The contribution of atmospheric N2 to Azolla spp. grown on flooded soils

Abstract The contribution of atmospheric N2 (Ndfa) to Azolla growth in flooded rice soils was studied by the 15N dilution method. Thirty days after the addition of 15N-labeled N to a soil (Maahas clay), Azolla pinnata was grown in three consecutive croppings with (2nd cropping) and without (first and third croppings) rice canopy. Lemna minor was used as a non-N2-fixing reference plant. When the carryover of N from the inoculum was not negligible, the 15N isotope composition in the harvested plant material was corrected by the equation Since the growth of Lemna was very poor in the first and third croppings of Azolla, the carryover of unlabeled N from the inoculum was large and the corrected 15N abundance value in Lemna was abnormal. Lemna, therefore, could not be used as a reference plant. In the first cropping green alga which grew as a contaminant was used as reference and the value of Ndfa was 88%. In the second cropping, where the growth of Azolla and Lemna was reasonably good, the value of Ndfa was e...

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