Improvement in use efficiency of N fertilizers can potentially better sustain agriculture by reducing N₂O emissions from soils, but little is known about its impact on soil CO₂ emissions. A study, involving both a field experiment and a laboratory incubation, was conducted in eastern Canada to determine the N fertilization effect on soil CO₂ emissions. In laboratory, we incubated nine different types of soil with and without 150 kg N ha^-1 as KNO₃ or (NH₄)₂SO₄. The N-fertilized soils had lower CO₂ emissions compared with the no-N control soils for six of them. Among fertilizer sources, emissions of CO₂ were on average 22% lower with KNO₃ than with (NH₄)₂SO₄. The field experiment conducted on a clay soil included three sources of N (urea-NH₄NO₃, CaNH₄NO₃, and aqua NH₃) at 0-200 kg N ha^-1 band-incorporated at the six-leaf corn stage. Under field conditions, most CO₂ was emitted between N application and grain maturity with cumulative seasonal soil emissions greater in the control (4.9 Mg C ha^-1) than in the N treatments (average of 4.0 ± 0.3 Mg C ha^-1). Evidence suggested that both heterotrophic and autotrophic respiration seemed affected, whereas the NO₃-based source had a more depressing effect on CO₂ emissions than did the NH₄ source.