Overall emissions for pepper production were lower than those found for the three lettuce cycles combined. This finding can be explained by the greater weight obtained with pepper production, making a direct comparison between crops difficult with a functional unit only accounting for the obtained yield. The results show that the long cycle of pepper and short cycles of lettuce fertilized with struvite did not differ greatly from each other in the uptake and use of P. We identified that the amounts accumulated in the plant biomass between treatments with the same struvite quantity  did not change substantially. This information reveals that little to no effect on struvite uptake can be attributed to the crop cycle duration or needs. This second idea is reinforced by the level of P found in the pepper biomass, corresponding to low concentrations and mirrored in the fruit P content . Although a clear P deficiency is shown in the plant biomass nutrient content, no such deficiency can be traced in the plant physiology or production capacity . Pepper fruit production increases with the given struvite, as well as leaf production and growth, showing significant differences that indicate the relevance of the given struvite amount to the plant. Related to the findings of Talboys et al. 2016 in 90-day experiments with struvite-fertilized crops, the amount of P taken by the plant is lower in the case of struvite but does not affect the final yield, being very similar to the more soluble triple superphosphate . This finding has been attributed to the struvite residual value in the substrate in comparison to TSP, enabling P uptake by the plant during a sustained timespan. The leachate P for lettuce and pepper plants was also shown to be a great indicator of the slow solubility of the fertilizer and increased with greater water flow when lettuce was harvested. The higher water demands of the pepper plants could therefore have been a defining factor contributing to low struvite dissolution, as seen in Fig. 5.

Although the plants had sufficient irrigation indicated by the daily water drainage, the leaching of phosphorous into the drained water only increased during the early stages of plant growth until 20 DAT. Once temperatures start to rise and drainage is reduced, hydroponic grow table the emissions of P into the drainage are also reduced. Although greater temperatures have been seen to increase struvite solubility , its use as a fertilizer unveils that irrigation plays a major role in plant phosphorus uptake . The greater variability obtained in the second lettuce cycle can also be attributed to the increasing temperatures enabling a greater dissolution of struvite in the perlite substrate as well as the slight reduction of the pH from the nutrient solution increasing the struvite solubility.The capacity of struvite dissolution, which has been attributed to different factors in previous literature, like the plant rhizosphere exudation , plant growth stage  and plant needs. These factors for greater struvite dissolution have not been reflected in these results, indicating a reduced uptake from the pepper plant compared to the lettuce crop. The idea of plant rhizosphere exudation being important for struvite dissolution was also questioned by Rech et al., 2018, who demonstrated the inefficiency of low-concentration root exudates to solubilize granular struvite. Overall, the quantity of P in the plant biomass  as well as the P leachate  in both crops indicate that the amount of dissolved P is very small. This information is reinforced by the analysis of the perlite substrate, indicating that a large amount of struvite remains undissolved in the substrate. This effect was also seen in previous literature with other crops, such as soybean and wheat  and common bean . These low dissolution percentages coincide with dissolutions in media with pH values ranging from 7.5 to 8 , which were mainly found in the present study. While the pepper plants did not reach adequate ranges of P in the biomass with struvite fertilization, the lettuce crops did not differ greatly from the control treatment, especially for 10LE and 20LE. This information reinforces the idea of further reusing the given struvite for consecutive cycles within the same substrate with short cycle crops, such as lettuce. On the other hand, the dissolution rate seems to be greater during the first plant cycle in all lettuce treatments.

The struvite crystal composition and available P could be more prone to dissolve earlier, progressively reducing the dissolution rate with consecutive plant cycles. This same dissolution trend was seen by Rech et al.  when observing the P concentration in the soil solution of wheat and soybean crops with the fertilization of three different struvite types. Concentrations of P were recorded for 40 days, showing a decrease and stability by the end of the experiment. The close dissolution rate of the second and third cycles could indicate this point of stability. The environmental analysis showed that the 5LE and 5P treatments had the highest impacts since they had the lowest yields. On the other hand, the greater use of struvite can also generate a greater discharge of P into the water system compared to treatments with less applied struvite. This finding is reflected in the case of the lettuce crops for the ME and FE impact categories. While greater yields were achieved for the 20LE treatment, greater P and N leachates were generated, increasing the environmental footprint in comparison to the other struvite treatments. Smaller crop growth in the case of 5LE and 5P can also increase the amount of leachates and discharge of N to the environment. This finding has been observed both for lettuce and pepper, where smaller crop growth leads to greater water and nutrient discharge. However, the P discharge in the struvite treatments was always lower than that in the control and thus impacted freshwater eutrophication. The impact of the struvite production compared to the monopotassium phosphate seem significantly smaller, being most noticeable in GW and FRS. The production of monopotassium phosphate on the other hand has a large impact on the MRS as predicted, due to the extraction of the finite phosphate rock. The impact of monopotassium phosphate is also noticeable in the ET, TA and FRS categories, responding to the emissions of chemical agents into the environment for the extraction and transport to site. The overall impacts seem to be more dominated by the production emissions associated to potassium sulfate, being present in almost all IC due to its major role in the nutrient solution. Takin in account the influence of the struvite slow solubility to reduce the emissions of P to water as well as the reduction of the impacts associated to the production of monopotassium phosphate, a great reduction of the impacts of fertilization can be seen.

