Population size of indigenous Rhizobium leguminosarum biovar trifolii in long-term field experiments with sewage sludge cake, metal-amended liquid sludge or metal salts: Effects of zinc, copper and cadmium

• Published in: Soil biology & biochemistry 2008-07, Vol.40 (7), p.1670-1680
• Main Authors: Chaudri, Amar, McGrath, Steve, Gibbs, Paul, Chambers, Brian, Carlton-Smith, Colin, Bacon, Jeffrey, Campbell, Colin, Aitken, Mark
• Format: Article
• Language: English
• Subjects: activated sludge; Agronomy. Soil science and plant productions; Bacteria; bioavailability; Biochemistry and biology; Biological and medical sciences; cadmium; Chemical, physicochemical, biochemical and biological properties; copper; dose response; field experimentation; Field experiments; Fundamental and applied biological sciences. Psychology; General agronomy. Plant production; Heavy metals; Liquid sludge; longitudinal studies; Metal salts; metals; microbial ecology; mortality; nitrogen-fixing bacteria; Other nutrients. Amendments. Solid and liquid wastes. Sludges and slurries; Physics, chemistry, biochemistry and biology of agricultural and forest soils; population size; Rhizobia; Rhizobium leguminosarum; Rhizobium leguminosarum bv. trifolii; salts; sewage; Sewage sludge; Soil and water pollution; soil bacteria; soil chemical properties; Soil science; Soil-plant relationships. Soil fertility. Fertilization. Amendments; Survival; symbionts; Trifolium repens; zinc
• ISSN: 0038-0717; 1879-3428
• DOI: 10.1016/j.soilbio.2008.01.026

There is conflicting evidence, and therefore continuing concern, as to whether metals in sewage sludge are deleterious to soil microbial processes and long-term agricultural productivity. Nine field experiments with sewage sludge cakes, three with metal-amended liquid sludges and three with inorganic metal salts were set up across Britain in 1994 to give individual metal dose–response treatments to try to answer this question. This study reports on the effects of Zn, Cu and Cd on the population size of Rhizobium leguminosarum biovar trifolii, a nitrogen fixing symbiont of white clover ( Trifolium repens), in soils from these experiments over 11 years. Significant ( P < 0.05) reductions in indigenous rhizobial numbers occurred on the Zn metal dose–response treatments at eight of the sludge cake sites in 2005, but few consistent effects were evident on the Cu or Cd metal dose–response treatments during the 11-year monitoring period. The soil total Zn concentrations where effects occurred were near to the UK statutory limit of 300 mg kg −1 for soils receiving sewage sludge. No significant reductions occurred in any treatments on the metal-amended liquid sludge or inorganic metal salt experiments in which the metals would be expected to be in a more bioavailable form, even after 11 years. The effects in the sludge cake experiments were related consistently with soil total Zn, with no recovery to date. The reductions in clover rhizobial numbers in the sludge cake experiments were due to Zn effects on free-living rhizobia in the soil, with gradual die-off over a long time with increasing soil total Zn concentrations. Currently, no consistent adverse effects on rhizobia have been seen at the UK limits for Cu and Cd of 135 and 3 mg kg −1, respectively.