Immobilized cyanobacterial inoculant for sustainable rice production

Nitrogen fixing cyanobacteria, both symbiotic and free-living types, are known to excrete ammoniacal nitrogen under natural ecological conditions and contribute to the nitrogen fertility of rice crop. The symbiotic N2 fixing cyanobacterium, Anabaena azollae - AS-DS-SK and the free-living N2, fixing cyanobacterium, Anabaena variabilis - SA0, were immobilized in solid matrices, including polyurethane foam (PUF) and polyvinyl foam (PVF). As a result, ammonia excretion was stimulated as compared to free non-immobilized cells. Anabaena azollae -AS-DS-SK immobilized with PVF in a photobioreactor was able to release ammoniacal N continuously. Incorporation of agrochemicals like the granular insecticide Carbofuran, the synthetic fungicides Bavistin/Benlate/Vitarax/ or the insecticides Butachlor/Thiobencarb at 5 ppm into N-free BG-II medium stimulated ammonia excretion by the cyanobacterial cultures immobilized in PUP, due to their inhibitory effect on the ammonia assimilating enzyme, glutamine synthetase. The influence of PUF - and sugarcane bagasse (SCB)-immobilized A.azollae with different forms of nitrogenous fertilizers on the N uptake of rice seedlings grown under hydroponics was investigated following the sup15 N dilution technique. Through the immobilized A. azollae with the nitrate form of N fertilizer contibuted higher cyanobacterial-derived N (29-36%), the total N-content was greater in rice seedlings receiving immobilized A. azollae and urea form of N. The effect of inoculation of immobilized cyanobacterial inoculum in the transplanted rice was quite significant. Similarly, inoculation of SCB immobilized strains of A. azollae -AS-DS-SK and A. variabilis -SA0 significantly increased the flood water ammoniacal N content (1,685 and 1,540 n moles/ml, respectively on the 16th day after inoculation as compared to 595 n moles/ml in the treatment 60 kg fertilizer N alone/ha) and also increased the grain and straw yield of rice. Hence, a carrier based immobilized cyanobacterial inoculant was developed to maintain better quality with improved shelf life and survival. Field evaluation studies indicated that the carrier based immobilized cyanobacterial inoculum at 2 kg/ha could be a better alternative for the conventional soil-based cyanobacterial inoculum, to achieve sustainability and higher productivity in rice farming.