An Appraisal of Urine Derivatives Integrated in the Nitrogen and Phosphorus Inputs of a Lettuce Soilless Cultivation System

Abstract

Reinforcing and optimizing sustainable food production is an urgent contemporary issue. The depletion of natural mineral resources is a key problem that is addressed by recycling mined potassium and phosphorus, and nitrogen, whose production depends on very high energy input. A closed-loop approach of fertilizer use asserts the necessity for efficient management and practices of organic waste rich in minerals. Human-derived urine is an underutilized yet excellent source for nitrogen fertilizer, and, in this study, processed urine fertilizer was applied to greenhouse soilless cultivation of lettuce (Lactuca sativa L.) cv. Grand Rapids. Biomass increase, biometric parameters, soil plant analysis development (SPAD) index, minerals, and organic acids content of lettuce were analyzed. From eight different urine fertilizer products generated, K-struvite, urine precipitate-CaO, and the liquid electrodialysis (ED) concentrate supported the growth of lettuce similar to that of commercial mineral fertilizer. ED concentrate application led to the accumulation of potassium (+17.2%), calcium (+82.9%), malate (+185.3%), citrate (+114.4%), and isocitrate (+185.7%); K-struvite augmented the accumulation of magnesium (+44.9%); and urine precipitate-CaO induced the highest accumulation of calcium (+100.5%) when compared to the control, which is an added value when supplemented in daily diet. The results underlined the potential of nitrogen- and phosphate-rich human urine as a sustainable source for the fertilization of lettuce in soilless systems.

Resource Recovery from Synthetic Nitrified Urine in the Hydroponic Cultivation of Lettuce (Lactuca sativa Var. capitata L.)

“…The nitrogen content in the biomass, regardless of the treatment solutions, was within the range given in the literature [25,[44][45][46]. Similar to nitrogen, the phosphorus content in each module was also in accordance with the scientific literature [46][47][48] and did not exceed the toxicity limit for lettuce (>10 g•kg DM −1 ).…”

“…The results were compared to the reference cultivation carried out with a commercial nutrient solution that is commonly used in large-scale commercial production. This research complements our knowledge on the cultivation of hydroponic lettuce on urine-based fertilizers that was previously conducted by Volpin et al [24] and El-Nakhel et al [25]. The cultivation of lettuce based on a fully nitrified stream (~0% NH 4 and 100% NO 3 /(NO 3 + NO 2 ) in effluent) obtained in an activated sludge reactor has not been performed in the past.…”

“…Therefore, an overabundance of sodium can be avoided when only partially nitrified urine is used as a nutrient solution. The high ammonium concentration resulting from this approach could significantly influence plant growth as severely as excessive sodium [24,25]. Volpin et al [24] treated urine in a membrane reactor without alkalinity correction and then created a concentrated urine product in membrane distillation.…”

“…The results showed that partially nitrified urine leads to very low yields, significantly lower than those obtained with nitrified urine in these studies (16.1 ± 2.9 g and 29.14 ± 11.52 g respectively). In turn, El-Nakhel et al [25] cultivated lettuce using electrodialysis (ED) concentrate obtained from fully nitrified urine and using commercial urine-based fertilizer acquired from partial nitrification. That fertilizer contained N in a proportion of NH 4 -N/NO 3 -N of 1/1.…”

Vertical Farming: The Only Way Is Up?

“…Human excreta are the primary source of essential nutrients in domestic wastewater, and source separation of urine for fertilizer production can reduce the environmental impact compared to synthetic fertilizers [41,42]. In addition, urine-derived fertilizers have been successfully applied in soilless cultures [43][44][45]. Thus, urine-derived fertilizers could provide a source of nutrients for crops produced in vertical farms in an urban environment, allowing us to transition toward a complete closed-loop nutrient cycling approach.…”