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

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 Ca-3(PO4)(2) was added as the sole P source. A hydroponic solution supplemented with three S concentrations (0, 20, and 1000 mu M) 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.

Automated summary

This study evaluated the effects of sulfur (S) application on phosphorus (P) solubilization by soybean roots and the involvement of rhizobia. The results indicated that S application can promote P solubilization by increasing the secretion of organic acids by roots, but the increase in nitrogen fixation is not involved in this promotion at least in the early vegetative growth period.