The aquatic fern Azolla, a small-leaf floating plant that lives in symbiosis with a nitrogen fixing cyanobacteria (Anabaena), is an outstanding plant, thanks to its high biomass productivity along with its tremendous rate per unit area for nitrogen-fixation.  The present study investigates the potential growth of Azolla in secondary effluents for removal of COD, phosphorus, and nitrogen. Results have shown that N and P removal at 100 ppm of each component in separate medium turned out to be 36% and 44%, respectively, whereas in case of a mixed solution of these two compounds, N and P removal declined to 33% and 40.5%, respectively. Moreover, results have suggested that in the presence of phosphorus nitrogen absorption decreased. Furthermore, Azolla has revealed a high potential of COD removal by 98.8% in 28 days. Finally, Azolla may be one of the most promising agents to remove COD and treat nitrogen-free and phosphorus-rich wastewaters.

We investigated the ability of the aquatic fern Azolla to take up ciprofloxacin (Cipro), as well as the effects of that antibiotic on the N-fixing process in plants grown in medium deprived (-N) or provided (+N) with nitrogen (N). Azolla was seen to accumulate Cipro at concentrations greater than 160 μg g⁻¹ dry weight when cultivated in 3.05 mg Cipro l⁻¹, indicating it as a candidate for Cipro recovery from water. Although Cipro was not seen to interfere with the heterocyst/vegetative cell ratios, the antibiotic promoted changes with carbon and nitrogen metabolism in plants. Decreased photosynthesis and nitrogenase activity, and altered plant's amino acid profile, with decreases in cell N concentrations, were observed. The removal of N from the growth medium accentuated the deleterious effects of Cipro, resulting in lower photosynthesis, N-fixation, and assimilation rates, and increased hydrogen peroxide accumulation. Our results shown that Cipro may constrain the use of Azolla as a biofertilizer species due to its interference with nitrogen fixation processes.

This study investigated the storage of nitrogen (N) and phosphorus (P) in the biomass, bed sediments and water column of representative reaches of a sub-tropical river, the upper Brisbane River (UBR), Queensland, Australia, and contrasted instream storage with total wet season exports. In reaches which contained accumulated fine sediments, more than 87% of total P and between 50% and 92% of total N were stored in the surface sediments. The lower proportion of N in sediment at some sites was attributed to substantial differences in the N/P ratios of sediments and macrophytes. At one site, the riverbed was dominated by cobbles and boulders and total nutrient stocks were comparatively low and dominated by the biomass. In reaches with a narrow channel and intact riparian cover, biomass N and P were stored predominately in leaf litter, while in wider unshaded reaches, macrophytes dominated. Total instream storage in the mid to lower reaches of the UBR was ∼50.9 T for N and ∼18.1 T for P. This was considerably higher than total wet season N (∼15.6 T) and P (∼2.7 T) exports from the UBR. The first flow event in the river after a prolonged period of no flow resulted in the export of free-floating, emergent species Azolla. The estimated biomass of Azolla in the mid to lower reaches of the river was equivalent to approximately 24% and 9% of the total N and P flux, indicating that this may be a significant, previously unaccounted for, source at peak flow.

Irrigated transplanted flooded rice is a major source of methane (CH₄) emission. We carried out experiments for 2 years in irrigated flooded rice to study if interventions like methane-utilizing bacteria, Blue-green algae (BGA), and Azolla could mitigate the emission of CH₄ and nitrous oxide (N₂O) and lower the yield-scaled global warming potential (GWP). The experiment included nine treatments: T₁ (120 kg N ha⁻¹ urea), T₂ (90 kg N ha⁻¹ urea + 30 kg N ha⁻¹ fresh Azolla), T₃ (90 kg N ha⁻¹ urea + 30 kg N ha⁻¹ Blue-green algae (BGA), T₄ (60 kg N ha⁻¹ urea + 30 kg N ha⁻¹ BGA + 30 kg N ha⁻¹ Azolla, T₅ (120 kg N ha⁻¹ urea + Hyphomicrobium facile MaAL69), T₆ (120 kg N ha⁻¹ by urea + Burkholderia vietnamiensis AAAr40), T₇ (120 kg N ha⁻¹ by urea + Methylobacteruim oryzae MNL7), T₈ (120 kg N ha⁻¹ urea + combination of Burkholderia AAAr40, Hyphomicrobium facile MaAL69, Methylobacteruim oryzae MNL7), and T₉ (no N fertilizer). Maximum decrease in cumulative CH₄ emission was observed with the application of Methylobacteruim oryzae MNL7 in T₇ (19.9%), followed by Azolla + BGA in T₄ (13.2%) as compared to T₁ control. N₂O emissions were not significantly affected by the application of CH₄-oxidizing bacteria. However, significantly lower (P<0.01) cumulative N₂O emissions was observed in T₄ (40.7%) among the fertilized treatments. Highest yields were observed in Azolla treatment T₂ with 25% less urea N application. The reduction in yield-scaled GWP was at par in T₄ (Azolla and BGA) and T₇ (Methylobacteruim oryzae MNL7) treatments and reduced by 27.4% and 15.2% in T₄ and T₇, respectively, as compared to the T₁ (control). K-means clustering analysis showed that the application of Methylobacteruim oryzae MNL7, Azolla, and Azolla + BGA can be an effective mitigation option to reduce the global warming potential while increasing the yield.

