Metal tolerance of isolates of Rhizobium leguminosarum biovar Trifolii from soil contaminated by past applications of sewage sludge

Abstract

We tested the tolerance to Cu, Ni, Cd and Zn of two isolates of Rhizobium leguminosarum biovar trifolii isolated from root nodules of clover plants grown in metal-contaminated sewage sludge treated plots (S-isolates) at Woburn Experimental Farm, and two isolates from root nodules of clover plants grown in uncontaminated control plots treated with farmyard manure (F-isolates). Survival of the isolates was compared in solutions containing different concentrations of Cu, Ni, Cd or Zn. The S-isolates from metal-contaminated soil were tolerant to larger concentrations of Cu, Ni, Cd and Zn compared to the F-isolates from uncontaminated control soil. The F-isolates were killed at concentrations of 0.002 μg ml −1 Cu, 0.2 μg ml −1 Cd 0.8 μg ml −1 Ni and 0.8 μg, ml −1 Zn within 72h, whilst the S-isolates survived, albeit in reduced numbers, at concentrations of 0.01 μg ml −1 Cu and 1.0 μg ml −1 Zn, but were killed by 1.0 μg ml −1 Ni and 0.8 μg ml −1 Cd within 72 h. A particularly strong increase in tolerance to Zn was shown by the S-isolates compared to other metals at the same concentrations. Thus, the order of decreasing toxicity in solution to the two F-isolates was Cu > Cd > Zn = Ni, but for the two S-isolates it was Cu > Cd > Ni > Zn. The S-isolates have multiple metal tolerance that enables them to survive in metal-contaminated soil, but they have lost their ability to fix nitrogen with Trifolium repens L. The F-isolates can fix nitrogen, but these do not survive in metal-contaminated soil because they lack tolerance to these metals.