Rhizobium in soils contaminated with copper and zinc following the long-term application of sewage sludge and other organic wastes

Abstract

Soils from six long-term sewage sludge-treated sites contaminated principally with Cu and Zn, but containing only small concentrations of Cd, were screened for the presence of effective Rhizobium leguminosarum biovar trifolii using a plant infection assay for nodulation with Trifolium repens L. (white clover). Copper and Zn were determined as the elements most limiting to sewage sludge recycling on agricultural land, potentially reducing symbiotic N 2-fixation in sludge-treated soils. Soil samples contaminated with Cu and Zn from past applications of pig slurry, and with Cu due to historical treatment with ‘pot-ale’ (whisky distillery waste), were also assessed. Associations between the presence or absence of nodulation and the concentrations of Cu, Zn and Cd in soil were examined using a general linear modelling procedure to account also for variations due to the pH value and organic matter content of soil samples as well as the inherent variability between the different sampling sites. Rhizobium, effective in N 2-fixation, were present in all the soils supporting the host plant, irrespective of metal concentrations in soil which increased to 300 mg Cu kg −1 and 2000 mg Zn kg −1. In contrast, several soil samples with no indigenous host plant failed to nodulate while clover in the infection test, but the statistical analysis indicated that this was not associated generally with increased metal concentrations in soil or with greater metal ‘availability’ measured by soil solution analysis and metal uptake by five-leaf-stage barley plants. However, there was some limited evidence implicating Zn in the absence of nodulation at one of the sludge-amended sites examined. This work demonstrated that nodulation and effective N 2-fixation by white clover occurs in sludge-treated soils above the current U.K. maximum permissible concentrations of heavy metals.