Impact of soil environmental factors on rates of N2-fixation associated with roots of intact maize and sorghum plants

We have evaluated the effects of oxygen partial pressure (pO₂), combined nitrogen, and the availability of organic substrates on nitrogen fixation (acetylene reduction) by bacteria associated with the roots of intact maize and sorghum plants. We also investigated the possibility of enhancing associative nitrogen-fixation by inoculating the soil in which the plants were grown with Azospirillum. Acetylene reduction (AR) activity was greatest when roots of intact plants were exposed to pO₂ between 1.3 and 2.1 kPa. Field-grown and greenhouse-grown plants supported similar levels of activity. Respiration inhibitors (2,4-dinitrophenol and sodium azide) eliminated AR activity at 2kPa O₂, whereas a fermentation inhibitor (sodium fluoride) only partially reduced the activity. Acetylene reduction activity was rapidly (1-3 h) inhibited by $NH_4^ + ,NO_3^ - $ and $NO_2^ - $ at concentrations of 4-20 mg NI⁻¹. Rates of AR varied substantially among individual plants in each experiment and between experiments. Amendment with any of several organic substrates greatly increased AR activity when rates were low, suggesting that the lack of activity was caused by a shortage of available carbon in the rhizosphere. Inoculation with Azospirillum failed to increase rates of AR associated with maize plants. In several experiments the indigenous bacteria associated with uninoculated plants exhibited greater activity than the bacteria associated with inoculated plants.