DYNAMICS OF THE DIAZOTROPH BACILLUS-POLYMYXA IN THE RHIZOSPHERE OF WHEAT (TRITICUM-AESTIVUM L) AFTER INOCULATION AND ITS EFFECT ON UPTAKE OF N-15-LABELED FERTILIZER

An experiment under glasshouse conditions with a sandy soil was conduc ted to evaluate the effect of the inoculation of wheat (Triticum aesti vum L.) roots with Bacillus polymyxa on N uptake by plants and to inve stigate 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 N-15 labelled ammonium nitrate; (ii) inoculated (2.5 10(7) s pores per seed and 0.5 10(9) spores kg(-1) soil) or not with B. polymy xa CF43. No promoting effect on wheat growth (dry yield) or N uptake w as observed after inoculation under these experimental conditions; 62 per cent of the N fertilizer was assimilated by plants during the firs t 33 days. Inoculation had no effect on the quantity of soil adhering to the roots. Soluble organic C and inorganic N concentrations were hi gher 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 r oot 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. polymy xa cells measured by an AB-ELISA method increased first in the bulk so il and later in the soil adhering to the roots after inoculation. Fert ilization with N decreased the B. polymyxa population in adhering soil , competitive suppression of diazotrophs by non-fixing bacteria probab ly being induced by the presence of large amounts of nutrients in the rhizosphere when N was added.