Moser, G.; Gorenflo, A.; Brenzinger, K.; Keidel, L.; Braker, G.; Marhan, S.; Clough, T.J. & Müller, C. (2018): <b>Explaining the doubling of N2O emissions under elevated CO2 in the Giessen FACE via in-field 15N tracing</b>. <i>Global Change Biology</i> <b>24</b>, 3897-3910<br>DOI: <a href="http://dx.doi.org/10.1111/gcb.14136" target="_blank">http://dx.doi.org/10.1111/gcb.14136</a>.
Resource Description
Title:
Explaining the doubling of N2O emissions under elevated CO2 in the Giessen FACE via in-field 15N tracing
Rising atmospheric CO2 concentrations are expected to increase nitrous oxide (N2O)<br/>
emissions from soils via changes in microbial nitrogen (N) transformations. Several<br/>
studies have shown that N2O emission increases under elevated atmospheric CO2<br/>
(eCO2), but the underlying processes are not yet fully understood. Here, we present<br/>
results showing changes in soil N transformation dynamics from the Giessen Free Air<br/>
CO2 Enrichment (GiFACE): a permanent grassland that has been exposed to eCO2,<br/>
+20% relative to ambient concentrations (aCO2), for 15 years. We applied in the field<br/>
an ammonium-nitrate fertilizer solution, in which either ammonium (NHþ<br/>
4 ) or nitrate<br/>
(NO<br/>
3 ) was labelled with 15N. The simultaneous gross N transformation rates were<br/>
analysed with a 15N tracing model and a solver method. The results confirmed that<br/>
after 15 years of eCO2 the N2O emissions under eCO2 were still more than twofold<br/>
higher than under aCO2. The tracing model results indicated that plant uptake of NHþ<br/>
4<br/>
did not differ between treatments, but uptake of NO<br/>
3 was significantly reduced under<br/>
eCO2. However, the NHþ<br/>
4 and NO<br/>
3 availability increased slightly under eCO2. The<br/>
N2O isotopic signature indicated that under eCO2 the sources of the additional emissions,<br/>
8,407 lg N2O–N/m2 during the first 58 days after labelling, were associated<br/>
with NO<br/>
3 reduction (+2.0%), NHþ<br/>
4 oxidation (+11.1%) and organic N oxidation<br/>
(+86.9%). We presume that increased plant growth and root exudation under eCO2<br/>
provided an additional source of bioavailable supply of energy that triggered as a priming<br/>
effect the stimulation of microbial soil organic matter (SOM) mineralization and<br/>
fostered the activity of the bacterial nitrite reductase. The resulting increase in incomplete<br/>
denitrification and therefore an increased N2O:N2 emission ratio, explains the<br/>
doubling of N2O emissions. If this occurs over a wide area of grasslands in the future,<br/>
this positive feedback reaction may significantly accelerate climate change.
Keywords:
| climate change | elevated CO2 | grassland | free air carbon dioxide enrichment | long-term response | N transformation | N2O emission | positive climate change feedback |
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