Explaining the doubling of N2O emissions under elevated CO2 in the Giessen FACE via in‐field 15N tracing |
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| Authors: | Gerald Moser André Gorenflo Kristof Brenzinger Lisa Keidel Gesche Braker Sven Marhan Tim J Clough Christoph Müller |
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| Institution: | 1. Department of Plant Ecology, Justus‐Liebig‐University Giessen, Giessen, Germany;2. Department of Biogeochemistry, Max Planck Institute for Terrestrial Microbiology, Marburg, Germany;3. Kiel University, Kiel, Germany;4. Department of Soil Biology, Institute of Soil Science and Land Evaluation, University of Hohenheim, Stuttgart, Germany;5. Department of Soil and Physical Sciences, Lincoln University, Canterbury, New Zealand;6. School of Biology and Environmental Science, University College Dublin, Dublin, Ireland |
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| 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 ( ) or nitrate ( ) 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  did not differ between treatments, but uptake of  was significantly reduced under eCO2. However, the  and  availability increased slightly under eCO2. The N2O isotopic signature indicated that under eCO2 the sources of the additional emissions, 8,407 μg N2O–N/m2 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 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. |
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| Keywords: | climate change elevated CO2 free air CO2 enrichment grassland long‐term response N transformation N2O emission positive climate change feedback |
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) or nitrate (
) 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 
did not differ between treatments, but uptake of 
was significantly reduced under eCO2. However, the 
and 
availability increased slightly under eCO2. The N2O isotopic signature indicated that under eCO2 the sources of the additional emissions, 8,407 μg N2O–N/m2 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 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.