资源环境科技发展态势分析平台

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 (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 CO2 Enrichment (GiFACE): a permanent grassland that has been exposed to eCO(2), +20% relative to ambient concentrations (aCO(2)), for 15years. We applied in the field an ammonium-nitrate fertilizer solution, in which either ammonium (NH4+) or nitrate (NO3-) was labelled with N-15. The simultaneous gross N transformation rates were analysed with a N-15 tracing model and a solver method. The results confirmed that after 15years of eCO(2) the N2O emissions under eCO(2) were still more than twofold higher than under aCO(2). The tracing model results indicated that plant uptake of NH4+ did not differ between treatments, but uptake of NO3- was significantly reduced under eCO(2). However, the NH4+ and NO3- availability increased slightly under eCO(2). The N2O isotopic signature indicated that under eCO(2) the sources of the additional emissions, 8,407g N2O-N/m(2) during the first 58days after labelling, were associated with NO3- reduction (+2.0%), NH4+ 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 N2O:N-2 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.