A 33-year study comparing maize (Zea mays) in forage legume or soybean (Glycine max) rotations to maize monocultures: Nitrogen fertilizer replacement values and maize yields

Context or problem

The benefits of legume-cereal rotations relative to cereal monocultures are generally known, but quantification of the rotational effect in terms of grain yield and mineral nitrogen (N) fertilizer replacement value (NFRV) is lacking.

Objective or research question

The objectives from a 33-year field experiment were to (1) compare the effects of mineral N fertilization on the grain yield of maize within maize monoculture, maize-annual legume (alfalfa/red clover), and maize-soybean rotations; and (2) determine the rotational effect of annual forage legume and soybean in terms of NFRV under various crop yield-N response patterns.

Methods

A long-term field experiment was established in Ottawa, Ontario, Canada to examine the effects on maize yield of mineral N fertilizer and crop rotation with either a forage legume (alfalfa or red clover) or soybean. Nitrogen fertilizer was applied in the maize year of the rotation. Years were categorized into low, intermediate, and high N response groups for maize yield. The rotational effect was estimated with the NFRV method. The rotational effect expressed as NFRV was also quantified with a range of synthetic N rates under different yield-N response conditions.

Results

Compared with maize monoculture, the rotational effect (Mean ± Standard Error of Mean) from soybean or annual forage legume, on grain yield of maize without synthetic N fertilization, was 1880 ( ± 630), 2280 ( ± 290) and 2810 ( ± 360) kg ha⁻¹, and 3420 ( ± 540), 4620 ( ± 420) and 5690 ( ± 300) kg ha⁻¹ under low, intermediate and high yield-N response conditions, respectively. With an addition of 50 kg N ha⁻¹ the rotational effect of soybean on maize yield significantly increased only under intermediate yield-N response condition (3350 ± 400 kg ha⁻¹). With further addition of synthetic N at either 100 or 150 kg N ha⁻¹, the rotational effect of soybean decreased to approximately 1340 ( ± 130) kg ha⁻¹. The rotational effect of annual forage legume on grain yield of maize supplied with 50 kg N ha⁻¹ was similar to that of zero-N application under low, intermediate and high yield-N response conditions, but with further additions of synthetic fertilizers at 100 or 150 kg N ha⁻¹ the effect was significantly reduced under the high yield-N response condition and similar under the intermediate yield-N response conditions. Under the low yield-N response condition, the rotational effect of annual forage legume on maize yield was greater with 150 kg N ha⁻¹ (3670 ± 820 kg ha⁻¹) than that with 100 kg N ha⁻¹ (1960 ± 610 kg ha⁻¹). In all rotation systems, NFRV decreased with increasing synthetic N application from 0 to 150 kg N ha⁻¹. Under the low, intermediate and high yield-N response conditions, NFRV varied from −106 to 31 kg N ha⁻¹, from −3 to 93 kg N ha⁻¹, and from 52 to 62 kg N ha⁻¹, with soybean; and from −48 to 77 kg N ha⁻¹, from 29 to 191 kg N ha⁻¹, and from 103 to 137 kg N ha⁻¹, with annual forage legume, respectively.

Conclusions

Based on the 33-year long-term study, we documented that the maize yield-N response curves under low, intermediate, and high N response conditions in different rotation systems. The effect of an annual forage legume on maize yields over maize grown in monoculture was greater than that of soybean. The magnitude of the effect of annual forage legume and soybean rotations on maize yield depended on the application rate of synthetic N fertilizer and the yield-N response pattern. Under more stressful yield-N response conditions, rotational effects of annual forage legumes or soybean were more pronounced under suboptimal synthetic N fertilizer application conditions.

Implications or significance

Our findings indicated the importance of considering leguminous crops, synthetic N application rates, and maize yield-N response curve/weather conditions for improving the climate resilience of maize production, while simultaneously meeting the demand for more sustainable nutrient management of soil resources.