Comparative effects of grain lentil–wheat and monoculture wheat on crop production, N economy and N fertility in a Brown Chernozem

Low commodity prices and a desire by producers on the Canadian Prairies to reduce fertilizer-N inputs have resulted in a marked increase in grain lentil (Lens culinaris medikus) production. Many producers grow lentil in rotation with spring wheat (Triticum aestivum L.). A 12-yr study carried out at Swift Current, Saskatchewan, on an Orthic Brown Chernozemic silt loam was used to compare the N economy of four monoculture wheat rotations, of various rotation lengths and levels of N fertilization, with that of a wheat–lentil (W–Len) rotation. Except for continuous wheat (Cont W) receiving mainly P fertilizer, all systems received N and P on the basis of soil tests. Neither grain nor straw yield of the associated wheat crop was influenced by lentil in the rotation; but grain- and straw-N concentrations of the wheat in W–Len were increased compared with those of monoculture wheat. Average N content of straw was greatest for grain lentil and for wheat grown on fallow (F) (14.2 kg ha −1 yr −1 ), followed by wheat in W–Len (11.8 kg ha −1 yr −1 ) and by stubble-wheat of well-fertilized monoculture systems (F–W–W and Cont W (9.5 kg ha −1 yr −1 )), and lowest for Cont W receiving mainly P (5.7 kg ha −1 yr −1 ). Nitrate N in the root zone (0–90 cm) in spring and at harvest was greatest under F–W, followed by W–Len and then by F–W–W and Cont W receiving N and P, and lowest for Cont W receiving mainly P. In the last 5 or 6 yr of study, there was a marked increase in the amount of soil-NO 3 N found in the root zone under the W–Len rotation. This was accompanied by a similar increase in the apparent net N mineralized (estimated by N balance) during the growing season. Concomitantly, there was a gradual reduction in the fertilizer-N requirement for W–Len, indicating a cumulative enhancement of the N-supplying power of the soil, although estimates of the initial potential rate of N mineralization on samples taken in 1990 did not confirm this trend. We concluded that soil-testing laboratories may need to adjust fertilizer-N recommendations downward for producers that regularly use a 2-yr W–Len rotation. There was less NO 3 N leached below the root zone of W–Len, probably because there was greater synchrony of N uptake in W–Len than in Cont W (N + P); this augurs well for the use of the W–Len rotation for sustainability.