Effect of Inoculation with N 2 -Fixing Spirilla and Azotobacter on Nitrogenase Activity on Roots of Maize Grown Under Subtropical Conditions

Abstract

Inoculated and non-inoculated seedlings of maize were grown in fertile clayloam soils of Egypt and Belgium under subtropical conditions provided in a greenhouse. Acetylene-reducing activity and microbial counts were determined during a period ranging from 6 to 12 weeks after sowing. Irrespective of soil origin, N 2 -fixing spirilla and Azotobacter were common under maize cultivation. Inoculation resulted in a transitional increase in their numbers at early stages of growth. Nitrogenase activity was not detected in the rhizosphere of young plants. The maximum activities measured (81 to 1,436 nmol of C 2 H 4 g −1 h −1 ) occurred close to the 50 to 70% silking stage. Inoculation with N 2 -fixing spirilla, particularly in Nile Delta soil, doubled the amount of N 2 fixed in a late period of growth (12 weeks), whereas inoculation with Azotobacter had no noticeable effect.