PLACEMENT EFFECTS OF ORGANIC SOURCES ON NITROGENASE ACTIVITY AND NITROGEN-FIXING BACTERIA IN FLOODED RICE SOILS

The placement effects of organic sources (cellulose and rice straw) at different depths on nitrogenase activity (acetylene reduction assay, ARA) associated with four tropical rice soils were investigated. Resul ts showed that placement of organic sources in the top profile (1-2 cm ) produced a higher nitrogenase activity; while placement in the 2-4 c m and 4-6 cm profiles significantly decreased the activity irrespectiv e of soil type. The effect of organic sources on nitrogenase varied de pending on the soil type. Cellulose and rice straw caused differential stimulation in response to the placement. Evidence was provided for t he creation of favourable redox potential supporting higher nitrogenas e activity in series receiving organic amendments in the top profile. Higher nitrogenase activity in the top profile was associated with gre ater populations of nitrogen-fixing Azospirillum sp., anaerobic nitrog en fixers and Azotobacter sp. Moreover, the lower amounts of reducing sugars and higher carbon dioxide evolution gave evidence of higher mic robial activity in the top profile. These results clearly indicate the positive relationship between nitrogenase activity, nitrogen-fixing p opulations and redox status in the top layers of flooded tropical rice soils.