Toxicity of organic compounds to the indigenous population of Rhizobium leguminosarum biovar Trifolii in soil

Abstract

A sandy loam was amended individually with 12 organic compounds at the largest concentrations found in sewage sludge to determine their effects on the indigenous population of Rhizobium leguminosarum biovar trifolii (rhizobia). After 5 weeks, only pentachlorophenol (PCP) at 200 mg kg −1 soil decreased the rhizobial population compared to the water and methanol controls (i.e. by >98% from 240,000 and 120,000 cells g −1 soil, respectively, to 2000 cells g −1 soil). After 6 months no rhizobia could be detected in the PCP-treated soil, whereas both controls contained 120,000 cells g −1 soil. In a separate dose-response experiment, PCP was added to soils at nine different concentrations and the rhizobial population enumerated after 6 months. The highest no observed adverse effect concentration (HNOAEC) for this species was 50 mg PCP kg −1 soil and the lowest observed adverse effect concentration (LOAEC) was 75 mg PCP kg −1 soil. At 75 mg PCP kg −1 soil, rhizobial numbers decreased by >99%.