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 N at harvest/N at harvest-N in inoculum ⨯ 15N abundance at harvest. 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 estimated at 86%. In another soil (Luisita sand), 15N-labeled N was added and one crop of rice was grown to reduce the temporal change of 15N abundance in available N. Then, A. pinnata, A. microphylla, and A. filiculoides were grown using Anabaena-free A. filiculoides and Lemna as non-N2-fixing reference plants. Since the growth of Anabaena-free Azolla was poor, this plant could not be used as a reference. The value of Ndfa among Azolla strains did not differ, and was about 80%. The 15N isotope composition in NH4+-N in the soil was similar to the isotope composition in the reference plant and could be used as a substitute of non-N2-fixing reference. In the same soil, Azolla and the reference plants were grown without the addition of 15N-Iabeled substrate. Based on the natural 15N abundance the value of Ndfa of A. filiculoides was estimated at 75% and that of A. nilotica was 96%. Determinations of the natural 15N abundance of Azolla and aquatic plant samples grown in ponds and rice fields in Japan and the Philippines also showed that the value of Ndfa was higher than 75%. © 1991 Taylor & Francis Group, LLC.