No abstract

The application of chemical fertilizers is costly and gradually lead to the environmental problems. Natural farming is becoming an increasingly important

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.