Nitrate leaching in the semiarid prairie: Effect of cropping frequency, crop type, and fertilizer after 37 years

A high NO 3 concentration in drinking water can be a health hazard. Further, leached N represents an economic loss to the producer. Although NO3 leaching to ground water would be minimal on the semiarid prairies, leaching can occur especially where frequent summer fallowing is practiced. We used a crop rotation study, initiated in 1967 on a medium-textured Orthic Brown Chemozem, at Swift Current, Saskatchewan, to determine the influences of fallow frequency, crop types, and fertilizer on nitrate leaching after 37 yr. Nitrate distribution was measured to 4.5 m depth by 0.3-m increments, in 10 cropping systems in fall 2003. We deduced that some NO3 leached beyond the rooting depth (1.2 m) of spring wheat (Triticum aetivium L.), especially under a fallow-wheat rotation receiving N and P fertilizer. The amounts of NO3 leached tended to be greater and to be located deeper in the soil profile as fallow frequency increased (e.g., fallow-wheat > fallow-wheat-wheat > continuous wheat, all receiving N and P fertilizer based on soil test). However, in this semiarid environment, NO 3 leaching was not great, being highest under fallow-wheat (N + P) (180 kg N ha(-1) leached in 37 yr). In fallow-containing systems inadequate fertilizer N or P resulted in reduced crop growth and N uptake leading to a tendency for greater leaching of the NO3 (about 145 kg N ha(-1) in 37 yr) mineralized during the fallow period than when such a system received N and P based on soil tests (about 66 kg N ha(-1) leached in 37 yr). In continuously cropped treatments there was little evidence of leaching. Replacing wheat grown on fallow with the shallow-rooted flax (Linum usitatissimum L.) in a fallow-wheat-wheat (N + P) rotation resulted in greater NO 3 leaching in the flax system (156 vs. 66 kg N ha(-1)) due to less N uptake by flax. In contrast, when the wheat grown on fallow was replaced with fall rye (Secale cereale L.) there was no leaching, perhaps because the fallow period was much shorter (12 mo compared with 20 mo for spring wheat) and also because the fall-seeded crop used soil NO3 in the fall and early spring reducing opportunities for leaching.