Seasonal change of N2fixing rate in rice field assayed byIn Situacetylene reduction technique

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Abstract
A technique to estimate the quantity of N2 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 N2 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 N2 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 C2H4/m2 in the wet season (163 days) and 300 nunol C.H,/ml in the dry season (168 days). Acetylene reduction activity associated with the rice plant (IR26) was calculated to be 90 nunol C2H4/m2 in wet season (163 days) and 50 mmol C2H4/m2 in the dry season (168 days, IR36). Similar rates were obtained at a second site in the Philippines. N2 fixation was associated with both the stems and roots of wetland rice.

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