Epiphytic nitrogen fixation on deepwater rice

Epiphytic N 2 -fixation in deepwater rice was studied by (1) microscopic examination of the rice plants at different growth stages, (2) measurement of the acetylene-reducing activity (ARA), and N 2 -fixing algal enumeration at the heading and maturing stages, and (3) bacterial enumeration at the maturing stage. A high rate of N 2 -fixing activity (5.1 μmolμplant -1 μhr -1 ) was observed mainly due to blue-green algae (BGA) which developed preferentially on submerged, decaying tissues of the host. A unique finding was the presence of BGA inside the leaf sheaths. The observations support the idea that the algal epiphytism and endophytism observed on rice are probably related to a mechanical effect, in relation to the roughness of the decaying tissues, rather than biotic relationships. Extrapolation of the N 2 -fixing activity to the field scale corresponded to an input of about 10-20 kg N μ ha -1 μcrop -1 . It can therefore be concluded that epiphytic N 2 -fixation on deepwater rice makes a substantial contribution to the ecosystem.