15N Dilution technique of assessing the contribution of nitrogen fixation to rice plant

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Abstract
An attempt to correlate the positive nitrogen balance in the rice-soil system with 15N dilution in the rice plant was done to see if isotope dilution can be used to assess the contribution of nitrogen fixation to nitrogen nutrition of rice. 15N ammonium sulfate and sucrose were added to the moist soil in pots to label biomass nitrogen fraction, and then the soil was air dried. It was moistened again, and planted to two crops of dryland rice or wetland rice. Dryland rice bad lower positive nitrogen balance and much higher 15N content than wetland rice. The rice-soil !Wstem with higher nitrogen gain had lower 15N content in the rice plant, suggesting inverse relationship of total nitrogen balance and 15N dilution. When the surface of pots was covered with black cloth, to suppress photodependent N2 fixation, no significant nitrogen gain was observed after two crops of rice. Significant gains were found in the rice-flooded soll system exposed to light and the 15N contents of plants decreased in allowing photodependent N3 fixation by blue-green algae or Azolla-Anabaena symbiosis. Contribution of plant nitrogen derived from photodependent N2 fixation was estimated to be 20-30% of the positive nitrogen gain in the system by the UN dilution technique using the rice-covered soil as reference system. Balance data of total nitrogen and UN enabled the estimation of the gross nitrogen gain in the rice-soil system UN dilution technique is useful in assessing the contribution of Nz fixation to nitrogen nutrition of wetland rice if proper reference system is taken.

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