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Differentiation of Clover Rhizobium Isolated from Biosolids-Amended Soils with Varying pH

Dep. of Agronomy, Univ. of Maryland, College Park, MD 20742

R. L. Chaney

USDA-ARS Environmental Chemistry Lab., Beltsville, MD 20705

P. van Berkum

USDA-ARS Soybean and Alfalfa Research Lab., Beltsville, MD

* Corresponding author ( ja35{at}umail.umd.edu).

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 low 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, and not heavy metal content, was important in the selection of rhizobia that formed ineffective N₂-fixing symbioses.

Received for publication March 18, 1996.

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