Nitrogen fixation byAzospirillium brasilensein soil and the rhizosphere under controlled environmental conditions

摘要

Acetylene reduction activity byAzospirillum brasilense, either free-living in soils or associated with wheat roots, was determined in a sterilised root environment at controlled levels of O2tension and with different concentrations of mineral N. In an unplanted, inoculated soil nitrogenase activity remained low, at approximately 40 nmol C2H4h-1per 2kg fresh soil, increasing to 300 nmol C2H4h-1when malic acid was added as a C source via a dialyse tubing system. The N2fixation byA. brasilensein the rhizosphere of an actively growing plant was much less sensitive to the repressing influence of free O2than the free-living bacteria were. An optimum nitrogenase activity was observed at 10 kPa O2, with a relatively high level of activity remaining even at an O2concentration of 20 kPa. Both NOinf3sup-and NHinf4sup+repressed nitrogenase activity, which was less pronounced in the presence than in the absence of plants. The highest survival rates of inoculatedA. brasilenseand the highest rates of acetylene reduction were found in plants treated with azospirilli immediately after seedling emergence. Plants inoculated at a later stage of growth showed a lower bacterial density in the rhizosphere and, as a consequence, a lower N2-fixing potential. Subsequent inoculations withA. brasilenseduring plant development did not increase root colonisation and did not stimulate the associated acetylene reduction. By using the15N dilution method, the affect of inoculation withA. brasilensein terms of plant N was calculated as 0.067 mg N2fixed per plant, i.e., 3.3 of the N in the root and 1.6 in the plant shoot were of atmospheric origin. This15N dilution was comparable to that seen in plants inoculated with non-N2-fixingPsudomonas fluorescens.