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Land use and agricultural practices can result in important contributions to the global source strength of atmospheric nitrous oxide (N 2 O) and methane (CH 4 ). However, knowledge of gas flux from irrigated agriculture is very limited. From April 2005 to October 2006, a study was conducted in the Aral Sea Basin, Uzbekistan, to quantify and compare emissions of N 2 O and CH 4 in various annual and perennial land-use systems: irrigated cotton, winter wheat and rice crops, a poplar plantation and a natural Tugai (floodplain) forest. In the annual systems, average N 2 O emissions ranged from 10 to 150 lg N 2 O-N m À2 h À1 with highest N 2 O emissions in the cotton fields, covering a similar range of previous studies from irrigated cropping systems. Emission factors (uncorrected for background emission), used to determine the fertilizer-induced N 2 O emission as a percentage of N fertilizer applied, ranged from 0.2% to 2.6%. Seasonal variations in N 2 O emissions were principally controlled by fertilization and irrigation management. Pulses of N 2 O emissions occurred after concomitant N-fertilizer application and irrigation. The unfertilized poplar plantation showed high N 2 O emissions over the entire study period (30 lg N 2 O-N m À2 h À1 ), whereas only negligible fluxes of N 2 O (o2 lg N 2 O-N m À2 h À1 ) occurred in the Tugai. Significant CH 4 fluxes only were determined from the flooded rice field: Fluxes were low with mean flux rates of 32 mg CH 4 m À2 day À1 and a low seasonal total of 35.2 kg CH 4 ha À1 . The global warming potential (GWP) of the N 2 O and CH 4 fluxes was highest under rice and cotton, with seasonal changes between 500 and 3000 kg CO 2 eq. ha À1 . The biennial cotton-wheat-rice crop rotation commonly practiced in the region would average a GWP of 2500 kg CO 2 eq. ha À1 yr À1 . The analyses point out opportunities for reducing the GWP of these irrigated agricultural systems by (i) optimization of fertilization and irrigation practices and (ii) conversion of annual cropping systems into perennial forest plantations, especially on less profitable, marginal lands. 20 0 S A J J M S A J J M Fig. 2 CH 4 and N 2 O flux rates of the rice plot from May to September 2005 and 2006. Arrows indicate the events of N (kg N ha À1 ) application to the plots (U, urea; AN, ammonium nitrate; AP, ammonium phosphate). Error bars indicate the standard error.