Effects of nutrient solution recycling on water and nutrient consumption patterns and lettuce growth

• Nutrient solution recycling improved water use efficiency, promoting zero-waste hydroponic systems with fixed water and nutrient input. • Reduced nutrient solution volume intensified nutrient imbalances, depleting N, P, and K rapidly. A targeted nutrient management strategy was proposed. • Nutrient recycling caused smaller leaf area, thinner leaves, and longer roots, while the total number of leaves remained unchanged. • Middle-position leaves, essential for photosynthesis, showed reduced performance under nutrient recycling. Leaf growth patterns were effectively modeled using a Gaussian function. Hydroponics is a promising cultivation method that increases crop yields per unit area while optimizing water, nutrient, and land use efficiency. However, nutrient recycling can alter solution composition due to preference nutrient uptake by plants, potentially affecting plant growth. This study investigated the effects of nutrient solution recycling on lettuce ( Lactuca sativa L.) growth, water use efficiency, and nutrient absorption patterns over three consecutive 21-day cycles (C 1 , C 2 , and C 3 ) using a deep flow technique system in a controlled environment. Results showed that nutrient solution recycling enhanced water use efficiency but led to nutrient imbalances in remaining solution: nitrogen (N), phosphorus (P), and potassium (K) were rapidly absorbed independent of plant transpiration, while magnesium (Mg), sulfur (S), and calcium (Ca) were absorbed more slowly. These imbalances resulted in morphological changes, as leaf area were reduced by 22.3 % in C 3 compared to C 1 , while primary root length increased by 34.6 %. Middle-position leaves, which are critical for photosynthesis, exhibited reduced light interception and photosynthetic capacity under nutrient recycling conditions. To optimize nutrient management, N, P, and K should be supplemented in multiple doses based on total plant demand, while Mg, S, and Ca should be supplied at standard concentrations with water. These findings highlight nutrient solution recycling as a viable strategy for improving resource efficiency in hydroponic systems.