Effects of thiamine and nitrogen fertilizer form on the number of N₂-fixing and total bacteria in the rhizosphere of maize plants

The effects of thiamine (vitamin B1) application as seed dressing and of N form supplied (NH₄⁺ versus NO₃^-) on rhizosphere pH and on rhizosphere microorganisms were evaluated in two different soils. Imbibition of maize (Zea mays L.) seeds with thiamine (1 g kg^-1) increased seed thiamine content by a factor of 370. Maize plants from untreated and treated seeds were cultivated in a growth chamber under controlled conditions for 10 d in a sandy loam soil, pH 7.1 (Mascherode soil) or in a sandy soil, pH 4.8 (Niger soil) fertilized with two different N sources (NO₃-N or NH₄-N with dicyandiamide, 100 and 250 mg N kg^-1 soil). The rhizosphere pH was not affected by thiamine, only slightly affected by N source in the Mascherode soil, but markedly affected in the Niger soil. Thiamine application and N source affected the most probable number (MPN) of diazotrophs and total bacteria isolated from the rhizosphere soil of 10 d old maize plants. In the Mascherode soil, thiamine application increased MPN of diazotrophs 4-fold and total bacteria 2-fold when the soil was fertilized with 100 mg NO₃-N compared to untreated seedlings. Compared to Mascherode soil, in the Niger soil, MPN of diazotrophs was extremely low, especially after NH₄-N treatment which significantly decreased pH of the rhizosphere. Thiamine application had only marginal effects on the MPN of diazotrophs and total bacteria. Total bacteria isolated from Niger soil fertilized with NH₄-N were about 10-fold lower compared to the soil from Mascherode. However, in the other two treatments, total bacteria were higher in the Niger soil compared to the Mascherode soil. In the Niger soil, apparently some of the heterotrophs (the Actinomycetes dominated in this soil) might have suppressed the diazotrophs. The results of the present study demonstrate that in many cases seed treatment with thiamine enhances MPN of diazotrophs and total bacteria in the rhizosphere of maize seedlings.