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

Two agriculturally important species of rhizobia, Rhizobium leguminosarum b iovar viciae (pea rhizobia) and R. leguminosarum bv. trifolii (white clover rhizobia), were enumerated in soils of a long-term field experiment to whi ch 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 ar e reported. Pea and white clover rhizobia were greatly reduced in soils con taining greater than or equal to 200 mg Zn kg(-1), and soil pore water solu ble 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 als o 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 f ree 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 ab out 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), respec tively. Pea seed yields were not significantly correlated with total soil C u (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 rhizob ia in the soil due to Zn toxicity, thus indirectly affecting N-2-fixation, and Zn phytotoxicity. These effects were exacerbated in slightly acidic soi ls due to increased solubility of Zn, and to some extent Cu, and an increas e 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 vi ew of the current study. None of these limits are based on toxicity thresho lds in soil pore water, which may have wider validity for different soil ty pes and at different pH values than total soil concentrations.