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. Results showed that placement of organic sources in the top profile (1-2 cm) produced a higher nitrogenase activity; while placement in the 2-4 cm and 4-6 cm profiles significantly decreased the activity irrespective of soil type. The effect of organic sources on nitrogenase varied depending on the soil type. Cellulose and rice straw caused differential stimulation in response to the placement. Evidence was provided for the creation of favourable redox potential supporting higher nitrogenase activity in series receiving organic amendments in the top profile. Higher nitrogenase activity in the top profile was associated with greater populations of nitrogen-fixing Azospirillum sp., anaerobic nitrogen fixers and Azotobacter sp. Moreover, the lower amounts of reducing sugars and higher carbon dioxide evolution gave evidence of higher microbial activity in the top profile. These results clearly indicate the positive relationship between nitrogenase activity, nitrogen-fixing populations and redox status in the top layers of flooded tropical rice soils.