DOI: https://doi.org/10.56669/XJIN6353

 

ABSTRACT

Global warming and climate change have rendered traditional agriculture ineffective in food production. The challenges are particularly acute in areas plagued by poor land management and unsustainable practices. The Bio-Circular-Green (BCG) economic model, which aims to minimize the use of chemical fertilizers while promoting resource recycling and environmentally sustainable methods, has been gaining traction. Azolla serves as a compelling example of BCG in sustainable rice cultivation due to its numerous advantages, including enhanced nitrogen use efficiency and rice yield, improved or sustained soil quality and fertility, decreased greenhouse gas emissions and ammonia volatilization, increased carbon sequestration, and the bioremediation of toxic pollutants. Nevertheless, Azolla has not achieved widespread acceptance among rice farmers globally, who continue to depend on rising applications of synthetic nitrogen fertilizers rather than leveraging Azolla's potential to enhance long-term soil fertility and health. This review outlines the multiple benefits of Azolla in sustainable rice production. This systematic literature review, along with the scientific evidence presented, could assist policymakers, scientists, and researchers in understanding the advantages, limitations, and innovative methods for utilizing Azolla as a cost-effective and environmentally friendly amendment in rice cultivation.

Keywords: nitrogen use efficiency; soil quality and fertility; greenhouse gas mitigation; bioremediation

INTRODUCTION

Climate change and population growth are driving an increase in global food demand, which is projected to rise by 35% to 56% between 2010 and 2050 (Van Dijk et al., 2021). One of the Sustainable Development Goals (SDGs) set by the UN (SDG 2) is to achieve food security and zero hunger through sustainable agriculture and better nutrition (Chanapanchai, et al., 2025). Practicing sustainable agriculture can be achieved through a bio-circular-green economy approach, which focuses on utilizing renewable biological resources or converting them into high-value bio-based products (Tan and Lamers, 2021).

Rice (Oriza sativa) accounts for 11% of all croplands worldwide and is the primary food crop for about half of the world's population (Das and Kim, 2024). The global consumption of milled rice is projected to rise from 480 million tons (Mt) in 2014 to approximately 550 Mt by 2030 (Yuan et al., 2021). Rice cultivation requires a large amount of mineral nutrients, including nitrogen (N), to ensure its growth, development, and yield. However, most rice-growing soils around the world are N-deficient, so N fertilizers must be applied to meet the N requirements of rice crops (Choudhury and Kennedy, 2004). In general, rice is grown with urea as a source of N. However, the use efficiency of added urea-N is relatively modest, often only 30% to 40%, and sometimes even lower (Choudhury and Kennedy, 2004). The primary causes of this low N-use efficiency are leaching losses, NH₃ volatilization, and denitrification (Ponnamperuma, 1972). NH₃ volatilization releases NH₃ and denitrification emits greenhouse gas (GHG) like N₂O, which is a much more potent GHG than CO₂. The global warming potential of N₂O is about 273 times that of CO₂. (Lee et al., 2025). Leaching of NO₃^- causes groundwater toxicity (Shrestha and Ladha, 1998). In addition to these environmental problems, prolonged or excessive usage of urea-N can cause nutritional imbalance, soil acidification, and organic matter loss (Tilman et al., 2001). Furthermore, rice cultivation with urea-N aggravates emissions of methane (CH₄), another potent GHG (Das and Kim, 2024). These issues are of great concern to soil and environmental scientists worldwide. To reduce the harmful effects of urea-N applications on agriculture and ecosystems, sustainable alternative solutions must be found and implemented.

Sustainable agriculture is defined as any set of agronomic practices that is economically viable, environmentally sound, and socially acceptable (Kollah et al., 2016). Sustainability in agriculture can be attained by: (1) enhancing the production of food grains to satisfy the population's demand; (2) reducing agricultural inputs to decrease the costs associated with agricultural production; (3) minimizing the use of inorganic or chemical fertilizers to mitigate their impact on the environment and soil biodiversity, which are essential for soil functionality; (4) innovating technologies to utilize wastewater resources for agricultural and other beneficial purposes to address the challenges posed by water scarcity; (5) investigating bioremediation technologies to repurpose contaminated or wastelands for agricultural and other applications; (6) employing eco-friendly methods to cultivate degraded lands, thereby diminishing the prevalence of waste or degraded land; and (7) formulating strategies to lower GHG emissions from agriculture (Kollah et al., 2016). Organic agriculture has been recognized as one of the effective strategies or alternative approaches to achieve sustainability, addressing the multifaceted challenges that not only aim to enhance agricultural productivity but also to preserve soil health and support biosphere functions. Organic farming emphasizes the importance of optimizing biological processes in soils and avoids the use of synthetic chemicals and fertilizers. (Chanapanchai, et al., 2025).

