EFFECTIVE RHIZOBIUM-LEGUMINOSARUM BIOVAR TRIFOLII PRESENT IN 5 SOILS CONTAMINATED WITH HEAVY-METALS FROM LONG-TERM APPLICATIONS OF SEWAGE-SLUDGE OR METAL MINE SPOIL

The presence of effective strains of Rhizobium leguminosarum biovar trifolii was determined in a range of soils contaminated with heavy metals resulting from long-term applications of sewage sludge and also metal mine spoil, using an in vitro plant inoculation technique with Trifolium repens. Effective Rhizobium were always present in soil from sites where the host plant was established, irrespective of the amount of metal contamination. With no indigenous clover present, effective Rhizobium were also isolated from soils with concentrations of metals which exceeded the maximum permissible concentrations for sludge-treated soil. This may indicate Rhizobium strains had developed tolerance to elevated metal conditions without losing the ability to fix N2 with white clover. However, elevated concentrations of Cd in soil apparently determined the absence of Rhizobium at one arable site as only half the soil samples taken nodulated white clover whereas increased concentrations of Zn and Cu had no effect in a soil low in Cd. Critical amounts of metals which resulted in a complete absence of Rhizobium in soil and nodulation could not be determined across the range of metal concentrations measured at each site. Therefore, detrimental effects of metals on Rhizobium were apparently site specific and there was no evidence that increased metal concentrations in soil selected for tolerant strains which were ineffective in N2-fixation with white clover.