Applications of 15N isotope techniques in Azolla-rice studies

Quantitative integrated values for biological N2 fixation by Azolla may be obtained through use of 15N in experiments employing the isotope dilution technique. Using Salvinia and Lemna as reference crops, Azolla has been found to derive as much as 80% or more of its N from N2 fixation, yielding 22-24 kg N/ha during a growth period of about 28 days. There are, however, variations in the %Ndfa and the amount of N2 fixed depending on the Azolla species or strain and the environmental and experimental conditions. The value of Azolla as biofertilizer for rice is realized only after its death and decomposition, when it releases N into the soil. The direct and indirect methods have been found to be equally good for assessing N recovery by rice from Azolla. Incorporation of 15N-labelled Azolla into paddy soil gave a 15N recovery by rice of about 40%. From a study undertaken in six countries it was found that on an overall basis under a wide range of environmental conditions the percentage 15N recovery by rice as well as the rice yield increases from Azolla application were not very different from those obtained from urea. Thus, Azolla may be considered as good as urea on a kg/kg N basis. An additional benefit is that an Azolla cover over the floodwater increases the fertilizer use efficiency of applied urea, and the rice yield. Several constraints, however, particularly those related to multiplication and culture under stress conditions, remain to be solved before the full potential of Azolla in lowland rice cultivation can be realized. (author). 22 refs, 3 figs, 2 tabs