Fluorine and white clover: Assessing fluorine's impact on Rhizobium leguminosarum

The soil fluorine (F) concentration in New Zealand agricultural soils has increased with time as a direct result of the widespread application of phosphate fertilizer to land. Elevated soil F concentrations may potentially harm soil microorganisms, which are important for nutrient cycling and soil formation. Rhizobium leguminosarum is a N₂ -fixing soil bacterium that is a fundamental component in New Zealand legume-based pastoral farming. Any impact of F on Rhizobium leguminosarum would have an adverse effect on New Zealand pasture production. In this study, F toxicity to Rhizobium leguminosarum was examined as a first step to develop F guideline values for New Zealand agricultural soils. Bottle-based experiments were conducted to examine the effect of the F^- ion on Rhizobium-white clover (Trifolium repens L.) symbiosis by observing nodule morphology and growth. Results indicate that the F^- concentration that causes 10% inhibition of Rhizobium respiration (IC₁₀ ) for F^- toxicity to Rhizobium leguminosarum was >100 mg F^- L^-1 . Significant morphological changes occurred when Rhizobium was exposed to F concentrations of 500 and 1000 mg L^-1 . Both light and transmission electron micrographs confirmed that the Rhizobium leguminosarum-white clover interaction was not influenced by F^- concentrations >100 mg L^-1 . The toxic F^- concentration for Rhizobium leguminosarum determined in this study is orders of magnitude higher than the F^- concentration in New Zealand agriculture soils under "normal conditions." There appears to be no indication of imminent risk of soil F to Rhizobium leguminosarum.