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

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 N-15 tracer study was carried out under controlled conditions where in parallel treatments either the NH4+ pool ((NH4NO3)-N-15) or the NO3- pool ((NH4NO3)-N-15) was enriched with N-15. Fluxes of CO2, CH4, and N2O as well as the N-15 enrichment of the N2O were measured. Denitrifying Enzyme Activity (DEA), total denitrification (N-2 + N2O) and N-2-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 N-2-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 N-2 to explain the increased N-2-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.