Genetic diversity of Sinorhizobium meliloti associated with alfalfa in Chilean volcanic soils and their symbiotic effectiveness under acidic conditions

A study was conducted with the aim of evaluating the genetic diversity of alfalfa rhizobia isolated from volcanic soils in southern Chile and their ability to establish an effective symbiosis with alfalfa. Rhizobial strains isolated from nodules were identified and selected based on PCR analyses and acid tolerance. Symbiotic effectiveness (nodulation and shoot dry weight) of acid-tolerant rhizobia was evaluated in glasshouse experiments under acidic conditions. The results revealed that Sinorhizobium meliloti is the dominant species in alfalfa nodules with a high genetic diversity at strain level grouped in three major clusters. There was a close relationship (r ² = 0.895, P ≤ 0.001, n = 40) between soil pH and the size of rhizobial populations. Representative isolates from major cluster groups showed wide variation in acid tolerance expressed on buffered agar plates (pH 4.5–7.0) and symbiotic effectiveness with alfalfa. One isolate (NS11) appears to be suitable as an inoculant for alfalfa according to its acid tolerance and symbiotic effectiveness at low pH (5.5). The isolation and selection of naturalized S. meliloti strains with high symbiotic effectiveness under acidic conditions is an alternative approach to improving the productivity of alfalfa and for reducing the application of synthetic fertilizers in Chile.