The Effects of Nitrogen Fertilizer Application on Co-Symbiosis of Rhizobium and Rhizophagus Irregularis in Clover (Trifolium repens L)

Legumes have dual symbiosis with arbuscular mycorrhizal fungi (AMF) and rhizobia. AMF are obligate biotrophs, known to play an important role in ecological processes. Legumes root forms nodules in symbiosis with rhizobia, which can fix atmospheric nitrogen. A factorial complete randomized block design with three replications was carried out under greenhouse conditions. Four levels of nitrogen (0, 2, 6 and 10 mM as nitrate), in Newman and Romheld nutrient solution were applied to the pots containing 1.5 kg sterile sand. Surface sterilized seeds of clover plants (Trifolium repens L.) were sown in pots and inoculated with arbuscular mycorrhizal fungus (Rhizophagus irregularis) and/or Rhizobium leguminosarum bv. trifoli. The control plants were left un-inoculated. The pots were daily irrigated with nutrient solution possessing the nitrogen levels. Clover plants were harvested after 12 weeks of growth and Shoot and root dry weights, root mycorrhizal colonization, nodule weights and numbers, nitrogen, phosphorus and potassium contents in shoot and root were determined. The highest amount of phosphorus was observed in mycorrhizal treatment at 0 and 2 mM nitrogen levels. At all levels of nitrogen, the presence of fungi increased the amount of root phosphorus. At all levels of nitrogen, the presence of bacterium significantly increased the amount of shoot nitrogen. The highest amount of shoot potassium (64.4 mg.pot-1) was recorded at 2 mM nitrogen and non-mycorrhizal root and the highest amount of root K (19.02 mg.pot-1) was seen at 2 mM nitrogen and in the presence of mycorrhizal fungus. The highest amount of shoot dry weight (4.46 g.pot-1) was recorded in the mycorrhizal plants treated with 2mM nitrogen. The rhizobial inoculation had pronounced effects on root mycorrhizal colonization. By increasing nitrogen level up to 6 mM, the root colonization increased and the highest root colonization (34.1%) was achieved in this treatment. The highest increase in number and fresh weight of nodules were obtained in co-inoculated plants. In the presence of mycorrhizal fungus, the number and fresh weight of nodules increased by 26.44 and 20.98%, respectively.