CH4 oxidation and emissions of CH4 and N2O from Lolium perenne swards under elevated atmospheric CO2

Emissions of N2O and CH4 and CH4 oxidation rates were measured from Lolium perenne swards in a short-term study under ambient (36 Pa) and elevated (60 Pa) atmospheric CO2 at the Free Air Carbon dioxide Enrichment experiment, Eschikon, Switzerland. Elevated pCO(2) increased (P < 0.05) N2O emissions from high N fertilised (11.2 g N m(-2)) swards by 69%, but had no significant effect on net emissions of CH4. Application of C-13-CH4 (11 mul l(-1); 11 at.% excess C-13) to closed chamber headspaces in microplots enabled determination of rates of C-13-CH4 oxidation even when net CH4 fluxes from main plots were positive. We found a significant interaction between fertiliser application rate and atmospheric pCO(2) on C-13-CH4 oxidation rates that was attributed to differences in gross nitrification rates and C and N availability. CH4 oxidation was slower and thought to be temporarily inhibited in the high N ambient pCO(2) sward. The most rapid CH4 oxidation of 14.6 mug C-13-CH4 m(-2) h(-1) was measured in the high fertilised elevated pCO(2) sward, and we concluded that either elevated pCO(2) had a stimulatory effect on CH4 oxidation or inhibition of oxidation following fertiliser application was lowered under elevated pCO(2). of Application (NH4NO3)-N-14-N-15 and (NH4NO3)-N-15-N-15 (10 at.% excess N-15) to different replicates enabled determination of the respective contributions of nitrification and denitrification to N2O emissions. Inhibition of CH4 oxidation in the high fertilised ambient pCO(2) sward, due to competition between NH3 and CH4 for methane monooxygenase enzymes or toxic effects of NH2OH or NO2- produced during nitrification, was hypothesised to increase gross nitrification (12.0 mg N kg dry soil(-1)) and N2O emissions during nitrification (327 mg N-15-N2O m(-2) over 11 d). Our results indicate that increasing atmospheric concentrations of CO2 may increase emissions of N2O by denitrification, lower nitrification rates and either increase or decrease the ability of soil to act as a sink for atmospheric CH4 depending on fertiliser management. (C) 2004 Elsevier Ltd. All rights reserved.