Effect of coconut waste and its biochar as hydroponics substrates on system performance and nitrogen transformation in aquaponics

Aquaponic systems combine aquaculture and hydroponics, creating a circular and sustainable agricultural model that optimizes nutrient utilization. Improving nitrogen use efficiency is crucial for the system's operational performance and nutrient cycling. This study explored the growth response of cherry tomatoes (Lycopersicon esculentum var. Cerasiforme) to different substrates, including coconut wastes and their biochar and medical stone, in the decoupled aquaponics system, focusing on reusing ammonia, nitrate and nitrite from crucian carp (Carassius auratus) farming effluent. Cherry tomato plants physiological information (plant height, fresh weight of the whole plant, and weight of tomato fruits, etc) and water quality parameters (ammonia nitrogen, nitrite nitrogen and nitrate nitrogen, pH) were monitored across different compartments. Results showed that using 6 mm columnar biochar as a substrate significantly improved the fresh weight, chlorophyll content, and overall weight of tomato plants, and the nitrogen conversion rate from aquaculture effluent to cherry tomatoes exceeded 89 %. Within 76 days, the relative removal rates of ammonia, nitrite and nitrate were above 84.20 %, 95.13 % and 83.98 %, respectively. The biochar composite with a 6 mm particle size demonstrated superior nitrogen immobilization and removal, suggesting a novel approach for the resource utilization of biomass materials. In conclusion, using biochar composite as a cultivation substrate improves nitrogen conversion rates and overall system efficiency of the aquaponic system, providing an effective and sustainable method for biomass material utilization.