Problems and prospects of cyanobacterial biofertilizer for rice cultivation

Abstract

Nitrogen (N₂)-fixing cyanobacteria are a dominant microflora in rice fields and are currently being used sporadically as a supplement to chemical nitrogen fertilizers for rice cultivation in rice-growing countries, including India and Bangladesh. This technology suffers from serious drawbacks and its use at the farm level is not gaining universal acceptance due to some major problems, which include development of a suitable production technology of biofertilizer for field use, establishment of the applied biofertilizer in the rice field and the sustainability of the technology. In order to significantly improve the efficient use of cyanobacteria as a N-based biofertilizer for rice cultivation, experiments were carried out in different dimensions both in the laboratory and field. Cyanobacterial strains were isolated, identified and quantified from a wide range of distinctively different types of soils, viz. acid, calcareous, saline, red and neutral soils under different agro-ecological zones (AEZ) of Bangladesh. The isolated strains were tested for their N₂-fixing capacity and growth rate under various stress conditions prevailing in the rice field e.g. pH, combined N, pesticides, salinity and nutrient availability in order to select suitable strains for use as biofertilizer. Large-scale cyanobacterial biofertilizer was produced with the strains showing high rates of growth and N₂ fixation both in liquid cultures under laboratory conditions and in soils of their habitats and non-habitats under open air. To assess the effectiveness of the produced biofertilizer, field trials at the selected locations were carried out on rice. Results of the field trials showed that cyanobacterial biofertilizer may reclaim the problem soils such as acid soils and saline soils, improve the fertility status and may supplement 25–35% N for rice cultivation in these soils. This biofertilizer may be used in improving the soil environment.