INOCULATION EFFECTS ON LEGUMES GROWN IN SOIL PREVIOUSLY TREATED WITH SEWAGE-SLUDGE

The effects on legumes of the long-term land application of sewage slu dge are not clear. Sludge-borne toxic elements, as well as essential n utrients and organic matter, complicate the response of legumes in ass ociation with their symbionts. To examine this problem, several strain s and species of Rhizobium and Bradyrhizobium were studied for their r esponse to the presence of heavy metals in agar growth media. Bradyrhi zobiwn japonicum was by far the most metal-tolerant of the organisms e xamined, often able to tolerate several times more Zn and Cd in agar m edia than more metal-sensitive organisms, such as Rhizobium leguminosa rum bv. trifolii. Soils were subsequently collected from metal-contami nated plots located at the Woburn Market Garden Experiment, and rhizob ia were inoculated into these soils and sown with the appropriate homo logous legume. Uninoculated controls and unamended soils were also sow n. White clover (Trifolium repens) in the uninoculated, sludge-amended treatment contained numerous, small white and ineffective nodules. In oculation enhanced nodulation and plant growth, but not to levels obse rved for plants grown on soil not amended with sludge. Inoculation wit h effective strains of rhizobia had little effect on plants grown in c ontrol 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. le guminosarum bv. trifolii in soils exposed to sludge-borne metals for m any years are ineffective. Microbial species, chemical characteristics of the soil into which the sludge was added and the length of time th e microbes were exposed to the metals each affected the response of th e macro- and microsymbionts.