Seasonal variability and mitigation options for N2O emissions from differently managed grasslands

Nitrous oxide emissions were measured from nine plots on an old grassland site near Giessen, Germany. The management regimes of the plots differed in the amount of nitrogen (N) fertilizer applied, in the cutting frequency and in the mean annual ground water table height. Emissions of N2O occurred mainly shortly after fertilization and during freeze-thaw periods. Additional field incubations tin jars) provided evidence that during frost periods N2O emissions as high as 22.000 ng N2O-N kg(-1) soil h(-1) originated directly from the frozen topsoil. For the highest fertilized plots the freeze-thaw period accounted for 43 and 52% of the total annual N2O emissions. Nitrous oxide emissions tended to increase with increasing N fertilizer application and decreasing water table depth. Furthermore, an increase in the number of cuttings per year reduced N2O emissions. The results suggest that the ability of plant roots to take up NO3- increases with increased cutting frequencies throughout the vegetation period, therefore reducing the amount of NO3- available for soil denitrifying microorganisms.