Explaining the doubling of N₂ O emissions under elevated CO₂ in the Giessen FACE via in-field ¹⁵ N tracing

Rising atmospheric CO₂ concentrations are expected to increase nitrous oxide (N₂ O) emissions from soils via changes in microbial nitrogen (N) transformations. Several studies have shown that N₂ O emission increases under elevated atmospheric CO₂ (eCO₂ ), but the underlying processes are not yet fully understood. Here, we present results showing changes in soil N transformation dynamics from the Giessen Free Air CO₂ Enrichment (GiFACE): a permanent grassland that has been exposed to eCO₂ , +20% relative to ambient concentrations (aCO₂ ), for 15 years. We applied in the field an ammonium-nitrate fertilizer solution, in which either ammonium ( ${\text{NH}}_4^{+}$ ) or nitrate ( ${\text{NO}}_3^{-}$ ) was labelled with ¹⁵ N. The simultaneous gross N transformation rates were analysed with a ¹⁵ N tracing model and a solver method. The results confirmed that after 15 years of eCO₂ the N₂ O emissions under eCO₂ were still more than twofold higher than under aCO₂ . The tracing model results indicated that plant uptake of ${\text{NH}}_4^{+}$ did not differ between treatments, but uptake of ${\text{NO}}_3^{-}$ was significantly reduced under eCO₂ . However, the ${\text{NH}}_4^{+}$ and ${\text{NO}}_3^{-}$ availability increased slightly under eCO₂ . The N₂ O isotopic signature indicated that under eCO₂ the sources of the additional emissions, 8,407 μg N₂ O-N/m² during the first 58 days after labelling, were associated with ${\text{NO}}_3^{-}$ reduction (+2.0%), ${\text{NH}}_4^{+}$ oxidation (+11.1%) and organic N oxidation (+86.9%). We presume that increased plant growth and root exudation under eCO₂ provided an additional source of bioavailable supply of energy that triggered as a priming effect the stimulation of microbial soil organic matter (SOM) mineralization and fostered the activity of the bacterial nitrite reductase. The resulting increase in incomplete denitrification and therefore an increased N₂ O:N₂ emission ratio, explains the doubling of N₂ O emissions. If this occurs over a wide area of grasslands in the future, this positive feedback reaction may significantly accelerate climate change.