The utilization of Azolla as a nutrient source for crops presents a promising alternative strategy for sustainable agriculture. Azolla can fix atmospheric nitrogen (N₂) and provide nitrogen to rice plants, thereby reducing the need for synthetic nitrogen fertilizers and lowering investment costs (Marzouk et al., 2023). Additionally, the application of Azolla in rice fields enhances sustainability, as these organisms absorb and sequester carbon dioxide (CO₂), thereby contributing to a reduction in GHG emissions (Brouwer et al., 2018). Despite numerous studies conducted in the past on the use of Azolla for soil and agricultural nitrogen management, a comprehensive review of its application to other global issues, such as GHG mitigation, ammonia volatilization, carbon sequestration, and bioremediation of potentially toxic elements, remains lacking. This review focuses on the sustainability of utilizing Azolla in rice production.

An overview of Azolla in rice production

Azolla is a free-floating aquatic fern generally found in freshwater ecosystems in tropical and subtropical regions (Marzouk et al., 2023). There are seven known species of Azolla: A. caroliniana, A. filiculoides, A. microphylla, A. pinnata, A. mexicana, A. rubra and A. nilotica (Figure 1). However, A. pinnata is the species mostly found in tropical and subtropical ecosystems (Marzouk et al., 2023). The special characteristic of Azolla is its symbiosis with the cyanobacteria (formerly known as blue-green alga (BGA)) Anabaena azollae. The Azolla–Anabaena symbiosis is unique because this relationship is established in every generation. The symbiotic activity occurs in the photosynthetic leaves, which contain large cavities covered with slimy mucilage sheaths, where the cyanobacteria filaments develop. In this symbiosis, the cyanobacteria or BGA, which fix nitrogen, provide sufficient nitrogen for itself and its host. The fern in turn provides a protected environment for the cyanobacteria and supplies it with a fixed carbon source (Chanapanchai et al., 2025). The symbiosis between Azolla and Anabaena possesses the ability to accumulate substantial amounts of fixed nitrogen, rendering it a promising biofertilizer for various crops (Choudhury and Kennedy, 2004). Nevertheless, rice (Oryza sativa L.) is particularly well-suited for the application of Azolla, as both thrive in flooded environments.

In paddy fields, Azolla can be cultivated either as a monocrop or in conjunction with rice (Bhuvaneshwari and Singh, 2015). Monocropping occurs before the cultivation of rice, while intercropping involves growing Azolla alongside rice, followed by its incorporation into the soil or harvesting for other uses. It is introduced into rice fields immediately after rice transplanting and can either be incorporated into the soil or allowed to decompose naturally. Adding Azolla to the soil before or after transplanting enhances soil fertility and reduces the need for synthetic fertilizers (Bhuvaneshwari and Singh, 2015).

The quantity of Azolla inoculum required ranges from 300 to 500 kg of fresh biomass per hectare to 20 tons of fresh biomass per hectare (Marzouk et al., 2023). The most commonly recommended inoculation rate of Azolla is between 1.5 and 3.0 tons of fresh biomass per hectare, as indicated by various studies, which can cover the field within 15 to 20 days post-inoculation and yield up to 40 kg of nitrogen per hectare (Bhuvaneshwari and Singh, 2015; Kollah  et al., 2016; Seleiman et al., 2022). The study conducted by Oyange et al. (2020) demonstrated that incorporating Azolla at the time of transplanting and 21 days after transplanting significantly increased the number of spikelets per panicle and grain weight when Azolla was incorporated one day prior to rice transplanting. Nyalemegbe et al. (2011) found that a single application of fresh Azolla at 2 tons per hectare, either as a basal application or as a dual application at 20 and 25 days after transplanting, resulted in yields of 30 to 40 kg of nitrogen per hectare. Additionally, it was noted that growing Azolla twice and adding it 35 and 50 days after transplanting resulted in 26 to 39 tons of biomass per hectare, fixing 44 to 61 kg of nitrogen per hectare (Bocchi and Malgioglio, 2010). When grown as a basal application and dual application after transplanting, Azolla can fix 76-94 kg of nitrogen per hectare (Bocchi and Malgioglio, 2010). A higher application rate achieves full coverage in a shorter timeframe; therefore, it is advisable to apply Azolla 7 days after transplanting to prevent damage to the rice (Bhuvaneshwari and Singh, 2015). However, at lower densities, Azolla may be outcompeted by other plants, including algae and weeds (Bhuvaneshwari and Singh, 2015).

