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Benefits of Azolla in lowland rice (Oryza sativa) cropping systems concerning N losses and fertilization

F. Hatt, V. Weber (2008)

Azolla, a free-floating aquatic fern having symbiotic association with the N2 fixing cyanobacterial symbiont Anabaena Azollae can fix 30-60 kg N/ha in 30 days. It is either incorporated as green manure at the beginning of the cropping season or grown as a dual crop along with rice, in the standing water of flooded fields. The fern is used to a great extend in China, Bangladesh and Vietnam as an important biological source to improve the N balance of rice fields. The nitrogen fixed by the cyanobacterial symbiont is either released upon decay of the incorporated Azolla or leached into the standing water from the growing Azolla and is available for uptake by the rice crop (Bharati, Mohanty et al. 2000). Additionally Azolla can promote aerobic transformations such as methane oxidation through enhanced aeration of the flood water in rice fields (Prasanna, Kumar et al 2002).

1. N fertilization by N2 fixing Azolla
Soils in traditional cultivation areas in South- and Southeast Asia are often nitrogen deficient. As a result N must be applied. For this mineral fertilizers e.g. urease and green manure e.g. Azolla is used. N fertilizer consumption for rice production stands are around 10 million t/a (Cissé and Vlek 2003).

2. Reduction of N losses by Azolla
2.1. Reduction of methane emission
Methane, the most abundant gaseous hydrocarbon in the atmosphere, is an important greenhouse gas that may account for approximately 15-20% of the total current increase in global warming (Bharati, Mohanty et al. 2000). Flooded rice paddy has been identified as one of the important sources of anthropogenic CH4. Flooded soils planted to rice are conductive to the production and emission of methane due to the presence of methanotrophs that utilize readily decomposable organic compounds under anaerobic soil condition (Bharati, Mohanty et al. 2000). The methanotrophs oxidize methane to carbon dioxide deriving energy from this reaction (Prasanna, Kumar et al 2002). With the intensification of rice cultivation to meet the needs for rising population, methane emission from this important ecosystem is likely to increase. However a reduction of 15-20% would result in stabilization of the methane concentration in the atmosphere to that of 1990 levels (Bharati, Mohanty et al. 2000). Azolla is an interesting opportunity for harvesting greenhouse gases such as carbon dioxide (Prasanna, Kumar et al 2002). The study of Bharati, Mohanty et al. (2000) indicated that dual cropping of Azolla reduced methane flux and yet increased grain yield similar to that of urea application. The decrease in methane efflux in plots with dual crop of Azolla could be related to the release of oxygen in the standing water by the growing Azolla leading to less reduced conditions in the soil (Bharati, Mohanty et al. 2000).

2.2. Reduction of NH3 volatilization
Ammonia volatilization and the gaseous emission of NH3 to the atmosphere are reportedly the major cause of low N fertilizer efficiency and important mechanisms for N losses in lowland rice fields. The low N fertilizer efficiency in rice cultivation is due to the low recovery of the fertilizer. In Asia the average N fertilizer recover efficiency in farmers fields is currently only about 30% (De Macale and Vlek 2004).
The fertilizer which is not recovered in plant material is lost as gas in form of NH3 to the environment. The most important factors in NH3 volatilization are high NH4 concentration of the floodwater combined with a high pH. Because of the high volatilization losses several options to increase urea efficiency were investigated. Therefore the NH4 concentration in the flood water was reduced using urease inhibitors, slow release products or urease super granules. Reduction of the floodwater pH was attempted by KCL addition or algicide to inhibit algal growth. Most of these approaches involve specially chemicals and are costly or laborious and have found little adoption in developing countries (Cissé and Vlek 2003). Aside from high losses as gas, N is also transported to the ground and surface waters. Both gaseous loss and transportation to water cause substantial economic loss to farmers and create negative impacts on the atmosphere and water quality. As a result the use of the aquatic fern Azolla in improving the efficiency of applied fertilizer has generated interest (De Macale and Vlek 2004). Azolla is used as surface cover in rice fields fertilized with for example Urea. Urea has a relatively high N-content, which facilitates its transportation Unfortunately Urea-N recovery reaches commonly not more than 30-40%. The poor efficiency of urea is related to NH3-volatilization and nitrification or denitrification (Cissé and Vlek 2003). The combined use of urea and Azolla is a possibility to reduce these losses.
In the study of De Macale and Vlek (2004) Azolla cover on the floodwater surface of lowland rice prevented at the time of urea application the rapid and large increase in floodwater pH and consequently NH3 volatilization losses (see Figure 2). The lower floodwater pH in the presence of Azolla cover is explained by the absorption of available light which is the most important limiting factor of the photosynthesis rate of algae. This is especially important at the time of Urea application because due to the fertilization the growth and photosynthesis rate of the algae increase. In turn the dissolved CO2 in the flood water is reduced during daytime leading to a rise in pH. As a consequence Azolla prevents the rise in pH due to the reduction of available light. Additionally the Azolla cover reduces the floodwater temperature significantly compared to the Azolla free plots. Flood water temperature affects the relative proportion of NH3 volatilization present at a given pH. An increase in the temperature from for example 20 to 30°C doubles the initial aqueous NH3 in a system which in turn increases the potential for NH3 loss. As a consequence the partial pressure of ammonia, which is an indicator of NH3 volatilization, is significantly depressed (De Macale and Vlek 2004). All in all the loss of N from the system with Azolla cover and Urea application amounted to no more than 15% (Cissé and Vlek 2003).
Besides the lower N losses the Azolla cover has positive effects on rice development. Rice is a poor competitor for available N early in the growing season. As a result Azolla effectively competes with young rice plants for applied urea, capturing nearly twice the urea-N than the rice plants. This leads to the conservation of urea-15N in the system, yielding a complete N recovery in the system early in the growing season. In the study of Cissé and Vlek (2003) six weeks after fertilization with 100 mg N/pot, altogether 50 mg of the applied urea-N was immobilized in the Azolla biomass. Due to decomposition of dead Azolla biomass nitrogen is released and is available for rice absorption. Around tillering time the rice plants are strong enough to compete with Azolla for applied nitrogen (Cissé and Vlek 2003). As a result of the adequate N supply during the rice development the tiller count in Azolla-covered plots in the study of Cissé and Vlek (2003) was significantly increased and the grain yield was likewise improved. In total the combination of Azolla and urease produced yields that were 10% higher than without cover. This prospect is especially attractive because of the high cost of N fertilizer and the growing need to improve grain yield with minimum adverse environmental effects associated with the intensive use of N fertilizer. All in all using Azolla as a surface cover in combination with urea can be an alternative management practice worth considering as a means to reduce NH3 volatilization as well methane losses and improve N use efficiency as well as rice yields (De Macale and Vlek 2004).

3. Literature
Bharati, K., S. R. Mohanty et al. (2000). “Influence of incorporation or dual cropping of Azolla on methane emission from a flooded alluvial soil planted to rice in eastern India.” Agriculture, ecosystems and Environment 79:73-83

Cissé M. and P. L. G. Vlek (2003). “Conservation of urea-N by immobilization-remobilization in a rice-Azolla intercrop.” Plant and Soil 250: 95-104

De Macale M. A. R. and P. L. G. Vlek (2004). “The role of Azolla cover in improving the nitrogen use efficiency of lowland rice.” Plant and Soil 263: 311-321

Prasanna, R., V. Kumar et al. (2002). “Methane production in rice soils is inhibited by cyanobacteria.” Microbiol. Res. 157: 1-6

June 14th, 2008
Topic: Crop Science, Plant nutrition, Sustainable production under environmental stresses Tags: None

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