Nutrient Flow in an Integrated Rabbit–Fish–Rice System in Rwanda

Abstract

two Abstract An analysis of nutrient flow, based on nitrogen (N) and phosphorous (P), was conducted on an integrated rabbit–fish–rice system (IRFR) system at the Rwasave Fish Farming Station (National University of Rwanda). Rabbits, stocked at 12 per are (1200 rabbits.ha -1 ) of pond, were reared over fishponds stocked with one and three male tilapia ( Oreochromis niloticus) per m 2 for pond treatments PT1 and PT2. Effluent fertilised by the rabbits was drawn away from the ponds by pipes installed at the bottom of the ponds to irrigate rice ( Oryza sativa L., variety Yuni Yin4 ) fields . There were six 400 m 2 ponds and nine 90 m 2 rice fields; three of the latter were irrigated by canal water and fertilised by NPK (200 kg.ha -1 .crop -1 , 2 applications; 100 kg.ha -1 .crop -1 , one application). The results showed that rabbit droppings supplied about 27% N and 79% P of the total N and P inputs, fertilising the ponds at a rate of 3.98 kg N and 1.94 kg P.ha -1 .d -1 . The fish recovered 18.5-7.6% N and 16.9-34.3% P of the total nitrogen (TN) and total phosphorus (TP) inputs. All water quality variables remained within good limits for tilapia aquaculture and nutrient distribution was not dependent on fish density. Large amounts of N and P accumulated in the water, sediment, and effluent fertilised rice fields at a higher rate (118.5 kg N and 27.2 kg P.ha -1 .d -1 ) than that of inorganic fertilizers, resulting in a slightly higher rice yield than that induced by NPK and urea. Tilapia effluent was thus able to substitute inorganic fertilisers completely, allowing savings to the farmers, and showing its potential as a fertiliser for fish and crop production rather than waste to be discharged, polluting the environment by its solids and organic matter component. Further studies involving a thorough analysis of nutrients lost and diversified uses of the nutrient-rich effluent are needed.