Understanding the ideal conditions for Azolla growth is crucial for enhancing biomass and nitrogenase activity in lowland rice cultivation (Marzouk et al., 2023). All vital macro and micro-nutrients, except combined nitrogen, are essential for optimal Azolla growth. Azolla has been successfully cultured in various nitrogen-free inorganic media (Adhikari et al., 2020). Deficiency in Calcium and Phosphorous negatively impacts both the growth and nitrogen fixation capabilities of Azolla (Adhikari et al., 2020).

When the relative humidity falls below 60%, Azolla becomes dry and brittle, and complete desiccation results in the fern's death (Adhikari et al., 2020). Although Azolla can thrive on moist dust surfaces or in damp peat debris, it prefers flooded conditions for optimal growth. Azolla flourishes in free-floating water levels ranging from 5 to 12 cm (Bhuvaneshwari and Singh, 2015). Conversely, Azolla migrates from the growing medium when there is excessive water, and water fluctuation during the early stages of rice growth can result in Azolla attacking the rice, leading to its decay and death (Adhikari et al., 2020). The development of Azolla is rapid during the first two weeks, after which growth slows as the plant matures (Adhikari et al., 2020).

Light intensity plays a significant role in Azolla growth, influencing photosynthetic activity, nitrogen fixation, and the symbiotic relationship between Azolla and the cyanobacteria (Marzouk et al., 2023). The intensity of light also governs the interplay of photoperiod, temperature, asexual reproduction, and other factors such as nitrogen and phosphorus supply, as well as pH levels. The optimal light intensity for Azolla growth is 20,000 lux, and growth diminishes rapidly under shaded conditions (below 1,500 lux) (Marzouk et al., 2023).

Temperature is another key factor affecting the growth and nitrogen fixation of Azolla (Kollah et al., 2016). Numerous studies have examined the effects of temperature on various Azolla species, revealing that both higher temperatures (above 30◦C) and lower temperatures (below 4◦C) hinder Azolla growth. The ideal temperature range for Azolla growth is between 18 °C and 28 °C. Furthermore, nitrogen fixation and the activity of the nitrogenase enzyme decrease significantly at 35-40◦C (Marzouk et al., 2023). However, it is important to note that different Azolla species exhibit varying degrees of temperature sensitivity. The optimal temperature requirement for A. mexicana, A. pinnata, and A. caroliniana is 30°C, while A. filiculoides requires 25°C (Adhikari et al., 2020). Azolla biomass experienced a 14-16-fold increase over 14 days at day/night temperatures of 30/25°C, 38/30°C, and 40/32°C, with nitrogenase activity peaking on the 7^th day at 30/25°C (Kollah et al., 2016).

The Influence of pH on the growth and nitrogen fixation of Azolla depends on various factors, including temperature, essential nutrients, and light intensity (Marzouk et al., 2023). The optimal growth of Azolla is observed within a pH range from 4.5 to 7.5. However, the ideal pH for Azolla growth can vary widely from 3.5 to 10.0, provided that all essential nutrients are sufficiently available (Marzouk et al., 2023). This observation suggests that while Azolla’s growth is pH-dependent, it is also influenced by other environmental and physicochemical factors.

Azolla: a promising biofertilizer for sustainable rice production

Azolla is recognized as an environmentally friendly, non-toxic, and preferred biofertilizer and green manure for lowland rice cultivation (Kollah et al., 2016). The capacity of the Azolla-Anabaena symbiosis to fix atmospheric nitrogen at accelerated rates renders it an exceptional agronomic option for rice cultivation. The following attributes of Azolla contribute to its effectiveness as a biofertilizer for rice:

1. A shallow freshwater habitat, such as that found in a flooded rice field, provides an ideal environment for Azolla.
2. Azolla can fix nitrogen at a fast rate.
3. It exhibits rapid growth.
4. As Azolla floats on the water's surface, it does not compete with rice for light and space.
5. In most climates, Azolla thrives best under the partial shade provided by vegetation, which a rice canopy can easily offer during its early and intermediate growth stages.
6. As rice approaches maturity, low light intensity beneath the canopy and nutrient depletion lead to the decomposition of Azolla, thereby releasing nutrients back into the medium.
7. Azolla decomposes quickly, which allows the nitrogen it has fixed, along with the phosphorus and other nutrients it may have absorbed from the water, to be swiftly released back into the medium, thus becoming available for uptake by rice during the development of grains.
8. Azolla possesses a superior capacity compared to rice for accumulating potassium in its tissues in low-potassium environments; consequently, after its decomposition, this nutrient becomes accessible to rice.
9. Unlike chemical nitrogen fertilizers, Azolla has long-term residual benefits, particularly improving soil fertility by increasing total nitrogen, organic carbon, available phosphorus, potassium, and other nutrients.
10. A dense layer of Azolla found in a rice field also provides the additional advantage of suppressing weed growth.

