Rice (Oryza sativa L.) plants growing in pots of flooded soil were exposed to a "'Nrenriched atmosphere for 3 to 13 days in a gas-tight chamber. The floodwater and soil surface were shaded with a black cloth to reduce the activity of phototrophic N2-fLXing micro-organisms. The highest 1"N enrichments were consistently observed in the roots, although the total quantity of 16N incorporated into the soil was much greater. The rate of 1"N incorporation into roots was much higher at the heading than at the tillering stage of growth. Definite enrichments were also found in the basal node and in the lower outer leaf sheath fractions after 3 days of exposure at the heading stage. Thirteen days was the shortest time period in which definite 16N enrichment was observed in the leaves and panicle. When plants were exposed to 1N2 for 13 days just before heading and then allowed to mature in a normal atmosphere, 11.3% of the total 16N in the system was found in the panicles, 2.3% in the roots, and 80.7% in the subsurface soil. These results provide direct evidence of heterotrophic N2 fixation associated with rice roots and the flooded soil and demonstrate that part of the newly fixed N is available to the plant.Nitrogen fixation by heterotrophic bacteria associated with rice (Oryza sativa L.) growing in flooded soil has been extensively studied, and this work has been summarized in a series of reviews (7,12,16). Most of these studies in the field and in the laboratory have used the acetylene reduction assay, and activity has been found in the lower portion of the stem, the roots, the rhizosphere soil, and in the bulk of the anaerobic soil (8,15,17,19). There have also been several studies which demonstrated incorporation of 15N2 into flooded rice soils without plants (5,11,14,19). Recently, Ito et al. (9) demonstrated incorporation of '5N2 into the roots, basal node, and decaying outer leaf sheaths of rice plants which had been transferred from the field and exposed to 1 N2 in N-free nutrient solution. These studies do not allow direct comparison of the activity in the soil versus that in the roots, and there is also no information on the fate of the fixed N. There have been few experiments done in which undisturbed grass soil systems were exposed to '5N2 and none with rice. De-Polli et al. (6) showed that the roots and rhizomes of Digitania decumbens and Paspalum notatum became enriched when soil cores containing these plants were exposed to '6N2 and presented some evidence for transport of the newly fixed nitrogen to the leaves of P. notatum. In the present study, rice plants growing in pots of flooded soil were exposed to

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