Nitrous oxide emissions from intensive agricultural systems: Variations between crops and seasons, key driving variables, and mean emission factors

Emissions of nitrous oxide from intensively managed agricultural fields wer e measured over 3 years. Exponential increases in flux occurred with increa sing soil water-filled pore space (WFPS) and temperature; increases in soil mineral N content due to fertilizer application also stimulated emissions. Fluxes were low when any of these variables was below a critical value. Th e largest fluxes occurred when WFPS values were very high (70-90%), indicat ing that denitrification was the major process responsible. The relationshi ps with the driving variables showed strong similarities to those reported for very different environments: irrigated sugar cane crops,pastures, and f orest in the tropics. Annual emissions varied widely (0.3-18.4 kg N2O-N ha( -1)). These variations were principally due to the degree of coincidence of fertilizer application and major rainfall events. It is concluded therefor e that several years' data are required from any agricultural ecosystem in a variable climate to obtain a robust estimate of mean N2O fluxes. The emis sions from small-grain cereals (winter wheat and spring barley) were consis tently lower (0.2-0.7 kg N2O-N per 100 kg N applied) than from cut grasslan d (0.3-5.8 kg N2O-N per 100 kg N). Crops such as broccoli and potatoes gave emissions of the same order as those from the grassland. Although these di fferences between crop types are not apparent in general data comparisons, there may well be distinct regional differences in the relative and absolut e emissions from different crops, due to local factors relating to soil typ e, weather patterns, and agricultural management practices. This will only be determined by more detailed comparative studies.