As in any agroecosystem, also in aquaponics the nitrogen (N) balance represents an important tool to evaluate sustainability, and to identify factors that can improve N use efficiency (NUE) and reduce N losses. In this respect, fish stocking density has been little investigated, hence this research aimed to evaluate the N balance of a low technology aquaponic (AP) system managed at two fish densities in comparison with a hydroponic system (HP). In the fish tanks common carp at two initial stocking densities were reared (2.5 and 4.6 kg m^–3 in low and high AP, hereafter named APL and APH, respectively) and the vegetated sector was cultivated with a leafy vegetable crop succession (Catalogna chicory, lettuce, Swiss chard). The N balance considered N input as fish feed or fertiliser, and N content in the initial water and the N output as N in the incremented fish biomass, in the harvested vegetables, in the sediments, and in the remaining water. Total N loss was estimated by difference. The total N input and the N loss through gas emission in the atmosphere were much higher in AP than in HP, particularly at high stocking density. The opposite trend was observed for the N input recovered in vegetable aboveground biomass. The N input recovered as fish biomass was slightly higher in APL compared to APH. The better results of APL than APH suggest that in low-tech AP system lower initial fish density should be adopted at the system start up to maximise both production and N recovery.

像在任何农业生态系统中一样，在水培法中，氮（N）平衡也是评估可持续性和确定可提高氮利用效率（NUE）并减少氮损失的重要工具。在这方面，很少进行鱼类种群密度的调查，因此，本研究旨在评估与水培系统（HP）相比，在两种鱼类密度下管理的低技术水培（AP）系统的氮平衡。在鱼缸中以两种初始放养密度饲养鲤鱼（2.5和4.6 kg m ^–3在低AP和高AP下分别命名为APL和APH），而无叶植物则种植有叶类蔬菜作物（Catalogna菊苣，生菜，瑞士甜菜）。氮平衡考虑将氮输入作为鱼饲料或肥料，初始水中的氮含量以及增加的鱼类生物量，收获的蔬菜，沉积物和剩余水中的氮输出作为氮。总氮损失通过差异估算。AP的总氮输入量和大气中通过气体排放引起的N损失要比HP高得多，特别是在高放养密度下。在地上蔬菜生物量中回收的氮输入观察到相反的趋势。与APH相比，APL中鱼类生物量的氮输入量略高。