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(M/EH005) Rhizobium-based bio-indicators to assess impacts of heavy metals in land-applied sewage sludge. Horswell, Jacqui1, Speir, Tom1, Van Schaik, Andrew1, 1 ABSTRACT- Application of sewage sludge to land provides a convenient method for disposal of a waste product, and has the beneficial aspect of adding plant nutrients and organic matter to the soil. However, sewage sludge also contains potentially toxic elements, such as heavy metals, and there is concern about the long-term effects of these metals on the soil. Symbiotic nitrogen fixation has been demonstrated as being sensitive to heavy metals in soils, this has received particular attention because N2-fixation by white clover can contribute more then 200 kg N ha-1 y-1 to agricultural soils. Current methods for assessing toxicity of heavy metals to Rhizobium are time consuming, require large numbers of replicates, and long incubation periods. Hence, there is a need to develop a rhizobial-assay that is simple, but sensitive enough to rapidly identify potentially contaminated soils. In this study we compared the response of a biosensor based on Rhizobium leguminosarum biovar. trifolii (Rhizotox-C), with more traditional techniques of measuring the presence of effective strains of Rhizobium (plant infection technique) and nitrogen fixation (delta15N natural abundance). The historically treated soils used in this study were from a pastoral farm which received large amounts of heavy metal contaminated sewage sludge over 10 years ago. There were significantly reduced numbers of Rhizobium, and N-fixation by clover in sewage treated plots, there was also significant toxicity to the genetically-marked Rhizobium in several of the sewage treated blocks. Preliminary results suggest some variability between the three methods, however, potential detrimental effects on Rhizobium in soil treated with heavy metal contaminated sewage-sludge has been demonstrated with each of the methods. Key words: Rhizobium, sewage sludge, heavy metals, biosensor |
