SEASONAL CHANGE OF N_2 FIXING RATE IN RICE FIELD ASSAYED BY IN SITU ACETYLENE REDUCTION TECHNIQUE : II. Estimate of Nitrogen Fixation Associated with Rice Plants

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A technique to estimate the quantity of N_2 fixed in the flooded water on the rice field and in association with the rice plant is described. Removal of the flood water and the surface soil and addition of fresh water eliminated the bulk of N_2 fixation associated with the blue-green algae. After the algal activity was eliminated, the acetylene reduction activity was higher in planted soils than that in fallow soils. Some N_2 fixation is, therefore, associated with the presence of the rice plant. In an IRRI unfertilized flooded rice field, algal acetylene reduction activity was estimated to be 200 mmol C_2H_4/m^2 in the wet season (163 days) and 300 mmol C_2H_4/m^2 in the dry season (168 days). Acetylene reduction activity associated with the rice plant (IR26) was calculated to be 90 mmol C_2H_4/m^2 in wet season (163 days) and 50 mmol C_2H_4/m^2 in the dry season (168 days, IR36). Similar rates were obtained at a second site in the Philippines. N_2 fixation was associated with both the stems and roots of wetland rice.
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