Miller, J. J., Beasley, B. W., Drury, C. F. and Zebarth, B. J. 2012. Denitrification during the growing season as influenced by long-term application of composted versus fresh feedlot manure. Can. J. Soil Sci. 92: 865-882. Application of composted (new practice) rather than fresh (current industry standard) feedlot manure to cropland is increasing in Alberta. We hypothesized that fall application of composted feedlot manure to cropland may lower growing season denitrification losses of nitrogen (N) to the atmosphere compared with fresh feedlot manure because of lower carbon (C) availability from labile C (water-soluble C, acetic acid) and total organic C. Treatments included soil amended with either fresh (FM) or composted manure (CM) containing straw bedding applied annually at 77 Mg ha^-1 yr^-1 from 1998 to 2009, as well as an unamended control. Surface soil denitrification was measured every 2 wk (May 20 to Sep. 25) for 4 yr (2007-2010) on undisturbed soil cores (0- to 10-cm depth) that were incubated in the field (24 h) using the acetylene inhibition method. Daily arithmetic mean denitrification rates for the 30 sampling dates ranged between 0.2 and 214 g N₂O-N ha^-1 d^-1 for the unamended control, 0.2 to 281 g N₂O-N ha^-1 d^-1 for CM, and 0.5 to 553 g N₂O-N ha^-1 d^-1 for FM. Cumulative denitrification fluxes were significantly (P≤0.05) lower for CM (0.7-1.4 kg N₂O-N ha^-1) compared with FM (3.2-5.1 kg N₂O-N ha^-1) for 2 (2007, 2008) of 4 yr, suggesting that cumulative denitrification fluxes over the growing season may be lower for CM in certain years. Total organic C, water-soluble C, and acetic acid contents in surface soil were generally similar for CM and FM, and we found no relationship between these organic fractions in the soil with daily denitrification rates during the 2010 growing season. Water-filled pore space had the most influence on soil daily denitrification rate. Although cumulative denitrification losses (0.7-10.5 kg N₂O-N ha^-1) for the amended soils in our study were negligible (<1%) compared with annual total N applied, the environmental consequences of this N loss should be taken into consideration when fresh or composted feedlot manure is applied to cropland.