Effect of ammonium nitrate on nodulation and nitrogen fixation (acetylene reduction) of the tropical legumeSesbania rostrata

Abstract

The effect of ammonium nitrate on nodulation and nitrogen fixation (acetylene reduction activity) was investigated usingSesbania rostrata, a tropical legume with the ability to nodulate both roots and stems. Plants were grown in plastic pots on silica sand and gravel, inoculated one month after sowing and then continuously irrigated with an automatic irrigation system. Four nitrogen treatments were applied: 0, 1.5, 3.0 and 6.0mm NH4NO3. Related symbiotic parameters were evaluated 20 days later. With 3.0mm NH4NO3, root nodulation was inhibited. At that concentration, stem nodulation was not affected, but related nitrogenase activity decreased 85% and was completely inhibited at 6.0mm NH4NO3. Increasing NH4NO3 concentration resulted in a diminution of stem nodule size. Stem and root acetylene reduction were strongly inhibited by high mineral nitrogen concentrations; however root nitrogen fixation was more affected than stem nitrogen fixation.