Optimization of Plant Nutrition in Aquaponics: The Impact of <i>Trichoderma harzianum</i> and <i>Bacillus mojavensis</i> on Lettuce and Basil Yield and Mineral Status

The present study aims to test the effect of a nutrient solution, with the addition of microbial inoculum, on the growth and mineral composition of ‘Hilbert’ and ‘Barlach’ lettuce cultivars (<i>Lactuca sativa</i> var. <i>crispa</i>, L.) and basil (<i>Ocimum basilicum</i>, L.) cultivated in a vertical indoor farm. These crops were grown in four different variants of nutrient solution: (1) hydroponic; (2) aquaponic, derived from a recirculating aquaculture system (RAS) with rainbow trout; (3) aquaponic, treated with <i>Trichoderma harzianum</i>; (4) aquaponic, treated with <i>Bacillus mojavensis</i>. The benefits of <i>T. harzianum</i> inoculation were most evident in basil, where a significantly higher number of leaves (by 44.9%), a higher nitrate content (by 36.4%), and increased vitamin C (by 126.0%) were found when compared to the aquaponic variant. Inoculation with <i>T. harzianum</i> can be recommended for growing basil in N-limited conditions. <i>B. mojavensis</i> caused a higher degree of removal of Na⁺ and Cl⁻ from the nutrient solution (243.1% and 254.4% higher, in comparison to the aquaponic solution). This is desirable in aquaponics as these ions may accumulate in the system solution. <i>B. mojavensis</i> further increased the number of leaves in all crops (by 44.9–82.9%) and the content of vitamin C in basil and ‘Hilbert’ lettuce (by 168.3 and 45.0%) compared to the aquaponic solution. The inoculums of both microbial species used did not significantly affect the crop yield or the activity of the biofilter. The nutrient levels in RAS-based nutrient solutions are mostly suboptimal or in a form that is unavailable to the plants; thus, their utilization must be maximized. These findings can help to reduce the required level of supplemental mineral fertilizers in aquaponics.