This study has investigated the potential of an Azolla-Anabaena symbiosis, a marriage between the cyanobacterium Anabaena azollae and the aquatic fern (Azolla), to remove ammonia from freshwater fish breeding areas. Experiments were carried out under artificial light of 20, 70, and 140 μmol m⁻² s⁻¹. We investigated three different water temperatures for the growing Azolla, ranging from sub-optimal to optimal temperatures (15, 22, and 28 °C). The capability of Azolla to remove ammonia from wastewater was demonstrated, and the highest ammonia concentration tolerated by the symbiosis between Azolla-anabaena without any toxic effect on the aquatic ferns was ascertained. The shortest time taken to remove ammonia from wastes, 2.5 cm deep and at 28 °C, was 40 min. The ammonia removal rate (A RR) was both light and temperature dependent and the highest rate (6.394 h⁻¹) was attained at light intensity of 140 μmol m⁻² s⁻¹ and at a temperature of 28 °C; the lowest (0.947 h⁻¹) was achieved at 20 μmol m⁻² s⁻¹ and 15 °C. The depth of the fish-wastewater pool also affected the A RR with the relation between A RR and the depth being a hyperbolic function.

Methane (CH₄) is an important greenhouse gas (GHG), and paddy fields are major sources of CH₄ emissions. This pot experiment was conducted to investigate the integrated effects of Azolla inoculation combined with water management and N fertilization on CH₄ emissions in a double-rice cropping system of Southern China. Results indicated that midseason aeration reduced total CH₄ emissions by 46.9%, 38.6%, and 42.4%, followed by N fertilization with 32.5%, 17.0%, and 29.5% and Azolla inoculation with 32.5%, 17.0%, and 29.5%, on average, during the early, late, and annual rice growing seasons, respectively. The CH₄ flux peaks and total CH₄ emissions observed in the late rice growing season were significantly higher than those in the early rice growing season. Additionally, CH₄ fluxes correlated negatively to soil redox potential (Eh) and dissolved oxygen (DO) concentration. Azolla inoculation and N fertilization greatly increased the rice grain yields, whereas midseason aeration had distinct effects on grain yields in both rice seasons. The highest annual rice grain yields of approximately 110 g pot⁻¹ were obtained in the Azolla inoculation and N fertilization treatments. In terms of yield-scaled CH₄ emission, Azolla inoculation combined with midseason aeration and N fertilization generated the lowest yield-scaled CH₄ emissions both in the early and in the late rice growing seasons, as well as during the annual rice cycle. In contrast, the highest yield-scaled CH₄ emission was obtained in the treatment employed continuous flooding, without Azolla and no N application. Our results demonstrated that Azolla inoculation, midseason aeration, and N fertilization practices mitigated total CH₄ emissions by 18.5–42.4% during the annual rice cycle. We recommend that the combination of Azolla inoculation, midseason aeration, and appropriate N fertilization can achieve lower CH₄ emissions and yield-scaled CH₄ emissions in the double-rice growing system.

Rice soil is a source of emission of two major greenhouse gases (methane (CH₄) and nitrous oxide (N₂O)) and a sink of carbon dioxide (CO₂). The effect of inorganic fertilizers in combination with various organics (cow dung, green manure (Sesbania aculeata) Azolla compost, rice husk) on CH₄ emission, global warming potential, and soil carbon storage along with crop productivity were studied at university farm under field conditions. The experiment was conducted in a randomized block design for 2 years in a monsoon rice (cv. Ranjit) ecosystem (June–November, 2014 and 2015). Combined application of inorganic (NPK) with Sesbania aculeata resulted in high global warming potential (GWP) of 887.4 kg CO₂ ha⁻¹ and low GWP of 540.6 kg CO₂ ha⁻¹ was recorded from inorganic fertilizer applied field. Irrespective of the type of organic amendments, flag leaf photosynthesis of the rice crop increased over NPK application (control). There was an increase in CH₄ emission from the organic amended fields compared to NPK alone. The combined application of NPK and Azolla compost was effective in the buildup of soil carbon (16.93 g kg⁻¹) and capacity of soil carbon storage (28.1 Mg C ha⁻¹) with high carbon efficiency ratio (16.9). Azolla compost application along with NPK recorded 15.66% higher CH₄ emission with 27.43% yield increment over control. Azolla compost application significantly enhanced carbon storage of soil and improved the yielding ability of grain (6.55 Mg ha⁻¹) over other treatments.