Alteration of size of wheat root systems and nitrogen fixation by associative nitrogen-fixing bacteria measured under field conditions

Abstract

Spring wheat was inoculated with Azospirillum brasilense Cd or Bacillus C-11-25 under field conditions. Use of a 32P measuring technique showed that the root systems of Azospirillum-inoculated wheat occupied 54% of the soil volumes compared with uninoculated wheat plant root systems. Bacillus-inoculated wheat root systems were 82% of the size of uninoculated wheat root systems. Inoculated and control plants did not differ in dry matter production or in total N content. Inoculated plants received between 5 and 10% of the N contained in their tissues from associative N2 fixation as measured by 15N isotope dilution. Reduction of root system size in inoculated plants explains how the inoculated wheat plants in this study, and in previously reported studies, are able to absorb atmospheric N2 via associative N2 fixation without increasing total plant N uptake.