In recent years, the use of Azolla has been recognized as more advantageous than inorganic fertilizers for the reduction of ammonia volatilization, mitigation of GHG emissions, remediation of heavy metals, support for economic development, and creation of job opportunities (Kollah et al., 2016; Chanapanchai et al., 2025), establishing it as an ideal candidate for sustainable rice production (Figure 2).

Azolla as an N fertilizer for improving rice yield

Azolla is commonly used for biological nitrogen fixation in paddy fields to reduce the need for chemical nitrogen fertilizers (Marzouk et al., 2023). The biological nitrogen fixation is carried out via nitrogenase enzymes, which convert atmospheric N₂ into usable ammonium (NH₄⁺) for rice plants (Chanapanchai et al., 2025). Azolla has been reported to fix N₂ at a rate of 1.1 t ha⁻¹ year⁻¹, which is higher than the fixing rate of legumes (0.4 t ha⁻¹ year⁻¹) (Marzouk et al., 2023). Less than 5% of the nitrogen sequestered by Azolla is available immediately to the growing rice plants. The remaining 95% remains in the Azolla’s biomass until the plant dies (Mandal et al., 1999). As the plant decomposes, its organic nitrogen is mineralized and released as ammonia (NH₃), which then becomes available as a biofertilizer for the growing rice plants (Mandal et al., 1999). During decomposition, organic nitrogen is mineralized rapidly during the first two weeks and then at a more gradual rate (Watanabe, 1982). Nitrogen is released mainly in the form of ammonium. The ammonium-nitrogen released was found to stabilize at approximately 1 mg NH⁴⁺-N g^-1 of fresh Azolla, which accounted for 26-28% of the total nitrogen content of Azolla (Watanabe, 1982).

Azolla holds significant agronomic importance for rice cultivation, where it is utilized as a dual crop alongside rice, contributing between 40 and 60 kg of nitrogen per hectare for each rice crop (Marzouk et al., 2023). Azolla pinnata is capable of fixing 75 mg of nitrogen per gram of dry weight each day and can yield biomass of 347 tons of fresh weight per hectare annually (Adhikari et al., 2020). This biomass comprises 868 kg of nitrogen, equivalent to approximately 1,900 kg of urea. When incorporated into rice fields, Azolla pinnata exhibits an average nitrogen-fixing capacity of 0.3 to 0.6 kg N/ha/day (Marzouk et al., 2023). In pot experiments, Azolla filiculoides introduced into paddy soil demonstrates a nitrogen fixation potential of 128 kg of nitrogen per hectare over a 50-day period (Adhikari et al., 2020). There is considerable variability in growth and nitrogen fixation capabilities among various strains of Azolla (Singh and Singh, 1987). Research shows that the inoculation of Azolla at a rate of 16.5 to 17.5 tons of fresh weight per hectare can fix as much as 52.5 to 55.1 kg of nitrogen per hectare (Singh and Singh, 1987). Likewise, the incorporation of 12.2 tons of dry weight per hectare can contribute up to 33.80 kg of nitrogen per hectare (Kimani et al., 2020). Table 1 shows the effectiveness of different species of Azolla in fixing N₂ and improving crop growth and yield in rice paddies. It has been observed that nitrogen fixation is influenced by several factors, including climate, nutrient levels in floodwater, the specific Azolla species, management practices, and the growth stages of rice (Marzouk et al., 2023).

Table 1. The effectiveness of different species of Azolla to fix N₂ and improve crop growth and yield in rice paddies.

