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Nitrous oxide (N2O) is a trace greenhouse gas that catalyses ozone destruction. It is also the major gaseous loss of N from the N-limited shrub-steppe ecosystem. We examined the spatial and temporal flux of N2O from small plots in an undisturbed shrub-steppe ecosystem having spatially heterogeneous plant cover. The N2O Aux from the soil surface and NH4+-N and NO3--N concentrations were measured periodically over 1 y from 44 points in 2.4 m(2) plots centered on individual Artemisia tridentata shrubs. Positive N2O flux occurred from March to October, with no detectable flux during the winter months. The spatial (plot) variability of N2O flux ranged from 23 to 130%, with lower variability as soil moisture increased. Temporal variability (March to October) was 171%, but decreased to 77% when calculated without the August sampling date. The measured N2O dux correlated positively with microbial activity (CO2 production), with moisture when the soil was fairly wet and usually with NO3-N concentrations. After a precipitation event on to dry soil, there was a pulse of N mineralization and N2O flux, which strongly correlated with proximity to vegetation. The estimated N2O flux occurring within 48 h after warm season precipitation events accounted for 20% of the total annual N2O flux from this ecosystem. Thus, small-scale spatial and short-term temporal variations can significantly affect annual estimates of ecosystem N2O flux and, thus, gaseous N loss from semi-arid ecosystems. Published by Elsevier Science Ltd.