Submitted to: Journal of Environmental Quality
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/30/1996
Publication Date: N/A
Citation: Interpretive Summary: A problem of urban development is the disposal of solid waste material since it contains heavy metals which may affect the environment if the waste is applied to agricultural fields. It is not clear how much waste may be disposed without significantly affecting the soil. Metal contamination of soil alters the genetic structure of soil microbes by the selection of those which are resistant and inhibiting the growth of those which are sensitive. There are some indications that rhizobia of white clover are indicator microbes for the levels of safe disposal of waste material because these soil bacteria are relatively sensitive to metals. In addition, rhizobia are important soil bacteria which have application in agricultural production because these bacteria establish symbioses with legumes used as forages and cover plants. In the case of white clover, authors of previous studies have suggested that metal contamination limited the biodiversity of rhizobia and was selective for survivors which were ineffective for providing nitrogen to the plant. The results presented in this paper contradicted those conclusions. It was demonstrated that soil pH was a factor which affected rhizobia survival more than metal content and that there was not selection for a single or a few genotypes. The survivors were not always ineffective for nitrogen fixation. This information will be useful for scientists and workers is urban waste disposal in the development of limits for waste disposal and of factors affecting bacterial diversity.
Technical Abstract: Metal contamination may alter the diversity of microbes residing in soil. The genetic structure and phenotypic characteristics of clover Rhizobium isolated from contaminated and control soils were compared. Plant infection and symbiotic competence tests were used for phenotypic characterization. Variation across isolates in fingerprint patterns determined with primers for Repetitive Extragenic Palindromic (REP) sequences and the Polymerase Chain Reaction (PCR) were used for genetic characterization. Two phenotypic groups of effective and ineffective isolates were identified using the symbiotic effectiveness test. Soil pH was the primary factor influencing this phenotypic characteristic. Effective isolates were associated with higher soil pH and ineffective isolates were associated with lower soil pH regardless of soil metal content. The isolates were genetically diverse. The variation of isolates from the different soils overlapped indicating that neither the heavy metals nor the lower soil pH resulted in the selection of a single genotype. Isolates from the most heavily contaminated soils were more variable than isolates from control soils. Soil pH, not heavy metal content, was important in the selection of rhizobia which formed ineffective nitrogen-fixing symbioses.