The effect of water table depth on emissions of N2O from a grassland soil

Nitrous oxide (N2O) emissions were measured by the closed chamber technique from five plots along a transect in a nitrogen-fertilised grassland, together with soil water content, soil temperature and water table depth, to investigate the effect of water table depth on N2O emissions. N2O fluxes varied from < 1 g N2O-N ha(-1) day(-1) to peaks of around 500-1200 g N2O-N ha(-1) day(-1) after N fertiliser applications. There was no significant difference in overall average water table depth between four of the five plots, but significant short-term temporal variations in water table depth did occur. Rises in the water table were accompanied by exponential increases in N2O emissions., through the associated increases in the water-filled pore space of the topsoil. Modelling predicted that if the water table could be managed such that it was kept to no less than 35 cm below the ground surface, fluxes during the growing season would be reduced by 50%, while lowering to 45 cm would reduce them by over 80%. The strong implication of these results is that draining grasslands, so that the water tables are only rarely nearer to the surface than 35 cm when N is available for denitrification, would substantially reduce N2O emissions.