Survival of Azospirillum brasilense in the Bulk Soil and Rhizosphere of 23 Soil Types

Abstract

The survival of Azospirillum brasilense Cd and Sp-245 in the rhizosphere of wheat and tomato plants and in 23 types of plant-free sterilized soils obtained from a wide range of environments in Israel and Mexico was evaluated. Large numbers of A. brasilense cells were detected in all the rhizospheres tested, regardless of soil type, bacterial strain, the origin of the soil, or the amount of rainfall each soil type received prior to sampling. Survival of A. brasilense in soils without plants differed from that in the rhizosphere and was mainly related to the geographical origin of the soil. In Israeli soils from arid, semiarid, or mountain regions, viability of A. brasilense rapidly declined or populations completely disappeared below detectable levels within 35 days after inoculation. In contrast, populations in the arid soils of Baja California Sur, Mexico, remained stable or even increased during the 45-day period after inoculation. In soils from Central Mexico, viability slowly decreased with time. In all soils, percentages of clay, nitrogen, organic matter, and water-holding capacity were positively correlated with bacterial viability. High percentages of CaCO 3 and fine or rough sand had a highly negative effect on viability. The percentage of silt, pH, the percentage of phosphorus or potassium, electrical conductivity, and C/N ratio had no apparent effect on bacterial viability in the soil. Fifteen days after removal of inoculated plants, the remaining bacterial population in the three soil types tested began to decline sharply, reaching undetectable levels 90 days after inoculation. After plant removal, percolating the soils with water almost eliminated the A. brasilense population. Viability of A. brasilense in two artificial soils containing the same major soil components as the natural soils from Israel did was almost identical to that in the natural soils. We conclude that A. brasilense is a rhizosphere colonizer which survives poorly in most soils for prolonged periods of time; that outside the rhizosphere, seven abiotic parameters control the survival of this bacterium in the soil; and that disturbance of the soil (percolation with water or plant removal) directly and rapidly affects the population levels. Azospirillum species survive for prolonged periods of time in the rhizosphere of numerous plant species (13). Colonization of roots is nonspecific, and bacteria migrate between different plant species (7, 9, 18). However, conflicting evidence has been reported for survival of Azospirillum spp. in the soil outside the rhizosphere. Azospirillum spp. occur in most soils of tropical (3, 22, 23, 48, 51) and some soils of temperate (26) regions, indicating a high survivability outside the rhizosphere. In contrast, in studies done mainly in temperate and semiarid zones (1, 4, 15, 21, 52, 53), but also in tropical regions (41), it was found that introduced Azospirillum spp. survived poorly in these soils and hardly lasted from one season to the next (27). In Israeli soils, Azospirillum spp. adsorbed firmly ...

Effects of Resource‐Island Soils, Competition, and Inoculation with Azospirillum on Survival and Growth of Pachycereus pringlei, the Giant Cactus of the Sonoran Desert

“…The effects of soil components, such as texture, pH, and nutrients, on the survival of Azospirillum are small and may be positive or negative (Bashan et al 1995) and those of soil structure are marginal at best (Y. Bashan, unpublished data). However, Azospirillum does not survive well in bulk soil, whereas it persists in the rhizosphere (Bashan et al 1995).…”

“…The effects of soil components, such as texture, pH, and nutrients, on the survival of Azospirillum are small and may be positive or negative (Bashan et al 1995) and those of soil structure are marginal at best (Y. Bashan, unpublished data). However, Azospirillum does not survive well in bulk soil, whereas it persists in the rhizosphere (Bashan et al 1995). A disappearance of inoculated azospirilla in soils where they are exotic, as was the case in our experiment, has been reported elsewhere (Albrecht et al 1983;Smith et al 1984).…”

Membrane potential, proton and sodium motive forces in Azospirillum brasilense Sp7-S

“…When the external pH was 4.5 the v8 was signi¢cantly depolarised. We suggest that this loss of homeostasis, re£ected in very long doubling times at this pH, explains the poor distribution of azospirilla in acidic soils [17,18].…”

“…In nature, A. brasilense are frequently isolated from neutral and alkaline soils, usually containing sodium [18,22,23]. Some strains of azospirilla, A. halopraeferens [24,22] and A. irakense [25], isolated from sodic, alkaline soils have a requirement for sodium in their growth media.…”

Soil factors exhibit greater influence than bacterial inoculation on alfalfa growth and nitrogen fixation

“…The strain A. brasilense Sp7 was found to survive well in the arable soil, even in the bulk soil (in contrast to L33). This result is surprising as Bashan et al (1995) found that A. brasilense declined in bulk soils after 15 days, probably because of nutrient deficiency. In the contaminated soil of the present study, the strain Sp7 was not able to survive, either with or without a plant.…”