Effects of Hydraulic Retention Time on Nutrient Film Technique Lettuce Production in a Decoupled Aquaponics System
Type of DegreeMaster's Thesis
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A series of 28-d hydroponic and aquaponic experiments were conducted at Auburn University from April 2019-March 2020 to determine the effects of limited nitrogen (N) and hydraulic retention time (HRT) on ‘Rex’ butterhead lettuce using nutrient film technique (NFT), respectively. PROC GLIMMIX was used to conduct an analysis of variance on all responses using SAS version 9.4. Hydroponic experiments analyzing the effects of limited N on lettuce growth observed N treatments to be statistically different in terms of size 7 days after planting (DAP). After 28 DAP, average size index was observed to decrease 12%, from 20.4in to 18.3in as N was decreased from 150 ppm N to 50 ppm N. Plant fresh mass was linear between treatments, ranging from 189.7 to 241.7g, with treatment 125 ppm N yielding the highest fresh mass on average at 241.7g. Dry mass of plants showed treatment 150 ppm N had a similar dry mass to treatment 125 ppm N, indicating the difference in treatment mass was due to higher water absorption by treatment 125 ppm N. Once N fell below 125 ppm, plant growth suffered in all treatments. These results suggest a target value between 125-150 ppm N is better suited for lettuce growth. Aquaponic experiments analyzing the effects of HRT on lettuce growth observed shorter HRT intervals (4d) exhibited better growth characteristics, producing more biomass and longer roots, over plants grown in longer HRT intervals (16d). After the initial experiment, iron supplementation was determined necessary to further evaluate HRT. In trial one without iron supplementation, aquaponic fresh mass and SPAD exhibited negative linear and quadratic trends 28 DAP respectively, decreasing 41% and 143%, from 203.43g to 143.81g and 18.7 to 7.6, as HRT increased from 4d to 16d. Foliar analysis revealed all HRT treatments absorbed excessive amounts of micronutrients. Shorter HRT intervals absorbed more micronutrients when compared to longer HRT intervals, with treatment 4d accumulating double the amount of Mn and Zn as treatment 16d. In trial two experiments with iron supplementation, analysis of lettuce SPAD and size index found plants became statistically different in terms of color and size 14 DAP, and by 28 DAP, treatment 4d was observed to have the lowest SPAD average and largest size index average. Plant fresh mass decreased by 10%, from 162.25g to 147.09g, as HRT was increased from 4d to 16d. Analysis of water variables showed average nitrate and pH values increased as HRT increased from 4d to 16d, from 373 mg L-1 to 404 mg L-1 nitrate and 6.94 to 7.25 pH. Although average nitrate concentrations were higher for the longer HRT intervals, foliar analysis showed plant N% decreased in longer HRT intervals. Iron supplementation eliminated iron deficiencies in plants up to 14 DAP, but by 28 DAP HRT treatments were observed to be iron deficient along with the elements magnesium, calcium, boron, and copper. However, in contrast with trial one experiments, iron supplementation was observed to considerably reduce the uptake of the divalent cations manganese and zinc in plant tissues. Our findings suggest that smaller quantities of nutrients may be able to grow plants in aquaponics provided that faster hydraulic retention times are used and all essential nutrients are of high enough concentration.