The effect that pesticides have on trace gas production and consumption in agricultural soils is often overlooked. Independent field and laboratory experiments were used to measure the effects that the commonly used herbicides prosulfuron and metolachlor have on trace gas fluxes (CO₂, N₂O, and CH₄) from fertilized soil of the Colorado shortgrass steppe. Separate sample plots (1 m²) on tilled and no-till soil at the sites included the following treatments: 1) a control without fertilizer or herbicide, 2) a fertilized (NH₄NO₃ equivalent to 244 kg ha⁻¹) control without herbicide, 3) and fertilized plots amended with an herbicide (prosulfuron equivalent to 0.46 kg ha⁻¹ 57% by weight active ingredient or metolachlor equivalent to 5.7 L ha⁻¹, 82.4% by weight active ingredient). During an initial study of one year duration, measurement of gas exchange revealed that prosulfuron-amendment stimulated N₂O emissions and CH₄ consumption by as much as 1600% and 1300% during a single measurement, respectively. During a second set of flux measurements beginning in August 2001, more frequent weekly measurements were made during a twelve week period. From this second study an increased N₂O efflux and CH₄ uptake occurred after a 7-week lag period that persisted for about 5 weeks. These changes in gas flux amounted to an overall increase of 41% and 30% for N₂O emission and CH₄ consumption, respectively. The co-occurrence of stimulated N₂O and CH₄ fluxes suggests a similar cause that is related to prosulfuron degradation. Evidence suggested that prosulfuron degradation stimulated microbial activity responsible for trace gas flux. Ultimately, prosulfuron-amendment led to an ∼50% reduction in the global warming potential from N₂O and CH₄ fluxes at this field site, which is equivalent to a reduction of the global warming potential of 0.18 mols CO₂ m⁻² d⁻¹ from these gases. Metolachlor application did not significantly affect the trace gas fluxes measured. These results demonstrate the potential impact that pesticides have on trace gas fluxes from agricultural soils, which could mean that the effects of other agricultural practices have been over or under estimated.