Optimization of Liquid Digestate Fertigation and Light Intensity for Lettuce Cultivation in Closed Plant Production System

Abstract

　Application of liquid digestate (LD), a by-product derived from anaerobic digestion of organic materials, was optimized for lettuce cultivation in closed plant production system (CPPS). LD can be dried to reduce transportation cost but ammonium, a major source of nitrogen in LD, is substantially lost during the process. In Experiment 1, it was shown that mixing of LD with lime juice at the ratio of 7:3 before drying was most effective in preventing ammonium loss. This redissolved mixture (RM) was used for fertigation in subsequent experiments. In Experiment 2, lettuce plant fertigated with RM had lower growth than those fertigated with chemical fertilizer when both treatments had the same nitrogen concentration. In Experiment 3, fertigation of RM resulted in lettuce plant with lower growth and higher accumulation of malondialdehyde than those fertigated with redissolved LD added with nitrate suggesting that growth suppression in the plant received RM could be attributed to ammonium toxicity. Reduction of light intensity causes an increasing of fresh and dry weight of lettuce plants in all treatments. Results from this study showed that RM can be used for lettuce cultivation in CPPS and condition of low light intensity with prolonged photoperiod can improve growth of lettuce plant.