Biological nitrogen fixation by epiphytic microorganisms in rice fields

Methods used included microscopic examinations, algal and bacterial counts, and measurements of acetylene-reducing activity (ARA). Both ARA and microbial colonization were found to be much higher on outer than on inner sheaths of shallow wetland rice, irrespective of the type of microorganism, possibly indicating that outer sheaths contain partially decomposing material. Deepwater rice revealed the presence of BGA, especially of nitrogen-fixing types, on the surface of exposed and inner roots, on culm, and both outside and inside the leaf sheaths. Epiphytic BNF in shallow wetland rice was mainly due to a visible growth of Gloeotrichia, which was predominant during the early stages of plant growth. Deepwater rice, however, showed no macroscopic epiphytism by Gloeotrichia, and both microscopic examinations and counts indicated no changes of the epiphytic algae along the cultivation cycle. While results could not fully explain the relationships between the algal epiphytes and the hosts, certain inferences were drawn: (1) Gloeotrichia seems to grow preferentially on rough surfaces; (2) in the case of microscopic epiphytism, most of the isolated epiphytic strains grew appressed to the surface of the culture vessels and rarely formed floating colonies; (3) the presence of BGA inside the leaf sheaths of deepwater rice was a unique finding of the study and has been confirmed by research conducted in Thailand; (4) the nitrogen input by organisms epiphytic on shallow wetland rice can be evaluated to 2-3 kg of N per ha per crop, mainly due to BGA; (6) epiphytic algae are protected from conditions which destroy nitrogen-fixing algal blooms and thus provide an inoculum which makes bloom regeneration possible; and (7) epiphytism and endophytism are probably related to a mechanical effect. It is concluded that epiphytic BGA makes little contribution to nitrogen input in shallow wetland rice fields (while playing an important role in inoculum conservation), but produces a substantial nitrogen input in deepwater rice.