A Study of Nitrogen Cycle in an Integrated Aquaponic System with Different Plant Densities

Abstract

Introduction: There is no doubt that a fundamental element like nitrogen is one of the most important components in both animal and plants cells. Within an integrated aquaponic system, nitrogen cycle has highest priority because it converts fish wastes into nutrients for plants. Aims: The main goal of this study is to quantify the nitrogen budget for an integrated rainbow trout - spinach aquaponic system, where three plants densities were used. The second objective is to determine a balanced plant density for optimal nitrogen removal rate and hydroponic vegetable production. Materials and methods: The experimental design consist in a recirculating aquaculture system with 12 growing units, mechanical and biological water treatment units and four aquaponic units. Three plants densities were used (V1 - 59plants/m 2 , V2 - 48plants/m 2 and V3 - 39plants/m 2 and a control variant V4). Fish were fed with two types of feed (41% and 50% protein), using 3 different feeding regimes. Water samples were taken and analysed by using Merck kits so that nitrate, ammonium and TAN retention rates will be observed. Water oxygen, pH and conductivity levels were also monitored. The meat, plants and faeces nitrogen content was determined by Kjeldahl method. The feed protein content was verified using the same method. Results: The amount of nitrogen removed from integrated aquaponic system through biological filtration and also by each of the three tested spinach biomass densities was determined apart. The nitrate, ammonium and TAN retention rates, as water passes through mechanical filter, were found to be insignificant (p>0,05), compared to ones from the biological filter that were higher. Differences between the retention rates for each of the three variants of tested plants densities were significant higher (p<0,05) at V1 compared to V3 and also higher at all three variants comparing them to the control variant. Also differences between plants nitrogen composition from V3 compared to V1 were found significant higher (p<0,05). The content of nitrogen from fish meat and fish faeces was found to be according to literature. Conclusion: In the present study, nitrate, ammonium and TAN retention rates were found to be related in a certain way with plants density and total nitrogen input in the aquaponic system, facts that also influence the plants nitrogen content. It is recommended higher plants densities to be used and also a better light intensity can be also used.