The Role of Magnesium Fertilizer in Shaping Soil Nitrogen Cycle and Microbial Community in Soybean Rhizosphere

Soybeans are an important grain and oil crop in China, and ensuring high yield and quality has long been a significant area of research. This study investigates the effects of different levels of magnesium (Mg) fertilizer on soybean yield, nutrient absorption, soil nitrogen cycling, and the composition of microbial communities, aiming to provide a theoretical foundation and scientific basis for improving soybean productivity and quality. Our field experiments included two influencing factors: MgO application levels and soybean cultivars. The specific treatments were as follows: MgO levels (Mg0, 0 kg/hm2; Mg15, 15 kg/hm2; Mg45, kg/hm2) and soybean cultivars (P1: HN0703; P2: SD085). The results showed that, compared to the Mg0 treatment, the Mg45 treatment significantly (P < 0.05) increased yield, number of effective seeds, and 1000-grain weight by 66.08%-236.38%, 36.03%-53.75%, and 18.37%-38.20%, respectively. Moreover, in cultivar P2, the Mg45 treatment significantly (P < 0.05) increased the nitrogen (N) content in leaves, pods, and grains by 13.35%, 34.71%, and 54.93%, respectively, compared to the Mg0 level. In addition, Mg fertilizer application affected soil fertility at different depths, particularly influencing the nitrogen cycle. Compared to the Mg0 treatment, the NO3-N content increased by 88.04%-96.38% under the Mg45 treatment, while the NO3-N content decreased by 23.28%-54.23%. Furthermore, urease activity increased by 12.77%-15.90%, while the activities of nitrate reductase (NR) and nitrite reductase (NiRs) decreased by 17.78%-20.69% and 38.67%-56.25%, respectively. Moreover, the soil N cycling genes are regulated by the application of Mg fertilizer, resulting in a decrease in genes related to N fixation and nitrification with the use of Mg fertilizer. The application of Mg fertilizer also altered the community composition of rhizosphere soil microorganisms and N cycling genes, particularly affecting the relative abundance of functional microbial taxa such as Nitrospirota and Gemmatimonadota, thereby influencing the N-fixing capacity of the soybean rhizosphere. Overall, our results demonstrate that the rational application of Mg fertilizer can enhance soybean yield and quality, improve soil fertility, and reshape microbial community composition, ultimately affecting soil N cycle in the soybean rhizosphere.