Survival of the indigenous population of rhizobium leguminosarum biovar trifolii in soil spiked with Cd, Zn, Cu and Ni salts

Solutions of Zn, Cd, Cu or Ni salts were added to a relatively uncontaminated soil, previously treated with farmyard manure (FYM), at six different concentrations for each metal. A further experiment compared the survival of the indigenous population of Rhizobium leguminosarum bv. trifolii in the same soil treated previously with FYM or metal-contaminated sewage sludge. No decline in rhizobial numbers occurred in either experiment after 2 months' exposure. After 18 months, the number of rhizobia in the control soils had declined by 90%. No rhizobia survived at Zn and Cd concentrations ≥385 and ≥7.1 μg g −1 soil, respectively. At these Zn and Cd concentrations, which are ca 1.3 and 2.4 times the U.K. limits, respectively, there was no N 2-fixation. At the greatest Cu concentration of 225 μg g −1 soil, which is 1.7 times the U.K. limit, there was a 99% decrease in rhizobial numbers compared to the control soil, but sufficient survived to give effective nodules on clover plants. Nickel had very little effect on the rhizobial population, even at the greatest concentration of 54 μg g −1 soil, ca 75% of the U.K. limit. The order of decreasing toxicity to rhizobia in the soil, and hence the indirect effect on N 2-fixation, was Cd > Zn > Cu. The soil treated with metal-contaminated sewage sludge contained 230 rhizobial cells g −1' soil after 18 months exposure. These could nodulate white clover, but were ineffective in N 2-fixation. Zinc and Cu concentrations in this soil were close to the U.K. limits, whereas Ni and Cd concentrations were half and 4 times the U.K. limits, respectively. Clover plants grown in soil spiked with Zn or Cd to give concentrations of ≥385 and ≥7.1 μg g −1 soil, respectively, or in the sludge-treated soil, after 18 months exposure were chlorotic and stunted, and contained less N than the control plants grown in FYM-treated soil. These effects were not caused by phytotoxicity, because they were overcome by the addition of inorganic nitrogen fertilizer. Rather, they resulted from a direct toxic effect of these metals on the indigenous effective rhizobial population. It took 18 months exposure of the moist soil at 20°C, in the absence of clover host plants, before the toxic effect became apparent.