Explaining the doubling of N 2 O emissions under elevated CO 2 in the Giessen FACE via in-field 15 N 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 (GiFACE): 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 ( NH 4 + ) or nitrate ( 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 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 NH 4 + did not differ between treatments, but uptake of NO 3 - was significantly reduced under eCO ₂ . However, the NH 4 + and NO 3 - 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 NO 3 - reduction (+2.0%), NH 4 + 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.
(© 2018 John Wiley & Sons Ltd.)