Trace gas fluxes of CO2, CH4 and N2O in a permanent grassland soil exposed to elevated CO2 in the Giessen FACE study
1Department of Soil and Environmental Sciences University of Azad Jammu and Kashmir, Faculty of Agriculture, Rawalakot Azad Jammu and Kashmir, Pakistan
2Department of Plant Ecology, Justus-Liebig University Giessen, Heinrich-Buff-Ring 26, 35392 Giessen, Germany
3School of Biology and Environmental Science, University College Dublin, Belfield, Dublin 4, Ireland
Abstract. Long-term field observations showed that N2O fluxes were not significantly affected by elevated CO2 shortly after N application 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). Based on the 15N enrichments of the N2O 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. 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 related to the dynamics of the N2O reductase activity.