Abstract
It is known that nitrogen is fixed by algae and photosynthetic bacteria at the surface soil of ponded paddy field. The biological nitrogen fixation is one of the main natural nitrogen supply in addition to the irrigation and rainwater supply. The active location of the bacteria and the amount of fixation, however, have not yet well stud-ied. In this report, at first, we investigated the penetrating depth of sunlight into the surface soil using a solar battery with radiometer. Then, we examined the distribution of to-tal nitrogen content under exposed and shielded conditions to the sunlight at interval of 2 mm from the surface. Total nitrogen contents determined by a NC-analyzer increased only in the uppermost (0 – 2 mm) layer from the ponded surface soil for the exposed condition. Since the sunlight reached down to 1 mm from the soil surface, we concluded that the biological nitrogen fixation occurred only at the thin surface layer with the penetrating sunlight.
