Water use efficiency and water and nitrate distribution in soil in the semiarid prairie: Effect of crop type over 21 years

In the semiarid prairie, available water is the most limiting and nitrogen the second most limiting factor influencing crop production. Although numerous studies have been conducted on the effect of management practices on water use efficiency (WUE), most have concentrated on monoculture wheat, the major crop grown in the region. Even those studies dealing with other crop types have mostly been short-term in nature. But precipitation is so variable in amount and distribution that such an assessment is best conducted in long-term experiments. We used the results of a 21-yr experiment, conducted in the Brown soil zone at Swift Current, Saskatchewan, to determine the influence of crop type on WUE, and used the distribution of water and NO3-N in the soil, and N uptake by the crop to assist in interpreting these results. Four crop rotations were compared: summer fallow-wheat-wheat (F-W-W), F-flax-W (F-Flx-W), continuous wheat (Cont W) and wheat-lentil (W-Lent). All received N and P fertilizer based on soil test. In the following presentation, the rotation phase shown in parentheses was the phase referred to. We used water and NO3-N measured in consecutive 0.3-m depth segments to 1.2 m in the soil, taken just prior to seeding and after harvest, and precipitation, to make this assessment. About 10 mm more water was conserved in the F-W-W rotation than in the F-Flx-W system during the 21-mo summer fallow period, and most of this difference in water was located in the 0.3- to 0.9-m depth. Soil water in the profile was 14 mm greater following flax harvest than following wheat harvest (mostly located in 0.6- to 1.2-m depth), because flax produces less biomass and has shorter roots than wheat. At harvest, wheat dried the soil to near the wilting point (154 mm), but flax and lentil left about 10 mm of available water in the profile (mostly in the 0.6- to 1.2-m depth), suggesting shallower rooting depths. Over the 9-mo winter period about 58 mm of water was stored in the soil after wheat and 41 mm after flax. Wheat stubble conserved more overwinter water than flax stubble because of its taller height. Lentil, with its much shorter stubble, conserved about 7 mm less water than wheat during winter. Because flax produces much less biomass and withdraws less N from the soil than wheat, it left more NO3-N in the soil (27 kg ha-1 more at seeding and 23 kg ha-1 more at harvest); most of the extra NO3 was in the 0.3- to 1.2-m depth reflecting flax's shallower roots. During the 9-mo overwinter period, 16 kg ha-1 of NO3-N was mineralized following wheat and 33 kg ha-1 following flax. In the spring, Cont W and stubble wheat in F-W-(W) had about 50% as much soil NO3-N as the W-Lent rotation, reflecting the cumulative benefits of N2 fixation by the pulse crop over the years . By harvest, soil NO3-N under (W)-Lent > W-(Lent) > F-Flx-(W) > F-W-(W) > Cont W. The excess NO3-N in the (W)-Lent compared to W-(Lent) was located in the 0- to 0.6-m depth suggesting excessive fertilizer application to the wheat phase of this rotation and implying a need for agronomists to reassess the criteria used for N recommendations for rotations containing pulse crops. Lentil used as much water as wheat even though its biomass was much less. WUE for wheat grown on summer fallow averaged 8.11 kg ha -1 mm-1, and for wheat grown on stubble 6.9 kg ha-1 mm-1. WUE for wheat was also higher when it followed flax than when it followed wheat. The WUE of flax and lentil averaged 50% and 64%, respectively, of wheat following wheat. A more meaningful way of expressing the efficiency of water use is as precipitation required per unit of produce from the complete cropping system (PUE). The PUE increased with cropping intensity on a yield basis (kg ha-1 mm-1): Cont W (4.6) > W-Lent (4.2) > F-W-W (4.1) > F-Flx-W (2.9) (opposite response to WUE). When PUE was calculated on a dollars produced per rotation basis ($ ha-1 mm-1): W-Lent (1.0) was higher than the other three rotations (0.6 to 0.7). Key words: Soil water, soil nitrate, lentil, flax, wheat, crop rotation, precipitation use efficiency