Enhancing zinc and iron bioavailability through crop rotation and organic farming: Insights from a long-term study

Agronomic biofortification of Zinc (Zn) and iron (Fe) in cereal grains is a critical strategy to address human deficiencies in these micronutrients. Our study examined the effects of crop rotation and farming system on Zn and Fe bioavailability in wheat grains, using data from 2011 to 2020 from the Glenlea Long-term Crop Rotation Study, Canada’s longest running study on organic farming. Two crop rotations, continuous annual (spring wheat, flax, oat, soybean) and annual-perennial (spring wheat, flax, alfalfa, alfalfa), managed both organically and conventionally, were fully phased. Compared to conventional farming, organic farming significantly increased grain Zn concentration in six out of ten years, with no significant effect on grain Fe. Additionally, grain Zn was also significantly higher in the annual-perennial rotation compared to the annual rotation in seven of the ten years. A significant interactive effect was observed for grain phytate, which was approximately 35 % lower in the annual-perennial rotation under organic farming than in other treatments. Consequently, the diversified rotation under organic farming significantly reduced the phytate/Zn and phytate/Fe molar ratios, indicating enhanced micronutrient bioavailability for human consumption. However, these effects were strongly associated with reduced phosphorus (P) availability in organic systems, particularly following alfalfa, which limited grain yields. Livestock manure-based compost application improved grain yield and nutrient uptake, but reduced Zn/Fe bioavailability due to increased phytate accumulation. This study demonstrates that diversified crop rotation and organic farming could improve micronutrient bioavailability, but at a cost of lower grain production tied to reduced P availability.