Magnesium-induced changes in soil chemical and microbial properties facilitate improvements in sugarcane yield and fertilizer use efficiency

Soil magnesium (Mg) deficiency limits sugarcane yield and quality, underscoring the need for optimized Mg fertilization. A field experiment was conducted to compare the effects of three types of Mg fertilizer (magnesium sulfate, silicon-magnesium, and boron-magnesium-zinc) on sugarcane yield, nutrient utilization, and the microbial community. Compared with the optimized fertilization treatment (OPT), all three Mg application treatments increased the 2-year average sugarcane yield, with the greatest increase observed in the OPT plus boron-magnesium-zinc fertilizer treatment (OPT+BMgZn). The use efficiencies of phosphorus, potassium, and Mg fertilizer were significantly greater in the OPT+BMgZn treatment than in OPT. Compared with the OPT, the bacterial Chao1 index increased in the Mg application treatments, whereas the fungal Chao1 and Shannon indices were unaffected (P < 0.05). Moreover, compared with OPT, the relative abundances of the plant growth-promoting microorganisms Bradyrhizobium, Acidothermus, and Conexibacter increased in the OPT+BMgZn treatment, whereas those of the pathogenic microorganisms Fusarium and Vanrija decreased in all Mg application treatments. Functional prediction revealed enrichment of predatory or exoparasitic functions in the bacterial community and reduced fungal pathotrophs in all Mg application treatments compared with OPT. Correlation analysis revealed that sugarcane yield and nutrient uptake were positively correlated with exchangeable Mg2+; bacterial community composition and function were primarily affected by available phosphorus and pH, whereas fungal composition and function were mainly influenced by available potassium and exchangeable Mg2+. Overall, integrating sugarcane yield, nutrient use efficiency, and microbial community structure and function identified OPT+BMgZn as the optimal Mg fertilization strategy for acidic red soils in South China.