A study of the impacts of Zn and Cu on two rhizobial species in soils of a long-term field experiment

Two agriculturally important species of rhizobia, Rhizobium leguminosarum biovar viciae (pea rhizobia) and R. leguminosarum bv. trifolii (white clover rhizobia), were enumerated in soils of a long-term field experiment to which sewage sludges contaminated predominantly with Zn or Cu, or Zn plus Cu, were added in the past. In addition to total soil Zn and Cu concentrations, soil pore water soluble Zn and free Zn2+, and soluble Cu concentrations are reported. Pea and white clover rhizobia were greatly reduced in soils containing greater than or equal to 200 mg Zn kg(-1), and soil pore water soluble Zn and free Zn2+ concentrations greater than or equal to 7 and greater than or equal to 3 mg l(-1), respectively, in soils of pH 5.9-6. Copper also reduced rhizobial numbers, but only at high total soil concentrations (> 250 mg kg(-1)) and not to the same extent as Zn. Yields of field grown peas decreased significantly as total soil Zn, soil pore water soluble Zn and free Zn+2 increased (R-2 = 0.79, 0.75 and 0.75, respectively; P < 0.001). A 50% reduction in seed yield occurred at a total soil Zn concentration of about 290 mg kg(-1), in soils of pH 5.9-6. The corresponding soil pore water soluble Zn and free Zn2+ concentrations were about 9 and 4 mg l(-1), respectively. Pea seed yields were not significantly correlated with total soil Cu (R-2 = 0.33) or soil pore water soluble Cu (R-2 = 0.39). Yield reductions were due to a combination of greatly reduced numbers of free-living rhizobia in the soil due to Zn toxicity, thus indirectly affecting N-2-fixation, and Zn phytotoxicity. These effects were exacerbated in slightly acidic soils due to increased solubility of Zn, and to some extent Cu, and an increase in the free Zn2+ fraction in soil pore water. The current United Kingdom, German and United States limits for Zn and Cu in soils are discussed in view of the current study. None of these limits are based on toxicity thresholds in soil pore water, which may have wider validity for different soil types and at different pH values than total soil concentrations.