Azolla species	Rate of application	Time of Azolla application	Findings	References
A. pinnata	16.5–17.5 t fresh weight ha-1 during the wet and 21.0–22.0 t ha-1 during the dry season	It was applied 15 days before and twice after transplanting.	Fixed 52.5–55.1 kg N ha-1 during wet and dry seasons, respectively, and increased the number of tillers and grain yield.	Singh and Singh, 1986
A. pinnata	Azolla compost (10 t ha-1) followed by PK fertilizer	Applied after one month of Transplanting.	Increased rice yield by 15% compared to the control.	Adhikary and Shrestha, 2018
A. pinnata	Combination of dried Azolla (90 kg N ha-1) and (NH4)2SO4	Applied one week after transplanting.	36.67% increase of total N and an increase of dry weight and grain yield.	Agbagba et al. 2018
A. pinnata	Five grams fresh Azolla combined with NK fertilizer	Applied 5 days after transplanting	Azolla replacement showed similar results with NPK fertilizer	Khair et al. 2021
A. pinnata	10%–30 % fresh Azolla with NPK	Applied at 4 days of sowing.	Higher grain weight than without Azolla.	Ahmad et al. (2024)
A. filiculoides	Azolla compost mixed with soil at 5% (w/w)	Azolla compost mixed with soil at the start of the pot experiment	13.8% increase of grain yield over control.	Razavipour et al. 2018
A. filiculoides	Fresh Azolla (2 t ha−1) with urea and urease inhibitor	Supplied in water one day before urea application.	Reduction of NH3 volatilization (54.6%), increase grain yield (9.0%–9.7%) and apparent N recovery efficiency (ANRE) (66.0–71.3%).	Yang et al. (2020)
A. filiculoides	12.2 t dry matter   ha-1 with biochar	Basal (1 day before transplanting)	Fixed 33.80 kg N/ha and increased grain yield	Kimani et al., 2020
A. filiculoides	20 t ha-1 (330 kg N ha-1 reduced to 15% + Azolla)	Applied at 10 days after transplanting	Fixed 120 kg N ha-1 and increased apparent N recovery efficiency (ANRE).	Yang et al., 2021
A. filiculoides	50 g fresh wt ha-1 equivalent to 22.8 kg dry wt ha-1	Incorporated at 46 days after transplanting	Fixed 40 kg N ha-1 and increased rice yield.	Talley and Rains, 1980
A. filiculoides	2 t ha-1  Azolla with 80 kg N t ha-1 and 40 kg N t ha-1	Dual at 25 and 40 days after transplanting	Increased uptake of N, P and S, SOC and grain yield.	Hossain et al., 2001
A. microphylla	100 g fresh Azolla	Applied at 10 days after transplanting	Increase survival rate of newly rice	Donayre et al. 2022
A. microphylla	Azolla dry matter (187.5 kg N ha−1)	Applied two weeks before transplanting.	Reduced NUE, while increased grain yield.	Suraphonphinit et al. 2023
A. microphylla	50-60% Azolla compost mixed with NPK fertilizer	Distributed with soil homogeneously before transplanting.	Reduced usage of synthetic fertilizers by up to 60% and increased grain yield.	Seleiman et al. 2022
A. microphylla	Fresh Azolla (12.2 g dry weight pot−1) combined with NPK fertilizer.	Applied one day before transplanting	Reduction of N2O emission (81.1%) and increase grain yield (36.3%).	Kimani et al. 2020
A. caroliniana	Fresh Azolla (100 kg N ha−1) with moderate N fertilizer (200 kg N ha−1 a−1)	Applied 7 days early and late rice transplanting followed by N fertilizer.	Reduced CH4 emission by 25.3% and yield-scaled CH4 emission while increased grain yield.	Xu et al. 2017
A. caroliniana	Fresh Azolla (1Mg ha−1)	Applied 7 d after transplantation.	Suppression of CH4 emission	Liu J et al. (2017)
A. pinnata and A. filiculoides	Fresh Azolla (30-90 kg ha−1)	Basal (before transplanting) or dual or basal plus dual	Improvement of rice growth, organic matter and potassium in soil.	Bhuvaneshwari and Singh, 2015

Azolla application has been shown to improve the number of tillers, the number of panicles per hill, the number of spikelets per panicle, and the percentage of filled grains (Watanabe, 1982). The incorporation of 8-10 tons of Azolla ha^-1 produced a 47% increase in grain yield over the control (Singh and Singh, 1987). An increase in grain yields of rice from 14 to 40% has been reported, with Azolla being used as a dual crop and by 15-20 % being monocropping during the fallow season (Samal et al., 2020). Utilization of Azolla increases yield in organic Basmati rice and improvement in grain and soil quality (Bhuvaneshwari, 2012). These findings showed that the application of Azolla as a biofertilizer resulted in a positive and significant improvement in rice yield.

Inevitably, the rising demand for N fertilizer has led to higher investment costs for farmers. In addition, this fertilizer has an efficiency rate of only 50%, as half of each application is lost and toxic to the environment. Moreover, excessive N application also negatively impacts rice production, as high levels of this nutrient can delay rice flowering, modify grain appearance, and reduce yield, nutritional content, and cooking quality (Chanapanchai et al., 2025).

Improving or maintaining soil quality and fertility

Azolla biomass contains higher quantities of nitrogen and potassium than other green manures, ranging from 3% to 5% and 3% to 6% on a dry-weight basis, respectively (Singh et al., 1988). Moreover, it has been reported that Azolla offers several beneficial long-term effects, including the enhancement of soil organic matter, an increase in total N, P, K⁺, Ca²⁺, and Mg²⁺, as well as an elevation in the activity of soil urease and phosphatase, and other biological properties (Prabakaran et al., 2022). The incorporation of Azolla has been shown to boost soil microbial populations, including bacteria, fungi, actinomycetes, and enzyme activities (Mandal et al., 1999).

