Epiphytic nitrogen fixation on deepwater rice

Abstract

Abstract Epiphytic N2-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 N2-fixing algal enumeration at the heading and maturing stages, and (3) bacterial enumeration at the maturing stage. A high rate of N2-fixing activity (5.1 μmolμplant-1μhr-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 N2-fixing activity to the field scale corresponded to an input of about 10–20 kg N μ ha-1μcrop-1. It can therefore be concluded that epiphytic N2-fixation on deepwater rice makes a substantial contribution to the ecosystem.