Inoculation effects on legumes grown in soil previously treated with sewage sludge

The effects on legumes of the long-term land application of sewage sludge are not clear. Sludge-borne toxic elements, as well as essential nutrients and organic matter, complicate the response of legumes in association with their symbionts. To examine this problem, several strains and species of Rhizobium and Bradyrhizobium were studied for their response to the presence of heavy metals in agar growth media. Bradyrhizobium japonicum was by far the most metal-tolerant of the organisms examined, often able to tolerate several times more Zn and Cd in agar media than more metal-sensitive organisms, such as Rhizobium leguminosarum bv. trifolii. Soils were subsequently collected from metal-contaminated plots located at the Woburn Market Garden Experiment, and rhizobia were inoculated into these soils and sown with the appropriate homologous legume. Uninoculated controls and unamended soils were also sown. White clover (Trifolium repens) in the uninoculated, sludge-amended treatment contained numerous, small white and ineffective nodules. Inoculation enhanced nodulation and plant growth, but not to levels observed for plants grown on soil not amended with sludge. Inoculation with effective strains of rhizobia had little effect on plants grown in control soils since an indigenous, effective population compatible with white clover was present in adequate numbers. Only soybeans (Glycine max) responded to inoculation since soybeans had never been cultivated in these soils. These results confirm earlier observations that R. leguminosarum bv. trifolii in soils exposed to sludge-borne metals for many years are ineffective. Microbial species, chemical characteristics of the soil into which the sludge was added and the length of time the microbes were exposed to the metals each affected the response of the macro- and microsymbionts.