Symbiotic dinitrogen fixation ‐ its dependence on plant nutrition and its ecophysiological impact

The cultivation of symbiotic legumes and the consequent small uptake of nitrate by the host plant depresses soil pH and impairs subsequent legume growth. The rate of soil pH decline depends on the soil H+buffer power, climatic conditions, soil permeability, and on the kind of legume cropping. It is shown for some leguminous crop species from the temperate climate that in many cases the N2fixation performance of Rhizobium/Bradyrhizobium symbiosis is more dependent on the growth conditions of the host plant than on the nitrogenase potential of the bacteroid. This is true for the supply of the host plant with phosphate, potassium and water as well as for light intensity. Nitrogen deficiency of the host plant because of insufficient nitrogenase activity was observed at low soil pH where nodulation and the development of the nitrogenase activity were delayed. Nodulation and nitrogenase activity are suppressed by high levels of available nitrogen in the soil. There are indications that the nitrogenase activity is very flexible and adjusts to the demand of the host plant. The mechanism of this regulation is not yet understood.