Abstract:
Rising atmospheric CO2 concentrations are expected to increase nitrous oxide (N2O)
emissions from soils via changes in microbial nitrogen (N) transformations. Several
studies have shown that N2O 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 (NHþ
4 ) or nitrate
(NO
3 ) was labelled with 15N. The simultaneous gross N transformation rates were
analysed with a 15N tracing model and a solver method. The results confirmed that
after 15 years of eCO2 the N2O emissions under eCO2 were still more than twofold
higher than under aCO2. The tracing model results indicated that plant uptake of NHþ
4
did not differ between treatments, but uptake of NO
3 was significantly reduced under
eCO2. However, the NHþ
4 and NO
3 availability increased slightly under eCO2. The
N2O isotopic signature indicated that under eCO2 the sources of the additional emissions,
8,407 lg N2O–N/m2 during the first 58 days after labelling, were associated
with NO
3 reduction (+2.0%), NHþ
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 N2O:N2 emission ratio, explains the
doubling of N2O emissions. If this occurs over a wide area of grasslands in the future,
this positive feedback reaction may significantly accelerate climate change.