The rise in phosphorus availability resulting from the incorporation of Azolla has been linked to the breakdown of Azolla’s biomass, which is rich in organic phosphorus, leading to the subsequent release of phosphorus in an available form (Mandal et al., 1999). Other potential mechanisms contributing to the increased phosphorus availability in soils following the decomposition of Azolla’s biomass may include reduction and chelation (Mandal et al., 1999). The enhanced phosphorus availability in soils due to Azolla incorporation promotes phosphorus uptake by rice plants (Singh and Singh 1987). According to Sampaio et al. (1984), the phosphorus in Azolla was comparable to that of algae and far more accessible than that in Ca(H₂PO₄)₂, the main ingredient in the popular phosphorus fertilizer triple superphosphate. It has been reported that Azolla-derived phosphorus was approximately 100 times more available than that found in triple superphosphate fertilizer (Kumarasinghe and Eskew 1993). Providing Azolla with more phosphorus (which should be added to rice and its subsequent crops, in particular) may enhance its availability and improve nitrogen fixation (Kumarasinghe and Eskew 1993). It was observed that rice plants grew more harmoniously and responded better to micronutrients (Fe, Mn, Zn, Cu, and Co) supplied through Azolla. However, mineral salts containing the same levels of micronutrients as those found in Azolla were toxic to rice plants (Singh and Singh 1987).

After decomposing, the algal biomass produces humus, which increases the amount of organic matter in the soil (Mandal et al., 1999). The persistence of such biomass in soil as organic matter, however, depends on its decomposability. The biomass of some algae is decomposed quickly, while that of others lasts longer (Watanabe, 1982). Decomposition susceptibility of algae is also related to their physiological growth stages. Mature Azolla biomass may persist in soil for a more extended period, and possibly results in an increase in the organic matter content of soils at a faster rate than is usually seen (Mandal et al., 1999). However, only a long-term inoculation/incorporation study can provide conclusive evidence on the rate of soil organic matter buildup from Azolla and its benefits, as in tropical and subtropical conditions, the majority of the organic carbon fixed by Azolla may be lost from the soil due to its rapid biochemical oxidation at the higher temperatures that prevail during the summer months (Mandal et al., 1999). Inoculation of rice fields with Azolla may improve soil structure. Azolla is known to excrete extracellularly several compounds like polysaccharides, peptides, lipids etc. during their growth in soils (Bertocchi et al. 1990). These compounds diffuse around soil particles and hold/glue them together in the form of microaggregates. Besides, these compounds, particularly polysaccharides, are composed of fibers that can also entangle clay particles and form clusters. These clusters or microaggregates, in turn, grow and take the shape of macroaggregates and subsequently of larger soil aggregates (Mandal et al., 1999). It has been reported that the humus formed by decomposition of Azolla biomass increases water-holding capacity, improves soil porosity, and decreases specific gravity (Mandal et al., 1999; Marzouk et al., 2023; Chanapanchai et al., 2025). All these findings suggest that the application of Azolla to paddy fields is most effective in improving the physical, chemical and biological properties of soil, thereby enhancing nutrient cycling and improving soil fertility.

Controlling the growth of aquatic weeds

In agricultural practices, controlling weeds in rice cropping systems is typically achieved using chemical herbicides such as Glyphosate, fenoxaprop-p-ethyl, propanil, and pyrazosulfuron-ethyl (Popp et al., 2013). Effective weed control is essential for maximizing rice production. Weeds compete with rice for light, nutrients, water, and space (Hossain et al., 2020). In the absence of weed management, yield losses can be estimated between 16% and 86%, and in some cases, they even reach 100% (Hossain et al., 2020). The extent of yield reduction is influenced not only by the level of infestation but also by the types of weed species present (Marzouk et al., 2023). However, these herbicides are potent chemicals that can have detrimental effects on both soil and water quality, impacting non-target organisms. Furthermore, the ability of Azolla to suppress weeds has been documented in literature globally since 1927, yet it has received limited attention in smallholder rice farming systems in many developing countries (Singh et al., 2021). Azolla cover has been shown to completely inhibit six weed species, including Scirpus juncoides Roxb. var. Hotarui Ohwi, Cyperus serotinus Rottb, Echinochla oryzicola Vasing, Eclipta prostrate L., and Monochoria vaginalis Burm. f. Presl var. Plantaginea (Roxb.) Solms-Laub (Biswas et al., 2005). There are two primary mechanisms through which this suppression occurs: the first and most effective is light-starvation of young weed seedlings due to sunlight blockage, while the second mechanism involves the physical barrier to weed seedling emergence created by a dense, interlocking Azolla mat, which does not hinder rice growth (Marzouk et al., 2023). A study conducted by Singh et al. (2021) evaluated the effectiveness of Azolla compared to herbicides in weed control and found that Azolla eliminated grasses (Echinochloa colona, Digitaria sanguinalis, Echinochloa crus-galli, and Panicum repens) and broad-leaved weeds (Fimbristylis miliacea, Digera arvensis, Commelina benghalensis, Cyperus rotundus, Convolvulus arvensis, and Cyperus esculentus), as well as reducing overall weed density.

