Algal Biofertilizer for Vital Soil and Free Nitrogen
The diazotrophic cyanobacteria substantially contribute to the fertility of the soil, especially under tropical rice field conditions. In addition to contributing up to 30kg N, these algae benefit the crop plants through excreting part of the biologically fixed nitrogen and secreting growth promoting substances like vitamins and amino acids, Algalization increases the fertilizer use efficiency of crop plants, reduces the loss of fertilizer nitrogen.
Introduction:
Soil is a dynamic system in which the physical, chemical, and biotic components are in a state of equilibrium. Application of chemical fertilizers without taking care of the other soil constituents disturbs this equilibrium which affects the productivity of the soil. Biofertilizers are the microorganism which helps in better crop nutrient management and maintenance of the soil health while working in perfect harmony with nature.
Under waterlogged conditions of rice fields, the diazotrophic cyanobacteria play a vital role in maintaining the soil fertility and sustaining the crop yield even in the absence of any nitrogenous fertilizer. Recently, algalization has been recognized as an important input in rice cultivation as it forms a perpetually renewable source of nutrients and improves soil health.
Nitrogen Fixation by Blue-green Algae :
The term Algal biofertilizer was coined in the early 70s to embody such blue-green algae that are capable of growing or molecular nitrogen. These algae, with the help of enzyme nitrogenase, bring about the reduction of molecular nitrogen tc ammonia at ambient conditions. This enzyme is inactivated by the presence of oxygen because of the oxidation of the labile sulphide groups and reduced synthesis of both the components. In cyanobacteria, under aerobic conditions, nitrogen fixation is carried out in specialized cells known as heterocysts, which provide necessary anaerobic conditions for the activity of nitrogenase. Heterocysts have thick walls that physically prevent the entry of oxygen; they have a high rate of respiration which scavenges the diffused oxygen and lacks photosystem ||, due to which there is no endogenous evolution of oxygen during photosynthesis. Till the findings of Wyatt and Silvey(1969), only the heterocystous cyanobacteria were considered to be capable of fixing nitrogen. Since then many non-heterocystous blue-green algal genera have been added to the list. Out of these, only Gloeocapsa is an aerobic nitrogen fixer while the rest need anaerobic or microaerophilic conditions. Stewart and Lex (1970) had opined that all the vegetative cells of the trichome of diazotrophic blue-green algae contain nitrogenase, but in the vegetative cells, it gets inactivated due to the presence of oxygen.
Like Haber- Bosch process, conversion of elemental nitrogen to ammonia is an energy-intensive process. The necessary for the process in the cyanobacteria is supplied through oxidative phosphorylation and photophosphorylation. When placed in an environment of combined nitrogen, the nitrogen-fixing cyanobacteria show accelerated growth and progressively reduce their dependence on molecular nitrogen. Thus it seems that the cyanobacteria resort to nitrogen fixation only under combined nitrogen-deficient conditions. Deficiency of combined nitrogen has been shown to cause degeneration of photosystem || in Anabaena variabilis and Mastigocladus laminosus, which leads to the activation of the nitrogenase because of the creation of anaerobic conditions. In presence of combined nitrogen, the enzyme nitrogenase remains repressed and this effect is more pronounced in the presence of ammonium nitrogen.
Similar to oxygen inhibition, chemical nitrogen inhibition of nitrogenase in blue-green algae is reversible. According to Stewart and Gallon (1980), this difference from the bacterial nitrogenase is not regulated by glutamine synthesis (GS), instead, the presence of ammonia increases ADP/ATP ratio and carbamyl phosphatase activity which in turn cumulatively and independently inhibit the nitrogenase activity. Removal of ammonium nitrogen stress reduces the ADP/ATP ratio to less than 0.5 and brings down the carbamyl phosphates activity which depresses the nitrogenase. The revival of nitrogenase activity takes place within about 24hour pf the removal of the stress.
Higher concentrations of combined nitrogen have been shown to inhibit both growth and nitrogen fixation. However, the tolerance limit for growth is much higher than for nitrogen fixation. Ammonium nitrogen is the putative inhibitor of nitrogen fixation and proves toxic for algal growth at 75ppm. Nitrate nitrogen is preferred by cyanobacteria and even at 100ppm, there is no detrimental effect on growth and only a marginal reduction is observed in nitrogen fixation. Stewart reported that in certain cyanobacteria, the nitrogen fixation continuous even at 50ppm ammonium nitrogen.
Nitrogen contribution by Blue-Green Algae :
Tropical conditions ensure increased incidence of cyanobacteria in the rice field soils and high humidity and temperature and shade provided by the crop canopy favor the luxuriant growth of cyanobacteria. Direct evidences showing enrichment of the soil nitrogen due to algal application have been deduced using 15N and the ARA technique has shown the addition of 18-45kg N ha-1 due to the activity of diazotrophic cyanobacteria. The availability of nitrogen fixed by the blue-green algae to the rice plant was shown with the help of 15N studies conducted by Reynault etal. Cyanobacteria have also been shown to liberate a variety of amino acids like aspartic acid, glutamic acid, and alanine, and their liberation had to be found maximum during the stationary phase. Being soluble in water, they form readily available sources of nitrogen for the crop plants.
Growth and yield of the rice crop provide an indirect estimate of the algal contribution. A series of multilocational trials conducted with different rice varieties under varying agro-climatic conditions have shown that algal inoculation can result in an addition of up to 30kg N ha-1 season-1. This, however, depends upon the agroecological conditions which regulate the establishment and activity of the introduced algae.
In addition to providing biologically fixed nitrogen, algal biofertilizer increases the availability of free nitrogen. The typical rice field condition favor denitrification resulting in rapid loss of the fertilizer nitrogen. Colonization of the rice field soils by the blue-green algae appreciably reduces this loss through the metabolization of the combined nitrogen. The nitrogen-fixing machinery of cyanobacteria is endowed with the unique property of temporarily shutting off the process of utilization of the molecular nitrogen when placed in an environment containing combined nitrogen. Under field conditions in presence of higher levels of fertilizer nitrogen, the cyanobacteria grow faster and may fix comparatively much less nitrogen. But they revert back to nitrogen-fixing when the combined nitrogen stress is removed due to progressive utilization and natural loss. The increased algal biomass is now expected to contribute much more nitrogen than what it would have contributed to the absence of chemical nitrogen. Thus the cyanobacteria prevent a part of the applied fertilizer nitrogen from being lost and make it available to the crop plants at a later stage.
Algalization and crop yield
Algal nitrogen fixation under no circumstances can completely substitute the nitrogenous fertilizers in rice cultivation. Algalbiofertilizer is recommended only as a supplement to nitrogenous fertilizers. The supplementation effect remains perceptible even in the presence of high levels of fertilizer nitrogen.
Conclusion
Algal biofertilizers can substantially contribute to making rice cultivation sustainable on long term basis. It helps the small and marginal farmers by providing a part of the much-needed nitrogen. It helps the small and marginal farmers by providing a part of the much-needed nitrogen. It enables the crop to utilize more of the applied fertilizer nitrogen and contributes to the vitality of the soil. The sustained addictive effect due to population build-up is an added attribute of this input. All that is needed is to identify and develop promising region specific stress compatible strains and place quality inoculum within easy reach of farmers.
1 comments:
Click here for commentsThere is a deep relationship between fertilizer and soil in the same way as there is between supplements and the human body. To maintain a healthy lawn, grow few vegetables, and manage the other crops we need proper fertilizer that can cover up the need for micro and macronutrients in the soil.
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