GROWTH AND NUTRIENT COMPOSITION OF AZOLLA-PINNATA R BROWN AND AZOLLA-FILICULOIDES LAMARCK AS AFFECTED BY WATER TEMPERATURE, NITROGEN AND PHOSPHORUS SUPPLY, LIGHT-INTENSITY AND PH

Experiments were carried out in a greenhouse to determine the potential use of two Azolla species (Azolla pinnata R. Brown and Azolla filiculoides Lamarck) as a source of biofertiliser nitrogen. The growth of these two species was studied in controlled water temperature baths maintained at 20, 25 and 30-degrees-C. Treatments included nitrogen nutrient levels up to 10 mg NH4NO3-N l-1, phosphorus nutrient levels up to 40 mg H2PO4--P l-1, pH levels of the nutrient medium from pH 4 to 8, and shading treatments from 100 to 30% of sunlight. For both species, 20-degrees-C was sub-optimal and 25-degrees-C was near optimal. At the latter temperature, maximum biomass occurred when A. pinnata received 1 mg l-1 of N and 5 mg l-1 of P, whereas A. filiculoides required 10 mg l-1 of N and up to 20 mg l-1 of P. Although both species grew well at pH values of 5 and 7, A. filiculoides grew much more poorly at pH values of 4 and 8 than A. pinnata. Both species responded similarly to shading; biomass yields of plants receiving 30% of greenhouse sunlight were less than one-third those of plants receiving no shading. As Australian surface waters are inherently low in nitrogen (N) and phosphorus (P) these results suggest that significant inputs of N and P will be required if these two Azolla species are used as a biological nutrient source, for example in rotations involving rice.