Mitigation of GHG emissions and NH₃ volatilization

Agriculture is responsible for the emission of three primary atmospheric GHGs, namely carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) (Das and Adhya, 2014). Among these, rice cultivation stands out as a significant anthropogenic contributor to CH₄, representing 22% of the total agricultural CH₄ emissions (Das and Adhya, 2014). According to Liu et al. (2017), the practice of dual cropping Azolla with rice enhances the dissolved O₂ levels at the soil-water interface, which in turn raises the soil redox potential and lowers pH, creating an acidic environment that is detrimental to methanogens, thereby effectively curtailing CH₄ emissions (Liu et al. 2017). Bharati et al. (2000) found that the dual cropping of Azolla alongside urea-N led to a 40% reduction in CH₄ flux compared to the application of urea-N alone. The observed decrease in CH₄ emissions in plots with dual cropping of Azolla may be attributed to the oxygen released into the standing water by the growing Azolla, which results in less reduced soil conditions (Bharati et al. 2000). Conversely, the incorporation of Azolla as green manure in conjunction with urea-N resulted in elevated CH₄ flux compared to the no-N control for up to 60 days. This variation was linked to carbon released from root lysis or exudates following flowering (Bharati et al. 2000). In laboratory incubation studies, Prasanna et al. (2002) observed a swift decline in CH₄ concentration in soil samples from rice fields treated with Azolla and urea-N, regardless of the urea-N application rate, compared to soil cores from plots treated solely with urea. The increased CH₄ oxidation was attributed to the aeration provided by the oxygen released from Azolla (Prasanna et al. 2002). Results indicate that Azolla can significantly reduce the production and oxidation of CH₄. Adhya et al. (2000) noted that the addition of Azolla could significantly reduce CH₄ emissions from rice fields compared to other organic amendments, such as Sesbania and farmyard manure (FYM).

The reduction of atmospheric CO₂ through the cultivation of Azolla has been the subject of extensive research, revealing that integrating Azolla into agricultural practices significantly contributes to the decrease of the global CO₂ budget. It has been estimated that promoting Azolla growth in Sri Lanka's wetland paddy fields can result in a reduction of 509,422 tons of CO₂ from the atmosphere. In a similar vein, research conducted in the UK demonstrated that A. filiculoides sequesters 32.54 metric tons of CO₂ per hectare per year, a rate that surpasses that of grasslands, forests, and algae (Dawson and Smith 2007).

Azolla serves as a nitrogen source, functioning as a slow-release nutrient that minimizes nitrogen loss and reduces N₂O emissions (Bharati et al., 2000). The primary factor contributing to NH₃ volatilization or gaseous nitrogen (N₂O) loss is the elevated concentration of available NH₄ and the high pH levels of the floodwater (Jeyapandiyan and Lakshmanan, 2014). To mitigate NH₄ concentration in the floodwater, urease inhibitors and slow-release products have been employed. The impact of an Azolla cover in urea-amended plots, treated with nitrogen rates of 0, 40, 80, 120, and 160 kg N ha⁻¹, was evaluated in comparison to plots receiving only urea, focusing on floodwater chemistry and NH₃ volatilization (Jeyapandiyan and Lakshmanan, 2014). The results indicated that a complete Azolla cover on the floodwater surface during urea application inhibited the rapid and significant rise in floodwater pH that typically accompanies urea hydrolysis and the algal photosynthetic processes. With the presence of an Azolla cover, the average floodwater pH decreased by as much as 1.9 units, maintaining the maximum pH below 8.3. Conversely, in the absence of an Azolla cover, the floodwater pH exceeded 8.5, reaching a peak of 10.1. Additionally, the floodwater temperature was reduced by up to 5 °C. Consequently, the partial pressure of NH₃, which serves as an indicator of potential NH₃ volatilization, was markedly lowered. The nitrogen loss observed in the Azolla-rice-soil system varied from 0.01 to 23%, while in the absence of an Azolla cover, nitrogen losses ranged from 21 to 49% (deMacale and Vlek 2004; Kollah et al., 2016).

