Objective Magnesium (Mg) is an essential mineral nutrient for plant growth. This study was aimed to investigate the growth and dynamic changes in three-dimensional root system architecture traits of soybean seedlings under different Mg concentrations.

Method The phosphorus-efficient soybean genotype ‘Yuechun 03-3’ was selected as the research object, and Mg concentrations were set in hydroponics as 0, 262.5, 525.0, 787.5 and 1 050.0 μmol/L to explore the effect of Mg nutrition on the growth and development of soybean seedlings. Furthermore, the optimized three-dimensional root quantification system was used to analyze the dynamic quantitative changes in the root system architecture traits of soybeans under control Mg treatment (525.0 μmol/L) and Mg deficiency treatment (0 μmol/L).

Result Compared with the control of 525.0 μmol/L Mg, the soybean shoot dry mass, root-to-shoot ratio, SPAD of old leaves, total root length, and total root surface area under the 0 μmol/L Mg deficiency treatment decreased by 89.04%, 48.67%, 51.42%, 93.36% and 94.31% respectively. Under other three Mg concentration conditions, the growth of soybeans showed relatively small differences compared with the control. The results of three-dimensional root system quantification found that compared with the control Mg treatment, Mg deficiency treatment significantly reduced the total root length, total root surface area, root centroid, number of root tips, convex hull volume, maximum root width, minimum root width, maximum root depth and maximum width/maximum depth of soybean roots with the extension of treatment time. However, it affected root solidity, bushiness and root volume distribution feebly.

Conclusion This study elucidates the wide adaptability range of soybeans to external Mg availability. By utilizing optimized three-dimensional root reconstruction techniques, it is found that Mg deficiency significantly reduces the total root length, number of root tips, root centroid and maximum root width of soybeans, while it does not significantly affect the dynamic changes in root solidity, bushiness and volume distribution. These findings have certain implications for rational use of Mg fertilizer and Mg nutrition diagnosis in soybeans.