Spatial heterogeneity of the N2O emission activity was characterized at a fertilized grassland by means of small chambers arranged in grids with spacings ranging from 0.11 m to 7.1 m. The fluxes exhibited marked spatial variability, approximately lognormally distributed. The variation decreased (1) over time within the 7.1-m grid, (2) when grid size was reduced, and (3) after glucose amendments. This decrease was probably caused by homogeneous soil aeration (1) when soil drains, (2) within minor areas, and (3) when overall soil respiration was enhanced by glucose. Isotropic semivariograms showed spatial autocorrelation of N2O fluxes beyond separations of 7.1 m and below those of 0.8 m. The small-scale autocorrelation disappeared in response to glucose. Substantial variation occurred below the scale of minimum separation. High local N2O fluxes at an 1.8-m resolution were coincident with wet depressions in the ground. We suggest that N2O flux patterns at the scale beyond 7 m was controlled by soil moisture variability governed by ground topography, whereas a patchy distribution of denitrifying microsites governed N2O emission at the scale below 1 m. The study indicates that even N2O fluxes obtained from ''megachambers'' (upward of 50 m2) should be interpreted carefully since they may not encompass the whole field variability. ¿ American Geophysical Union 1994