While the pepper crop shows a clear reduction in emissions related to fertilization with the use of 10 g and 20 g of struvite, sustained production is unclear due to the low content of P in plants. While the production of pepper continues and demands on P can increase, its dissolution and uptake might not be sufficient in time. On the other hand, the lettuce needs were covered for all three cycles for all treatments, showing a P content similar to that of the control treatment. The idea of sustained production for longer periods of time corresponds to the findings of Bonvin et al., 2015 and Rech et al., 2018 urging for the definition of the residual value of the remaining struvite after the initial crop production. To understand the environmental impact of one year of lettuce cycles, several assumptions were made. To generate the nine-cycle scenario  that would correspond to yearly production, the three initial cycles for our three treatments were taken as references to generate correlations for the P uptake in biomass from the initial P given, as well as the potential yield produced with the P content in the plant biomass .Further on, the error detected in this last correlation was subjected to a sensitivity analysis adding a standard deviation of a total 46% to the yield production for a 9 cycle production of lettuce in all treatments. The control was also given a standard deviation of 10%, flood tray taking in account that the P fertilization was consistent over time. The P loss through the leachates was estimated from the average obtained in all treatments due to its direct relation with irrigation. With the following prediction, the total biomass content of yearly production as well as the resulting yield and emissions to water were obtained to further extend the environmental outcome . The control treatment was estimated with the generated yields and emissions from the three initial cycles. All other fertilizers for all treatments were based on the NS used for the three initial cycles extended for nine production cycles. The obtained emissions were then divided by the obtained total yields. The LCA for the year’s production with the same initial struvite shows a slight emission increase for all ICs, especially for the 5LE and 10LE treatments. The changes observed indicate that the productions obtained for the 5LE and 10LE treatments decrease to a point where the functional unit is reduced and consequently emissions are increased. On the other hand, control treatment yields were sustained in time and maintained close to identical emissions of the three lettuce cycles. The prospective production obtained for the 20LE treatment was similar to that of the control, obtaining results that reduced the environmental emissions for all impact categories compared to the control treatment except ME.

The 20LE treatment maintains the capacity for competitive production in time compared to the other struvite treatments which can also be seen in the sensitivity analysis in fig 11 in the supplementary information, staying below the control emissions in lower production scenarios, especially for FE and MRS. This, however, implies a potential greater emission to water, as reflected in the FE and ME impact categories generated by the leaching of the struvite containing N and P. The use of the discharged water for less demanding crops  can further reduce nutrient leaching into the urban water cycle as well as a reduction and adjustment of the nutrient solution N content with the addition of struvite. Further loss into the environment can be assessed with a specific analysis of the struvite nitrogen emission factor to the air in the form of ammonia, N2O and NOx in soilless systems, which is strongly encouraged to determine the GW impact more accurately. This result has been viewed as both interesting and necessary research to understand whether slow dissolution can discourage emission to air or if the composition of N struvite in the form of ammonia will further induce processes of nitrification and denitrification in the substrate. The findings in this work point out that the successful reuse of struvite in hydroponic production is possible and is been growing in importance, even being used in the fertirrigation for other crops achieving equal results to conventional fertilizers . Similar work has been made with source separated urine, integrated into hydroponic production as nutrient source, and also using phytoremediation systems for yellow water treatment . These works have found promising results on the reuse of urine although its application can be considered controversial . This new way to find circularity in urban ecosystems is deemed as necessary and imposed specially in the waste treatment sector. The capacity to find an added value to the outcome of urban waste can help achieve new environmental goals like the compulsory recovery of P in certain regions of the EU . The local P recuperation and local administration can increase the local resilience to P pricing and distribution; therefore the P precipitation and struvite production should be encouraged in WWTP. The societal and political interest in more sustainable and circular food production systems is increasing and in parallel to this development, the focus in waste treatment is being directed towards increased resource recovery. It is imperative to reduce food loss to increase food security, however, this is complex as losses occur in the whole production and supply chain.