Abstract

A glasshouse experiment was carried out to study the effect of ammonium-N on the nodulation, growth and N-uptake of Sesbania sesban (L.) Merr. inoculated with an effective Mesorhizobium (S. sesban) strain.  Ammonium-N was supplied twice weekly as 100 ml of nutrient solution at concentrations of 0, 100, 200, 400, 600 and 800 mg N/l.  The seedlings were grown in pots with sterilized sand substrate and assessed at 5, 7 and 9 weeks after planting (WAP).  Nitrogen supply significantly improved nodulation (number of nodules and nodule dry weight/plant) with treatment 100 mg N/l compared with 0 mg N/l at 5 and 7 WAP, thus indicating synergism for the N₂-fixation symbiosis. However, with treatments ³ 200 mg N/l nodulation was either depressed or inhibited.  Seedlings treated with 100-400 mg N/l generally exhibited better growth (shoot and root dry weight/plant) and N-uptake than the other treatments (0, 600 and 800 mg N/l).  This study has demonstrated that S. sesban seedlings are tolerant to relatively high levels of N, and that treatment with 100 mg N/l (20 mg N per week) is necessary to stimulate an early and effective N₂-fixing symbiosis.

Key words: Ammonium, improved fallow, Mesorhizobium, N₂-fixation, nodulation,Sesbania sesban.