstract. Long-term field observations showed that N₂O fluxes observed shortly after N application were not significantly affected by elevated CO₂ in the Giessen Free Air Carbon dioxide Enrichment (FACE) study. To further investigate this unexpected result a ¹⁵N tracer study was carried out under controlled conditions where in parallel treatments either the NH₄⁺ pool (¹⁵NH₄NO₃) or the NO₃⁻ pool (NH₄¹⁵NO₃) was enriched with ¹⁵N. Fluxes of CO₂, CH₄, and N₂O as well as the ¹⁵N enrichment of the N₂O were measured. Denitrifying Enzyme Activity (DEA), total denitrification (N₂ + N₂O) and N₂-to-N₂O ratios were quantified in separate experiments. Over the 57 day incubation, N₂O fluxes averaged 0.090 ng N₂O-N g⁻¹ h⁻¹ under ambient and 0.083 ng N₂O-N g⁻¹ h⁻¹ under elevated CO₂ (not significantly different). The N₂O production processes were identified by a two-source model. Results showed that N₂O must have also been produced by a third source – possibly related to organic N transformation – which was stimulated by elevated CO₂. Soil CO₂ fluxes were approximately 20 % higher under elevated CO₂ than soil from ambient but the differences were not significant. CH₄ oxidation rates were on average −1.75 ng CH₄-C g⁻¹ h⁻¹ in the elevated and −1.17 ng CH₄-C g⁻¹ h⁻¹ in the ambient indicating that elevated CO₂ increased the CH₄ oxidation by 49 % compared to ambient CO₂ under controlled conditions. N fertilization increased CH₄ oxidation by 3-fold in both CO₂ treatments. CO₂ did not have any significant effect on DEA while total denitrification and N₂-to-N₂O ratios increased by 36 and 33 %, respectively. The results indicate that shortly after N application elevated CO₂ must have stimulated both the N₂O production and reduction to N₂ to explain the increased N₂-to-N₂O ratio and at the same time explain the non-responsiveness of the N₂O emissions. Thus, the observed variation of the CO₂ effect on N₂O emissions throughout the year is possibly governed by the dynamics of the N₂O reductase activity.