Conditions in soils of south-coastal British Columbia during the fall and winter favor both NO3− leaching and denitrification. Estimates of NO3− leaching were made on a coarse-textured soil having a high water table (Sumas) and a well-drained, medium-textured soil (Agassiz) in each of 1991 and 1992, following application of 0, 300 and 600 kg total N ha−1 as dairy cattle slurry at the end of September. Leaching was calculated by measuring NH4+ and NO3− concentrations biweekly to 90 cm depth and subtracting denitrification losses. Denitrification estimates were made using the acetylene blockage technique on intact cores taken to 43 cm depth at biweekly intervals from the 0 and 600 kg N ha−1 treatments. Most of the NO3− disappeared from the 90 cm soil profile by January in both years. At the high rate of manure application, denitrification losses were 29 and 20 kg N ha−1 at Agassiz in 1991 and 1992, and 48 and 17 kg N ha−1 at Sumas in 1991 and 1992, respectively. Denitrification losses in the non-manured treatments ranged from 3 to 16 kg N ha−1 Significant denitrification losses occurred at the 15- to 28-cm and the 30-cm to 43-cm depths, indicating that denitrification measurements at these depths are important. Leaching losses ranged from 60 to 188 kg N ha−1 from the control treatments, and from 102 to 241 kg N ha−1 following the highrate of manure application. Denitrification accounted for up to 17% of the NO3− loss from the soil profile, indicating that the majority of the NO3− was leached. Key words: Acetylene blockage technique, denitrification rate, air-filled porosity, manure
