Nitrous Oxide Emissions from Grassland in Manawatu, New Zealand

Nitrous oxide (Np) is a minor component of the atmosphere which acts as a potent greenhouse gas and also causes ozone depletion. Soil bacteria are the most important agents of N2O production, but both the New Zealand and the international data on N2O emission rates from soils are very poor at present. In order to help define the possible range of emission ranges from pasture soils· we selected sites with characteristics that would make then "high" or "low" emitters of N2O. The "high" emission site was a fine-textured, fertile but poorly drained flood plain soil. The "low" emission site was located in an area of broken terrain supporting low fertility pastures, Within this area, l"'ndscape units that received high nutrient inputs from animal camping behaviour were also monitored. Events in which significant amounts of N2O were emitted were episodic while low rates of emission were often recorded. The large events (S0-100+ g N/ho/day) were associated with: heavy rain after long dry periods, and grazing when soils had high water contents. Emission rates tended to be highest in late winter and early spring. At the "low" emission sites rates rarely exceeded 5 g N2O­N/ha/day and no extreme events were recorded. Rates also tended to be low in the area affected by animal excreta, despite having many attributes favouring Np formation. Soil nitrate levels and temperature were not well correlated with emission rate.