EFFECTS OF A WHEAT–SUMMER-FALLOW ROTATION ON SUBSOIL NITRATE

Abstract

Effects of length of cultivation, soil texture, and the presence of a water table on soil nitrogen was investigated in semiarid southwestern Saskatchewan. Cores from as deep as 8 m were taken in 1974 from fields that had been sampled in 1939 when they were broken, and in 1953 after they had been cropped for 14 yr to a wheat–summer-fallow rotation. Virgin soils were almost devoid of nitrate. After cropping for 14–18 yr, nitrate accumulated in the subsoil. After 35 yr, nitrate was still accumulating in the Sceptre clay, but the nitrate bulge had disappeared from the Wood Mountain loam leaving the nitrate uniformly distributed in the subsoil. Only 43% of the nitrate that was present in the Wood Mountain profile after 14 yr of cultivation was still present after 35 yr. This loss could be due to subsoil nitrate being leached beyond the sampling depth, or the result of periodic bidirectional movement of nitrate. Nitrate accumulated at two depths in the Sceptre clay. The accumulation at the lower depth was ascribed to precipitation entering the clay through large cracks before wetting the subsoil, and the upper accumulation to infiltration through the soil surface. In the first 14 yr of cultivation, total nitrogen was lost from the Wood Mountain loam at an average rate of 1.8%/yr (relative to the virgin soil); in the following 21 yr the rate was 0.65%/yr.