Sulfur application enhances secretion of organic acids by soybean roots and solubilization of phosphorus in rhizosphere

ABSTRACT

Nitrogen (N) fixation by rhizobia plays an important role in N assimilation of soybean (Glycine max). Nitrogenase, the enzyme responsible for N fixation in rhizobia, contains a large amount of sulfur (S) molecules and requires phosphorus (P) in adenosine triphosphate. Thus, S and P are essential for plant growth and N fixation by rhizobia. However, P in soils forms insoluble PO₄^{3 –} salts with metals. Plant roots solubilize these PO₄^3- salts by secreting organic acids and absorb solubilized PO₄^3-. In this study, the effects of S application on P solubilization by soybean roots and the involvement of rhizobia were evaluated. Soybean plants inoculated with Bradyrhizobium diazoefficiens USDA110 were grown in vermiculite, and Ca₃(PO₄)₂ was added as the sole P source. A hydroponic solution supplemented with three S concentrations (0, 20, and 1000 µM) was applied, and plants were grown for 5 weeks. Root fresh weight, the soil-plant analysis development (SPAD) values, and shoot and root S concentrations increased significantly with S application, but root nodule numbers and nitrogenase activity were not significantly affected. Organic acid secretion by roots was drastically increased by S application, and shoot P concentration and content also increased significantly. These results indicated that S application can promote P solubilization by increasing the secretion of organic acids by roots and increase in N fixation is not involved in this promotion at least in early vegetative growth period.

KEY WORDS:

• Nitrogen fixation
• organic acid
• phosphorus
• soybean
• sulfur

Disclosure statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Additional information

Funding

This study was supported by the Japan Science and Technology Agency, Strategic International Research Cooperative Program, EIG CONCERT-Japan, and Grants-in-Aid for Scientific Research, Fostering Joint International Research (B) (20KK0136) from the Japan Society for the Promotion of Science to N. O-O;Strategic International Collaborative Research Program [Concert-Japan];