NITRATE CONCENTRATION OF SUBSURFACE DRAINAGE WATER FROM A CORN FIELD IN SOUTHERN NEW-BRUNSWICK

The subsurface drainage discharge and associated nitrate-nitrogen (NO3 -N) concentration emanating from a continuously cropped corn field wer e monitored year round for four consecutive crop years, May 1989 to Ma y 1993. Annual nitrogen (N) inputs were approximately 90 kg.ha-1, from a combination of manure and fertilizer N sources. The experimental si te consisted of an on-farm field which was systematically subdrained b y three equally sized (1 ha) plots, each with independent outlets. Ove r the four duration of the study, maximum mean annual values of cumula tive subsurface drainage discharge, NO3-N lost in the drainage dischar ge, and flow-weighted NO3-N concentration in the drainage discharge we re 960 mm, 30.0 kg.ha-1, and 5.0 mg.L-1, respectively. The former two values were incurred the same crop year that maximum annual precipitat ion (1418 mm; 134% of normal) was received. The maximum mean seasonal flow-weighted NO3-N concentration, which was computed for four distinc t time periods of each crop year, was 13.4 mg.L-1 and occurred during the May to September period in 1992 when heavy rains followed shortly after fertilizer application. For the entire study period, approximate ly 85% of the total drainage discharge and 70% of the total NO3-N leac hing occurred outside the active (May to September) growing season.