Trace gas fluxes of CO 2 , CH 4 and N 2 O in a permanent grassland soil exposed to elevated CO 2 in the Giessen FACE study

Abstract. Long-term field observations showed that N 2 O fluxes observed shortly after N application were not significantly affected by elevated CO 2 in the Giessen Free Air Carbon dioxide Enrichment (FACE) study. To further investigate this unexpected result a 15 N tracer study was carried out under controlled conditions where in parallel treatments either the NH 4 + pool ( 15 NH 4 NO 3 ) or the NO 3 − pool (NH 4 15 NO 3 ) was enriched with 15 N. Fluxes of CO 2 , CH 4 , and N 2 O as well as the 15 N enrichment of the N 2 O were measured. Denitrifying Enzyme Activity (DEA), total denitrification (N 2 + N 2 O) and N 2 -to-N 2 O ratios were quantified in separate experiments. Over the 57 day incubation, N 2 O fluxes averaged 0.090 ng N 2 O-N g −1 h −1 under ambient and 0.083 ng N 2 O-N g −1 h −1 under elevated CO 2 (not significantly different). The N 2 O production processes were identified by a two-source model. Results showed that N 2 O must have also been produced by a third source – possibly related to organic N transformation – which was stimulated by elevated CO 2 . Soil CO 2 fluxes were approximately 20 % higher under elevated CO 2 than soil from ambient but the differences were not significant. CH 4 oxidation rates were on average −1.75 ng CH 4 -C g −1 h −1 in the elevated and −1.17 ng CH 4 -C g −1 h −1 in the ambient indicating that elevated CO 2 increased the CH 4 oxidation by 49 % compared to ambient CO 2 under controlled conditions. N fertilization increased CH 4 oxidation by 3-fold in both CO 2 treatments. CO 2 did not have any significant effect on DEA while total denitrification and N 2 -to-N 2 O ratios increased by 36 and 33 %, respectively. The results indicate that shortly after N application elevated CO 2 must have stimulated both the N 2 O production and reduction to N 2 to explain the increased N 2 -to-N 2 O ratio and at the same time explain the non-responsiveness of the N 2 O emissions. Thus, the observed variation of the CO 2 effect on N 2 O emissions throughout the year is possibly governed by the dynamics of the N 2 O reductase activity.