Factors effecting biological dinitrogen fixation by associative symbiosis in temperate regions

Several observations have indicated that plant-root/bacteria associations can contribute significantly to the nitrogen economy of some plants. However, there are factors limiting nitrogenase activity. It is essential in actual practice to know these limiting factors. The authors present factors related to the bacterial and vegetal part of the association and discuss the environment in which the association occurs. It has been demonstrated that there exists a well defined interrelation between plant species and bacterial inoculant. It seems that the potential of denitrification by the bacterial strain is important for nitrogenase activity in monoxenic wheat cultures. Winter-wheat cultivars grown under greenhouse conditions exhibit significantly different nitrogenase activity on their root system, which suggests that plant genotype is important. Finally, it is shown that active associative symbioses appear to be stimulated by a starter nitrogen fertilization. It is also suggested that the use of 15N will be very effective in determining the actual contribution of N2 fixation to plant N. (author)