Useful carriers for cyanobacteria: their response to cyanobacterial growth, acetylene-reductase activity, cyanobacterial grazers and paddy yield in calcareous soil.

摘要

Snails and nematodes, the potential cyanobacterial grazers, differ in their choice for cyanobacterial diet. Snails prefer non-mucilaginous forms while nematodes prefer mucilaginous forms. Such differences in feeding choice between the cyanobacteria suggests that it may not be possible to select strains of diazotrophic cyanobacteria that are resistant to all grazers. The potential consumption of cyanobacteria at an average field density of 20 000 snails ha-1 was estimated to be about 50 kg (fresh weight) ha-1 day-1. Dorylamus sp. was most dominant nematode associated with cyanobacterial consumption. Phytoextracts of neem (Azadirachta indica), bel (Aegle marmelos) and tobacco (Nicotiana tabacum) were effective in controlling these cyanobacterial grazers. The minimum concentration of neem, bel and tobacco phytoextract in water for 100% mortality of snails were 0.1, 2.0 and 0.05%, respectively. However, trepellent level was only 0.01% for neem and tobacco phytoextract. Complete mortality of nematode (Dorylamus spp.) required a higher concentration level (2%) even in the most effective tobacco phytoextract. Lower levels of phytoextract (0.1%) were found to stimulate growth and nitrogen fixation of cyanobacteria. Application of these plant biomasses resulted in significant increase in cyanobacterial acetylene-reducing activity (ARA) and rice yield and a significant decrease in snail and nematode population. Augmentation of cyanobacterial acetylene-reducing activity was two to three times higher in comparison to the control in both the years of experimentation. Rice yield also increased between 3.8 and 58.5% over the control, depending on the quantity and nature of plant biomass. Tobacco waste was significantly superior in comparison to neem and bel biomass as carrier of cyanobacterial culture..