CHANGES OF SOIL NITRATE-NITROGEN AND DENITRIFICATION AS AFFECTED BY NITROGEN-FERTILIZER ON 2 QUEBEC SOILS

Corn (Zea mays L.) production may result in NO3- -N accumulation in so ils because of high N fertilizer applications. Nitrate-N in soil may b e lost by denitrification and leaching. Prediction of the amount lost would be helpful in minimizing these losses. Field experiments were co nducted to investigate the effects of N fertilizer rates on soil NO3- -N under corn on two soils of contrasting texture from 1988 to 1991 in southwestern Quebec. Denititrification as affected by previous N fert ilizer rates was estimated for the nongrowing season of 1990-1991. Soi l NO3- -N levels in the fall increased linearly with increasing N fert ilizer rates above the normal N rate both on clay and sandy clay loam soils. However, NO3- -N could have been leached out of the rooting zon e on the coarser Chicot soil (fine-loamy, mixed, frigid Typic Hapludal f) in the late growing season, depending on rainfall. The changes in s oil NO3- -N over winter were a function of both soil NO3- -N levels th e previous fall and over-winter precipitation. Denitrification losses varied from 7 to 24 kg ha-1 during the nongrowing season, increasing w ith N fertilizer rates only on the Ste. Rosalie clay soil (fine, mixed , frigid Typic Humaquept). In comparison with total NO3- -N disappeara nce over the same period, losses of NO3- -N due to denitrification wer e relatively small. Thus, significant portions of NO3- -N losses were probably due to leaching both on clay and sandy clay loam soils.