Maize (Zea mays L.) plays an important role in food production in the world. Maintaining adequate micronutrient concentrations in maize grains is of great importance for food quality and human health. The aim of this study was to investigate the spatial distribution and bioavailability of micronutrients in soils and maize grains in major crop production areas in China, where knowledge is limited, and further to identify the main environmental factors affecting micronutrient concentrations in maize grains.

Totally, 995 maize grain and soil samples from famers’ field were collected in 2017 from 202 counties in four major maize agro-climatic regions of China. Information on grain yield and cultivation management were also collected for each field. Concentrations of Fe, Zn, Mn and Cu in grain and soil properties including pH, total N, Olsen-P, available K, organic matter, and concentrations of DTPA-extractable Fe, Zn, Mn and Cu Were determined.

Concentrations of DTPA-extractable micronutrients varied greatly with maize production areas, being higher for soils in southwest of China which were acid soils with high organic matter content. Analyses of variance across different areas revealed significant genotypic and environmental effects for grain micronutrient concentrations and grain yield. Average micronutrient concentrations in maize grain were 17.3 mg kg^-1 (range 5.0-51.7) for Fe, 17.4 mg kg^-1 (range 6.2-35.1) for Zn, 1.49 mg kg^-1 (range 0.07-4.66) for Mn and 4.86 mg kg^-1 (range 1.90-10.50) for Cu, respectively. Compared to the biofortification target values of 60 mg kg^-1 for Fe and 38 mg kg^-1 for Zn, all samples were considered inadequate to meet dietary requirement for human. Grain Fe and Zn concentrations correlated significantly and negatively with grain yield, and positively with growing degree days (GDD) and soil DTPA-Fe and DTPA-Zn, respectively. The significantly positive correlation between Zn and Fe concentrations suggest good prospects of simultaneous improvement for both micronutrients.

Our study provided the current status of bioavailability of micronutrients in soil and maize grains in China. Quantifying the effect of environmental factors on concentrations and bioavailability of micronutrients in maize grains should be addressed in future research.