The proportional fraction of ammonia released in each treatment, determined through direct measurement in the leachates, was considered when calculating the emission factor as well as the nitrogen given in the irrigation. For the same reason, TA  was also chosen to reflect the direct emissions due to the application of ammonia as well as other acidifying agents generated during transportation and manufacturing of fertilizers . FE  and ME  have been regarded as the most relevant impact categories when analyzing fertilization methodologies, especially considering nitrogen and phosphorous . These impact categories are especially relevant in this study since slow struvite dissolution can provide insight into the possible reduction of P leaching into fresh and marine waterbodies, and again, the addition of N through struvite can also be reflected in the leachate quantities. FRS  was added as a relevant impact category to reflect fossil energy-related emissions that could arise due to struvite precipitation and transport compared to mineral P. Finally, MRS  was chosen to reflect the extraction of finite mineral resources, especially focused on phosphate rock extraction versus the recycling and reuse of phosphorous in the form of struvite. The resulting productions for the three lettuce cycles can be observed in Table 1. Here, we can appreciate the average yields obtained for all three harvests and all treatments for their fresh and dry weights. Further information on the specific production within the marked pots  can be seen in Table 2 in the supplementary information. We identified a general decrease in yield during the third harvest, most likely due to a remarkable decrease in the overall temperature during 54 DAT and 81 DAT in contrast to the previous crop cycles . This variation in the climatic conditions can be observed in Fig. 1 in the supplementary information with the recordings of humidity, radiation and temperature during all three cycles. While temperatures still ranged between 20 ◦C and 25 ◦C, the sudden change in comparison to the previous two crop cycles may have caused a delay in lettuce growth.

While no great differences can be seen in the overall yield of our lettuce cycles, the close monitoring of our pots can give us the variability of the obtained yield for the lettuces grown with the same initial struvite. This finding means that from the same pot, we can monitor the obtained yield in all three cycles. Table 2 in the supplementary information provides us with such information showing a general decline in production,grow table with the most acute decrease in yield in the 5LE treatment with a − 11% difference between the first harvest and the second. On the other hand, the decline for treatments 10LE and 20LE was less pronounced, with − 2% for both. In the case of pepper plant growth and production, Tables 2 and 3 provide insight into the differences spotted between treatments. Table 2 provides the main measurements made of the pepper plants at the end of the experiment . While no significant differences were seen for the stem weight, an increase in the fresh and dry weight was observed with the increasing amount of struvite applied. The same increase was noted for the leaf weight, number, and LAI, showing significant differences in all but the latter. The yield produced by the pepper plants  showed a greater total weight for the 20P treatment. While the total number of fruits was also higher for the 20P treatment, the weight per fruit did not differ greatly from that of the other treatments. The results shown in Fig. 2 depict the P content in the lettuce crop after 27 days of growing in the greenhouse for all treatments. The amount of P found in the lettuce biomass is directly related to the amount of struvite given, being lowest for the 5LE treatment followed by the 10 L and finally 20LE treatments. The amounts of P found in the 5LE and 10LE treatments decrease noticeably over time in the second and third cycles, while the 20LE treatment does not experience a great reduction during the second cycle but rather on the third cycle. It is important to point out that the results found for the second crop cycle show a much greater variability than the first and third ones. The remaining struvite content in the perlite and therefore P remaining in the substrate were analyzed and plotted in Fig. 2. Here, we can appreciate a great difference between the struvite fertilization treatments and the control, since the nutrient content in the perlite slowly increases over time for the latter, while the P content in the struvite treatments fluctuates and slowly decreases due to its dissolution. Here, again, a much greater variability in the results was observed for the second cycle. Fig. 3 depicts the P content in pepper biomass, fruit and perlite, showing great variation between struvite fertilization treatments and the control. While our treatments showed a low P content of 1.2 mg/g in leaves and 0.7–0.8 mg/g in the plant stem, giving ranges of 0.02 to 0.03 g of P in the total dry biomass, the control treatment showed values within adequate ranges of 2.1 mg/g . The amount of P in the harvested fruits reveals the differences between treatments based on the great mobility within the plant. Fruits are an ultimate sink of the phosphorous content in plants, and this result is reflected with a very clear relation to the struvite treatment. The great variability seen in these results derives from the great difference found between harvests within the same treatment, while the first pepper fruit harvest contained greater P concentrations, the third suffered a great reduction for all treatments, even the control . Finally, the amount of P found in perlite responds to the initially given struvite.

