NO AND N2O EMISSIONS FROM SAVANNA SOILS FOLLOWING THE FIRST SIMULATEDRAINS OF THE SEASON

Data on the emissions of oxides of nitrogen from the soil during the e arly part of the wet season are reported for nutrient-rich and nutrien t-poor sandy soils at Nylsvley, South Africa. The emissions of NOx and N2O following the first wetting event of the season are elevated rela tive to subsequent events. The observed high emission rates (76 ng N-N O m(-2) s(-1)) are partially attributed to the sandiness of the soil, which permits NO to diffuse out of the soil rapidly. The pulse of high emissions following wetting is maintained for approximately 72 hours, thereafter continuing at around 20 ng NO m(-2) s(-1) while the soil r emains moist. The initial pulse is suggested to be due to the accumula tion of a substrate pool during the dry period, coupled with an inabil ity of plants and microbes to use it effectively during the first few days after wetting. There were no significant differences in the peak or subsequent emission rates for either NO or N2O between two sites of differing nitrogen mineralisation potentials. N2O emissions averaged 8% of NOx emissions. The enhanced emissions of NOx which follow the fi rst wetting after a prolonged dry period do not make a very large cont ribution to the annual gaseous N emission budget, but could be a signi ficant contributor to the high tropospheric ozone levels observed over southern Africa in springtime.