The role of Azolla in lowering the pH of simulated floodwater

Abstract

Rice is the single most important source of food for people and Azolla could play a very important role in rice production. For centuries Azolla and its nitrogen-fixing partner, Anabaena, have been used as "green manure" in China and other Asian countries to fertilize rice paddies and increase production. In recent decades Azolla has all but disappeared in rice cultivation. In this paper we present evidence from growth-chamber studies on the potential of Azolla to reduce the pH values in floodwater, particularly with respect to the CO2 equilibrium and metabolism, in the presence and absence of green algae and/or nitrate. Reduced pH values in floodwater could significantly reduce the volatilization of ammonia from applied urea fertilizer. The results show that aqueous systems containing unicellular green algae blooms of Chlorella kessleri and Chlamydomonas reinhardtii exhibited pH increases to levels in excess of pH 9, and even higher in a nutrient solution amended with NaNO3. The presence of Azolla inhibited the pH rise, due to the reduction of the photosynthetic activity of algae caused by the Azolla cover. Azolla alone did not alter the pH or CO2 behavior in water. The ratio of measured and calculated dissolved inorganic carbon was near unity in the presence of Azolla, suggesting an equilibrium of the CO2 in solution with the atmospheric CO2. C-14 assimilation by Azolla was primarily from the atmosphere, but nearly half of it was respired in the subsequent dark period. Some leakage of CO2 into the solution occurred during the day, possibly as a result of photo-respiration. The results suggest that the use of Azolla to prevent a significant increase in pH might require an efficient cover to assure shade.