Explaining the doubling of N₂O emissions under elevated CO₂ in the Giessen FACE via in‐field ¹⁵N tracing

Rising atmospheric CO 2 concentrations are expected to increase nitrous oxide (N 2 O) emissions from soils via changes in microbial nitrogen (N) transformations. Several studies have shown that N 2 O emission increases under elevated atmospheric CO 2 ( eCO 2 ), 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 CO 2 Enrichment (Gi FACE ): a permanent grassland that has been exposed to eCO 2 , +20% relative to ambient concentrations ( aCO 2 ), for 15 years. We applied in the field an ammonium‐nitrate fertilizer solution, in which either ammonium ( ) or nitrate ( ) 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 eCO 2 the N 2 O emissions under eCO 2 were still more than twofold higher than under aCO 2 . The tracing model results indicated that plant uptake of did not differ between treatments, but uptake of was significantly reduced under eCO 2 . However, the and availability increased slightly under eCO 2 . The N 2 O isotopic signature indicated that under eCO 2 the sources of the additional emissions, 8,407 μg N 2 O–N/m 2 during the first 58 days after labelling, were associated with reduction (+2.0%), oxidation (+11.1%) and organic N oxidation (+86.9%). We presume that increased plant growth and root exudation under eCO 2 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 N 2 O:N 2 emission ratio, explains the doubling of N 2 O emissions. If this occurs over a wide area of grasslands in the future, this positive feedback reaction may significantly accelerate climate change.