DENITRIFICATION DURING THE GROWING-SEASON FOLLOWING DAIRY-CATTLE SLURRY AND FERTILIZER APPLICATION FOR SILAGE CORN

Denitrification losses during the growing season may reduce the availa bility of manure and fertilizer N for crop production. Denitrification losses were measured during the growing seasons of 1992 and 1993 foll owing spring manure or fertilizer application on a sandy soil with a h igh water table (Sumas) and a well-drained silt loam soil (Agassiz), b oth cropped to silage corn (Zea mays L.). Dairy cattle slurry (600 kg total N ha(-1)) was surface applied and incorporated in April. Ammoniu m nitrate (200 kg N ha(-1)) was surface applied at planting in early M ay. Denitrification rates were measured using the acetylene blockage t echnique on intact soil cores taken from 0- to 13-cm, 15- to 28-cm and 30- to 43-cm depths biweekly following manure application until mid-J une, then monthly until after harvest in September. Denitrification ra tes were measured weekly in the control and manured treatment in 1993. In 1993, average denitrification rates were significantly higher foll owing manure application (485 g N ha(-1) d(-1)) than following fertili zer application (214 g N ha(-1) d(-1)), and were higher at 15- to 30-c m depth (213 g N ha(-1) d(-1)) than at 0- to 15-cm depth (22 g N ha(-1 ) d(-1)) and the 30- to 45-cm depth (44 g N ha(-1) d(-1)). Denitrifica tion losses during the growing season in 1993 were 75 and 39 kg N ha(- 1) in the manured and fertilized soil at Sumas, compared with 68 and 3 4 kg N ha(-1) in the same treatments at Agassiz. Denitrification losse s during 1992 were 156 and 107 kg N ha(-1) in the manured and fertiliz ed soil at Sumas, compared with 95 and 27 kg N ha(-1) in the same trea tments at Agassiz. This research demonstrates that denitrification los ses can be substantial during the growing season, and can be underesti mated if denitrification rates are measured only to 15 or 20 cm depth.