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Effect of Rhizobial Extracellular Polysaccharide on Solution Phosphate Levels of Acid Soils¹

ABSTRACT

Previous work has shown a positive correlation between the ability of a strain to colonize acidic agar and its production of extracellular polysaccharide (EPS) in solution culture. This paper examines the relationship between EPS production and phosphate levels in solution, determining: (i) the interaction of EPS with an oxide (gibbsite) surface, (ii) the effect of EPS on phosphate sorption, and (iii) the effect of EPS on previously sorbed phosphate. Rhizobial EPS was readily adsorbed onto gibbsite. When EPS was present before the addition of phosphate, both the quantity of EPS and its source significantly influenced the amount of phosphate sorbed. Addition of standard amounts of EPS from different strains to a gibbsite suspension with previously sorbed phosphate produced solution phosphate levels which differed significantly. The pH of the EPS added to the gibbsite-phosphate suspension was also varied; added EPS decreased solution phosphate levels if the EPS was more acidic than the gibbsite suspension, and increased solution phosphate levels if it was more alkaline. Rhizobium in an acid soil may be able to influence the phosphate level of their immediate micro-environment through the excretion of extracellular polysaccharide at an appropriate pH.

¹ Partially supported by a grant from the US Agency for International Development and subcontract from Univ. of Hawaii/NifTAL project.

² Former Graduate Student and Professor, respectively; Dep. of Land, Air and Water Resources, Univ. of California, Davis, CA.

Received for publication July 5, 1984. Accepted for publication October 1, 1984.