Global analysis of nitrogen fertilization effects on grain zinc and iron of major cereal crops.

Abstract

Human zinc (Zn) and iron (Fe) deficiencies can partly be alleviated by enhancing cereal concentrations of these micronutrients. Soil nitrogen (N) levels codetermine cereal grain yields and Zn and Fe nutrition of plants and grains. Grain Zn and Fe concentrations have been reported to be affected by both yield dilution and enhanced acquisition and grain allocation of Zn and Fe. A global meta-analysis of 100 publications concerning wheat, maize, and rice providing 785 records of Zn and 506 records of Fe allowed us to assess their relative importance and quantify the concentrations and bioavailability of Zn and Fe in major cereal grains over a wide range of N fertilization levels. Compared with the no N controls, N application significantly increased grain Zn and Fe concentrations in all crops except maize Zn. The increase in grain protein concentration correlated positively with the increases in Zn and Fe concentrations in all cereals except Zn in maize. In rice, the grain Zn and Fe concentration increase was independent of the rate of N applied. Grain concentrations of Zn and Fe in wheat and Fe in maize were positively correlated with N rate but were only higher than those in the controls above 40–60 kg N ha−1. At lower N rates, the dilution effect was thus stronger than the enhancement effect. N supply had a larger effect on Zn and Fe concentrations in loamy textured soils or at lower soil available N and phosphorus (P), or higher soil organic matter and available Zn contents or with P and Zn fertilization, but the effect sizes differed among crops. Reductions in phytic acid concentration after N fertilization occurred in wheat, potentially improving micronutrient bioavailability. Thus, our findings indicate that N fertilization could be managed in ways that simultaneously support high grain yields and enhance nutritional quality for major cereals.