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

Abstract

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°C. Treatments included nitrogen nutrient levels up to 10 mg NH 4NO 3 N I −1, phosphorus nutrient levels up to 40 mg H 2PO 4 −1 P I −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°C was sub-optimal and 25°C was near optimal. At the latter temperature, maximum biomass occurred when A. pinnata received l 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.