Dynamics of the diazotroph Bacillus polymyxa in the rhizosphere of wheat (Triticum aestivum L.) after inoculation and its effect on uptake of ¹⁵N-labelled fertilizer

An experiment under glasshouse conditions with a sandy soil was conducted to evaluate the effect of the inoculation of wheat (Triticum aestivum L.) roots with Bacillus polymyxa on N uptake by plants and to investigate the effect of N fertilization on the dynamics of this strain. Different treatments were considered: (i) amended (100 kg N ha⁻¹) or not with ¹⁵N labelled ammonium nitrate; (ii) inoculated (2.5 10⁷ spores per seed and 0.5 10⁹ spores kg⁻¹ soil) or not with B. polymyxa CF43. No promoting effect on wheat growth (dry yield) or N uptake was observed after inoculation under these experimental conditions; 62 per cent of the N fertilizer was assimilated by plants during the first 33 days. Inoculation had no effect on the quantity of soil adhering to the roots. Soluble organic C and inorganic N concentrations were higher in the soil adhering to the roots than in bulk soil. However, the nitrate concentration in soil adhering to the roots was influenced by inoculation at the beginning of plant growth, nitrate supply to the root system being delayed by its probable retention in the rhizosphere. Furthermore, total bacteria in roots, soil adhering to the roots and bulk soil was not affected by N fertilization. The number of B. polymyxa cells measured by an AB-ELISA method increased first in the bulk soil and later in the soil adhering to the roots after inoculation. Fertilization with N decreased the B. polymyxa population in adhering soil, competitive suppression of diazotrophs by non-fixing bacteria probably being induced by the presence of large amounts of nutrients in the rhizosphere when N was added. (author)