Denitrification during the growing season following dairy cattle slurry and fertilizer application for silage corn

Denitrification losses during the growing season may reduce the availability of manure and fertilizer N for crop production. Denitrification losses were measured during the growing seasons of 1992 and 1993 following spring manure or fertilizer application on a sandy soil with a high water table (Sumas) and a well-drained silt loam soil (Agassiz), both cropped to silage corn (Zeamays L.). Dairy cattle slurry (600 kg total N ha−1) was surface applied and incorporated in April. Ammonium nitrate (200 kg N ha−1) was surface applied at planting in early May. Denitrification rates were measured using the acetylene blockage technique 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-June, then monthly until after harvest in September. Denitrification rates were measured weekly in the control and manured treatment in 1993. In 1993, average denitrification rates were significantly higher following manure application (485 g N ha−1 d−1) than following fertilizer application (214 g N ha−1 d−1), and were higher at 15- to 30-cm 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). Denitrification 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 34 kg N ha−1 in the same treatments at Agassiz. Denitrification losses during 1992 were 156 and 107 kg N ha−1 in the manured and fertilized soil at Sumas, compared with 95 and 27 kg N ha−1 in the same treatments at Agassiz. This research demonstrates that denitrification losses can be substantial during the growing season, and can be underestimated if denitrification rates are measured only to 15 or 20 cm depth. Key words: Acetylene blockage, soil cores, nitrogen, manure, maize