Azolla Biofertilizer Is an Effective Replacement for Urea Fertilizer in Vegetable Crops

Azolla spp. is a water fern that hosts Anabaena azolla, an N-fixing cyanobacterium, in its dorsal leaf cavities. Azolla occurs naturally in freshwater bodies in warm-temperate and tropical regions, and they have commonly been grown in rice paddies as a living fertilizer, providing N to the rice crop. We evaluated the potential use of Azolla harvested from freshwater bodies and applied as a biofertilizer to dryland vegetable crops. Two-thirds of the greenhouse gas emissions from crop production is attributed to N fertilizer, including fossil fuels used in fertilizer production and transportation. Azolla grown in on-farm ponds could remove CO2 from the atmosphere and minimize the use of fossil fuels in fertilizer production and transport. A 140-d laboratory incubation was used to compare the N mineralization of Azolla biofertilizer with compost and cyanobacterial biofertilizer treatments. Azolla treatments had the greatest N availability at the end of the incubation (73.0%), with compost demonstrating the least N availability (15.5%), and the cyanobacterial biofertilizers moderate in N release (31.6%). A greenhouse study evaluated the N uptake and yield of kale (Brassica oleracea) receiving Azolla biofertilizer compared to urea and organic fertilizers. The nitrogen uptake by kale followed the same pattern as in the incubation study, with the Azolla treatments highest among the organic fertilizers, and urea the greatest overall. Compost yielded better than the control but was the lowest yielding among the fertilizer treatments. Finally, we compared the agronomic effect of Azolla biofertilizer with urea and manure applied at the same N rates to spinach (Amaranthus cruentus) and radish (Raphanus sativus) crops grown in the field on alluvial and peat soils. Fertilizer treatments affected the spinach yield at both locations but did not affect the radish yield. The manure treatment resulted in the highest spinach yields (18–27 t ha−1), and the Azolla treatment applied at the same N rate as the manure yielded the same as the manure treatment on the peat soil and had the highest leaf and branch numbers. Azolla shows promise as a biofertilizer for dryland vegetable crops; however, an economic feasibility analysis is needed prior to encouraging the widespread adoption of on-farm Azolla production and use.