Rhizobium inoculants for alfalfa in acid soils: A proposal for Uruguay

Abstract

Ensifer meliloti establishes symbiosis with Medicago sativa (alfalfa) and other perennial species of Medicago that grow in soils with neutral to alkaline pH, whereas Ensifer medicae makes symbiosis with annual medics adapted to moderately acid soils. The new species Rhizobium favelukesii, whose strain is LPU83, belongs to an alfalfa group of inefficient rhizobia, known as the Oregon type, initially represented by Rhizobium sp. strain Or191. R. favelukesii is considered a potential risk in the acid soils where alfalfa is grown, and could explain the inefficient nodulation observed in different countries. In acidic soils from the «Dairy Basin» of Uruguay, producers inoculate alfalfa with E. melliloti U143 strain. This edaphic condition is often marginal because the maximum potential of rhizobia-alfalfa symbiosis is not achieved at acid pH. Although Uruguay has an outstanding position in the production and use of rhizobial inoculants, the commercial strains currently used in Trifolium, Lotus and alfalfa were selected about 50 years ago in different conditions that the present ones as a consequence of: i) the displacement of cultivated pastures to other sites, ii) the sowing method, and iii) the use of new cultivars. In this review, alfalfa inoculation is analyzed in some countries and a strategy for the development of an inoculant suitable for Uruguayan acid soils is proposed. This strategy is based on the selection of efficient and competitive strains, as the first selection criteria, and persistency in soil as the second one.

Efecto de bacterias promotoras del crecimiento vegetal (Medicago sativa L.) en dos tipos de suelo, cobre y composta

“…Los factores de variedad-composta presentan interacción en la longitud del tallo, variedad-cobre en altura de la planta y variedad-cepa-cobre en el diámetro del tallo. Las variedades pueden presentar preferencia con las cepas, como lo mencionan Tabares-da Rosa et al (2019), quienes encontraron el establecimiento de una relación simbiótica entre Medicago sativa, Medicago littoralis y Medicago tonata, alfalfas anuales que crecen naturalmente en suelos de pH neutro a alcalino con rizobios de Sinorhizobium meliloti y Sinorhizobium medicae, basándose en una caracterización genotípica y bioquímica, donde se adaptaban a diferentes especies del género Medicago. También se menciona que puede deberse al origen de la alfalfa o al tipo de suelo presente como en suelos ácidos; el rendimiento de los genotipos de alfalfa es consecuencia de su tolerancia a la acidez donde se pueden ver beneficiadas las cepas inoculantes de rizobios.…”