SEASONAL-VARIATIONS IN NITROUS-OXIDE LOSSES FROM MANAGED GRASSLANDS IN THE NETHERLANDS

Seasonal and interannual variations in nitrous oxide (N2O) losses from agricultural soils hamper the accurate quantification of the N2O sour ce strength of these soils. This study focuses on a quantification of seasonal and interannual variations in N2O losses from managed grassla nds. Special attention was paid to N2O losses during the growing seaso n and off-season as affected by grassland management. Fluxes of N2O fr om grasslands with three different types of management and on four dif ferent soil types in the Netherlands were measured weekly during two c onsecutive years, using flux chambers. There were distinct seasonal pa tterns in N2O losses, with large losses during spring, summer, and aut umn but relatively small losses during the winter. These seasonal vari ations were related to fertilizer N application, grazing and weather c onditions. Measurements of N2O concentrations in soil profiles showed that a rise in groundwater level was accompanied by increased N2O conc entrations in the soil. Disregarding off-season losses would underesti mate total annual losses by up to 20%, being largest for unfertilized grassland and smallest for N-fertilized grazed grassland. Total annual N2O losses ranged from 0.5 to 12.9 kg N ha(-1) yr(-1) for unfertilize d grasslands to 7.3 to 42.0 kg N ha(-1) yr(-1) for N-fertilized grazed grasslands. Despite the considerable interannual variations in N2O lo sses, this study indicates that the results of measurements carried ou t in one year have predictive power for estimating N2O losses in other years.