Aims: 2The in¯uence of carbon-nitrogen and nitrogen-phosphorus ratios of input fertilizers, and that of pond water, on the growth of heterotrophic and phosphate-solubilizing bacteria of water and sediment, was examined in relation to fertilizer mineralization indices using different modes of fertilization through inorganic and organic sources.
Methods and Results:The ®rst experiment used carbon-nitrogen-phosphorus ratios varying from 12 : 2 : 1 to 151 : 6 : 1, applied at the rate of 0á043 g l ±1 week ±1 , whereas in the second ratios varied from 25á6 : 6á2 : 1 to 150 : 12 : 1 applied once at the rate of 3á33 g l ±1 . Different fertilizers (cattle dung, poultry droppings, urea, single superphosphate and starch) were mixed in different proportions to achieve the desired carbon-nitrogen-phosphorus ratio. The heterotrophic and phosphate-solubilizing populations were more responsive to an early manuring phase than later, implying that pond fertilization was microbiologically more dynamic in the earlier phase. The carbon-nitrogen and nitrogen-phosphorus ratios of 11á8 (88á6 : 7á5) and 7á5 (7á5 : 1), respectively, of input fertilizers favoured growth of both heterotrophic and phosphate-solubilizing bacterial populations much better than the other ratios tested. Likewise, water carbon-nitrogen and nitrogen-phosphorus ratios of 11á9 and 3á34 induced bacterial growth. The carbon-nitrogen ratios of 12á63 (101 : 8) (input fertilizer)±4á54 (water), and nitrogen-phosphorus ratios of 8 (8 : 1) (input fertilizer)±2á93 (water), gave gross primary productivity values higher than the remaining ratios, exhibiting overall curvilinear relationships. The values of gross primary productivity were the direct function of values of fertilizer mineralization indices for carbon, nitrogen and phosphorus. Conclusions: It is concluded that the mixed fertilizer (carbon-nitrogen-phosphorus-88á6 : 7á5 : 1) comprising cattle dung (95%), poultry droppings (2á5%), urea (2%) and single superphosphate (0á5%), applied at the rate of 23 000 kg ha ±1 year ±1 , was a suitable cost-effective fertilization option for aquaculture practices. Signi®cance and Impact of the Study: As chemical fertilizers are expensive and cause some adverse effects on the soil structure, composition, micro¯ora and other characteristics of the pond, mixed combinations of inorganic and organics with narrow range of carbon-nitrogenphosphorus ratio can be suitable and cost-effective fertilization tools in aquaculture practices, which is to be linked with the microbial activities of the pond.
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