There is a growing interest in the adoption of conservation tillage systems [no-till (NT) and reduced tillage (RT)] as alternatives to conventional tillage (CT) systems. A 2-year study was conducted to investigate possible environmental consequences of three tillage systems on a 2.4-ha field located at Macdonald Research Farm, McGill University, Montreal. The soil was a sandy loam (0.5 m depth) underlain by a clay layer. Treatments consisted of a factorial combination of CT, RT, and NT with the presence or absence of crop residue. Soil NO 3 --N concentrations tended to be lower in RT than NT and CT tillage treatments. Denitrification and N 2O emissions were similar among tillage systems. Contrary to the popular assumption that denitrification is limited to the uppermost soil layer (0?0.15 m), large rates of N 2O production were measured in the subsurface (0.15?0.45 m) soil, suggesting that a significant portion of produced N 2O may be missed if only soil surface gas flux measurements are made. The N 2O mole fraction (N 2O:N 2O+N 2) was higher in the drier season of 1999 under CT than in 2000, with the ratio occasionally exceeding 1.0 in some soil layers. Dissolved organic C concentrations remained high in all soil depths sampled, but were not affected by tillage system .