Bioremediation of hazardous pollutants

Azolla facilitates the bioremediation of trace metals through processes of accumulation and biosorption (Jafari et al. 2010). The presence of heavy metals in wastewater can be effectively managed through bioremediation using Azolla. Following bioaccumulation, the biomass of Azolla is readily harvestable due to its rapid desiccation (Sachdeva and Sharma 2012). The bioaccumulation characteristics of Azolla render it an ideal candidate for bioremediation systems (Sachdeva and Sharma 2012), which can be employed to treat contaminated waters or sewage for agricultural applications. The potential of various Azolla species to bioremediate trace metals differs significantly. For instance, A. pinnata has been reported to remove 70-94% of trace metals (specifically Hg and Cd) from ash slurry and chlor-alkali effluent in Singrauli, India, with the concentration of these trace metals in Azolla tissues ranging from 310 to 740 mg kg^-1 dry mass (Rai, 2008). In a hydroponic study, it was observed that Azolla thrived under arsenic (As) concentrations varying from 29 to 397 mg kg⁻¹ dry mass. Among the species, Azolla caroliniana exhibited the highest accumulation of As at 284 mg kg⁻¹ dry weight, while A. filiculoides accumulated 54 mg kg⁻¹ dry weight. Furthermore, A. filiculoides was noted to accumulate trace metals such as Cd, Cr, Cu, and Zn at concentrations of 10,000, 1,990, 9,000, and 6,500 ppm, respectively (Sela et al. 1989). Different species of Azolla have great differences in their ability to absorb trace metals. It was noted that the highest bioconcentration potentials for Pb²⁺, Cu²⁺, Mn²⁺, and Zn²⁺ were 94% in A. microphylla, 96% in A. filiculoides, 71% in A. pinnata, and 98% in A. microphylla, respectively (Jafari et al. 2010). The biomass generated after phytoremediation can serve as a source for bioenergy production or as a bio-ore for the recovery of commercially valuable trace metals (Chanapanchai et al., 2025). Additionally, the residual biomass after heavy metal extraction can provide a rich source of protein for animal feed or can be utilized as green manure (Chanapanchai et al., 2025). There are numbers of other hazardous pollutants that have been effectively remediated by various species of Azolla, including petroleum products (Diesel hydrocarbon, BTEX, crude oil), antimicrobial pharmaceuticals (Sulfadimethoxine) and dyes (Acid red 88 (AR88) (Kollah et al., 2016).

CONCLUSIONS

For centuries, Azolla has been acknowledged as a promising biofertilizer in the production of lowland rice. The use of Azolla presents a cost-effective solution that offers substantial long-term benefits for environmental conservation, as well as a significant capacity to provide fixed nitrogen and other essential nutrients such as potassium (K⁺), sulfur (S), magnesium (Mg²⁺), and phosphorus (P) in lowland rice production systems. When Azolla is properly adopted and utilized, it enhances nitrogen use efficiency, thereby reducing nitrogen loss and the reliance on synthetic nitrogen fertilizers. Moreover, dual cropping with Azolla has been shown to decrease CH₄ emissions by 40% compared to the use of urea alone, while also promoting the oxidation of CH₄. Owing to its rapid growth rate, Azolla has proven advantageous in enhancing soil organic carbon levels, increasing microbial biomass, and improving nutrient cycling. Additionally, it demonstrates significant bioremediation capabilities for heavy metals such as cadmium (Cd), chromium (Cr), copper (Cu), and zinc (Zn), as well as for petroleum products including diesel hydrocarbons, BTEX compounds, and crude oil, along with antimicrobial pharmaceuticals like sulfadimethoxine and dyes such as Acid Red 88 (AR88). The incorporation of Azolla in agricultural practices contributes to improved socioeconomic conditions by lowering agricultural input costs and creating job opportunities for small-scale industries involved in its cultivation. However, the application of Azolla for sustainable practices remains limited due to several factors: (1) social and political barriers that hinder the establishment of initiative programs to provide subsidies or credits for adopting this technology in many developing nations; (2) a lack of awareness programs for end users, particularly farmers, necessitating intensive extension programs; and (3) the need for scientific research initiatives aimed at developing efficient strains tailored for specific applications. In addition to these challenges, the propagation of Azolla is primarily confined to tropical regions; however, if strains capable of thriving at lower temperatures are identified, they could potentially be utilized in temperate climates. The review advocates for increased collaborative research efforts to optimize the utilization of this vital natural resource for sustainable agricultural practices.

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