Non-symbiotic Nitrogen Fixation in the Rhizo- Spheres of Rice, Maize and Different Tropical Grasses

Nitrogenase activity estimated in the rhizospheres of rice, maize and different tropical grasses grown under controlled laboratory conditions was shown to depend upon plant species. High nitrogenase activity (2000-6000 nmoles CzH4 h-' g-' dry root) occurred in rice rhizosphere, this activity being only 10 times lower than that of symbi0ticsystems;inthe rhizosphere of many other grasses grown in a similar way nitrogenase activity was as low as 10 nmoles C2H4 h-l g-' dry root. The influence of soil type on nitrogenase activity was impressive; but the exact nature of the factors implicated could not be established. A rather weak flush of nitrogenase activity in the rhizosphere occurred in the early stage of the plant growth; it was probably due to the exudation of compounds from the seedandlasted 2 or 4 daysaccording to the size of the seed. When the plant entered into its intense photosynthetic phase, the nitrogenase activity gradually increased. When the shoots were severed, nitrogenase activity in the rhizosphere ceased. Nitrogenase activity in the rhizosphere responded greatly to light intensity. Extrapolation of these laboratory findings to the field is discussed.