Optimizing the fertilizer N rates at different irrigation levels for optimum yield of wheat and corn at reduced nitrate leaching losses

Optimum irrigation and fertilizer nitrogen (N) rates are important to improve crop yield at reduced environmental risks in the form of NO3-N leaching losses, without any financial loss. The study aimed to investigate the impact of rational irrigation and nitrogen management on wheat and maize crop yield vs. NO3-N leaching losses, with field experiments conducted at the experimental station, University of Agriculture Faisalabad, Pakistan, for two years, with wheat-fallow-corn seasons each year. Suction lysimeter were installed for collection of leachates while soil water balance was computed using the HYDRUS-1D model. We explored the various management strategies, including three irrigation and N levels (sub-optimal, optimal, and supra-optimal, referred to as I1, I2, and I3 for irrigation, and N1, N2, and N3 for nitrogen, respectively) for wheat and maize crops. The three irrigation levels were 325, 400, and 475 mm for wheat, and 375, 525, and 675 mm for maize crops. The three N-levels were 100, 130, and 160 kg ha-1 for wheat, and 220, 270, and 320 kg ha-1 for maize. The results indicated that increasing the irrigation and nitrogen levels significantly improved the growth and yield of both crops during both seasons. The highest grain yield of wheat (4.0 t ha-1) and maize (7.8 t ha-1) was observed with I3N3; however, I2N3 showed statistically no difference in yield, while showing significantly reduced (28.6%) annual NO3-N leaching losses of 23.8 kg ha-1, and the highest financial benefits of 780$. Sub-optimal levels of irrigation and N, though reduced the NO3-N leaching losses, caused significant yield losses, generally unacceptable to the farmers. Irrigation water use efficiency (WUEi) also improved by 12% in wheat and 20% in maize under I2 than that of the I3 level. Besides, considering economic profit, the highest value cost ratio (1.64 and 2.04 in wheat and maize, respectively) was achieved under the I2N3 treatment, as opposed to the other treatments. Based on comprehensive analysis, the I2N3 treatment is recommended for sustainable yield and minimal environmental risk in the wheat-maize cropping system. Moreover, it was observed that the rainy fallow period contributes 14.0-31.5% of the total NO3-N leaching losses. Further investigation is needed to minimize NO3-N leaching losses, especially during the rainy fallow period, by early maize sowing and increasing the efficiency of N fertilizer (such as fertilizer coating) under the flood irrigation system, to achieve the potential goals of sustainable productivity and environmental security.