Nitrous oxide emissions from grass swards during the eighth year of elevated atmospheric pCO₂ (Swiss FACE)

Emissions of N-2 O were measured during the growth season over a year from grass swards under ambient (360 muL L-1 ) and elevated (600 muL L-1 ) CO2 partial pressures at the Free Air Carbon dioxide Enrichment (FACE) experiment, Eschikon, Switzerland. Measurements were made following high (56 g N m(-2) yr(-1) ) and low (14 g N m(-2) yr(-1) ) rates of fertilizer application, split over 5 re-growth periods, to Lolium perenne , Trifolium repens and mixed Lolium /Trifolium swards. Elevated p CO2 increased annual emissions of N-2 O from the high fertilized Lolium and mixed Lolium /Trifolium swards resulting in increases in GWP (N-2 O emissions) of 179 and 111 g CO2 equivalents m(-2) , respectively, compared with the GWP of ambient p CO2 swards, but had no significant effect on annual emissions from Trifolium monoculture swards. The greater emissions from the high fertilized elevated p CO2 Lolium swards were attributed to greater below-ground C allocation under elevated p CO2 providing the energy for denitrification in the presence of excess mineral N. An annual emission of 959 mg N-2 O-N m(-2) yr(-1) (1.7% of fertilizer N applied) was measured from the high fertilized Lolium sward under elevated p CO2 . The magnitude of emissions varied throughout the year with 84% of the total emission from the elevated p CO2 Lolium swards measured during the first two re-growths (April-June 2001). This was associated with higher rainfall and soil water contents at this time of year. Trends in emissions varied between the first two re-growths (April-June 2001) and the third, fourth and fifth re-growths (late June-October 2000), with available soil NO3 (-) and rainfall explaining 70%, and soil water content explaining 72% of the variability in N-2 O in these periods, respectively. Caution is therefore required when extrapolating from short-term measurements to predict long-term responses to global climate change. Our findings are of global significance as increases in atmospheric concentrations of CO2 may, depending on sward composition and fertilizer management, increase greenhouse gas emissions of N-2 O, thereby exacerbating the forcing effect of elevated CO2 on global climate. Our results suggest that when applying high rates of N fertilizer to grassland systems, Trifolium repens swards, or a greater component of Trifolium in mixed swards, may minimize the negative effect of continued increasing atmospheric CO2 concentrations on global warming.