Nitrous oxide (N2O) is a greenhouse gas which destroys ozone in the stratosphere and accounts for about 10% of the radiative-forcing of gases produced by human acitivity to the atmosphere in Canada. Agricultural soils account for the majority of nitrous oxide (N2O) emitted by anthropogenic sources. Most emissions occur during a brief period (1-3 weeks) in which soil frozen in winter thaws in spring. To identify the factors driving emissions during thawing of soil a series of laboratory experiments were conducted. A loamy calcareous soil was packed into rings, moistened to 85% water-filled pore space, placed for 2 days at -20oC and then placed into 1.5 L sealer jars at room temperature to determine N2O emission rates over a 7-day period. Control treatments were treated similarly except were placed at 5oC for 2 days prior to placement at room temperature. The laboratory assay effectively reproduced enhanced N2O emission upon thawing of frozen soil. N2O emissions were highly associated with the rise of soil temperature from freezing to room temperature. Of the factors tested, NO3- addition enhanced N2O emission from thawing soil more than addition of sucrose (available C), liquid hog manure, and water. Increasing bulk density (1.1, 1.2, and 1.25 g cm-3) of soil amended with NO3- decreased N2O emission upon thawing of soil. The results imply that NO3- level in soil is the predominant factor affecting N2O emission from thawing soil.