Occurrence and survival of Azospirillum spp. in temperate regions

Abstract

In order to evaluate the suitability of Azospirillum spp. as a crop inoculant in temperate regions, the natural occurrence, distribution and survival of Azospirillum after seed inoculation in Belgian agricultural soils was studied. Azospirillum was present in most of the fields examined, but concentrations never exceeded 1000 cfu per g soil or per g roots. Under field conditions none of the known species was found to be localized inside the roots of barley, wheat, rye, maize or grasses. Also, the distribution of A. brasilense SpBr14 within the root system of hydroponic-grown wheat was studied by immunofluorescence. From the rhizosphere samples of the field crops investigated, a number of microaerophilic, diazotrophic bacteria were isolated and identified as A. lipoferum, found only on maize and grass roots, and A. brasilense, present under all crops. In contrast to A. brasilense, A. lipoferum was able to use different amino-acids and some derivatives as sole carbon and nitrogen sources. Use of a peat-based seed inoculant resulted in the establishment of the Azospirillum spp. in the rhizosphere of field-grown winter barley and winter wheat. The established population survived during winter without appreciable change in numbers, but there was no indication of active growth during spring or summer.