The resulting phosphorous concentrations found in the leachates were calculated for the total outgoing water weekly per plant,generating the patterns found in Fig. 4. The accumulation of P in the leachates for the lettuce and pepper crops can be seen in supplementary information Fig. 6. The results for the lettuce crop show the discharge of phosphorous during all three cycles, recognizing a clear pattern before and after each harvest. This pattern was highly noticeable for the CLE treatment, where the phosphorous content in the leachates decreased with the growth of the plant and rose once the plant was harvested and replaced with a seedling. This same pattern can be observed for all struvite fertilization treatments for lettuce, finding greater amounts for 20LE and less for 10LE and 5LE. The phosphorous content in water, on the other hand, differs greatly when observing the CP and the 5P, 10P and 20P treatments. The biomass growth, climatic conditions and subsequent irrigation amount define the loss of phosphorous in the CP treatments, showing an overall decrease in the concentration with a peak at approximately Day 37 after transplanting. All treatments with struvite showed very low concentrations in the leachates, especially after 20 DAT. The results obtained in the previous sections enable us to generate the nutrient balance for P during these cycles for all treatments. This understanding helps us estimate the P flows into the plant, substrate and water. These nutrient balances were calculated for the P flows in the lettuce and pepper crops  and averaged to obtain data for one plant. In addition, the water balances per plant are given in Figures 9 and 10 in the supplementary information. The nutrient balance is subjected to potential inaccuracies given through the sampling of substrate, water and biomass and the generation of mean values for all samples generating approximate values close to 100%. The balance for lettuce gives an overall picture of the obtained results of the phosphorous flows into the plant biomass as well as leachates. Compared to the control treatment, the phosphorous flow into the outgoing water was approximately 10 to 14 times lower for the 10LE and 5LE treatments, respectively, while the flow into the plant biomass remained similar. The remaining phosphorous in perlite remained high in the 5LE and 10LE treatments, while more than half was reduced in the 20LE treatment. We also appreciate an accumulation of P in the perlite of the CLE treatment. For pepper,ebb flow table the biomass flows were divided between the fruits produced and the generated biomass on the day the plants were cut and weighed. Here, we can appreciate the great quantity of phosphorous found in the pepper fruits, which equals the total phosphorous found in the plant leaves and stem. The total biomass showed a great difference between the CP treatment and the struvite fertilization treatments, revealing a much greater P content in the control.

Due to the greater irrigation needs of pepper plants compared to lettuce plants, the CP treatment received an overall greater amount of P through irrigation compared to the CLE treatments. Therefore, although the P in the perlite and leachates is lower in CP than in the CLE treatments in terms of percentage, the absolute amounts are greater. In the case of the pepper plants fertilized with struvite, the P in the leached water was similar and even smaller than the amount found in the lettuce crop. The outgoing P in the leachates of the pepper plants was 10, 19 and even 35 times lower than that in the control treatment  for the 20P, 10P and 5P treatments, respectively. The calculated dissolution rates for the applied struvite in lettuce and pepper are shown in Fig. 5. The struvite dissolution was estimated by the P contained in the water leachates as well as P in the plant biomass. This dissolution has a direct impact on the P uptake by the plant that was estimated as the P contained in the P biomass. The results for lettuce show greater dissolution with a greater initial amount of struvite. The dissolution of the struvite was also higher during the first lettuce cycle , showing smaller differences between the second and third cycles . The dissolution rate found in the pepper crop was smaller than that in the lettuce crop but followed the same pattern as seen before, with greater dissolution with higher amounts of struvite. Fig. 6 shows the results for lettuce, and Fig. 7 shows the results for the environmental assessment of the fertilization treatments. Since only the fertilization of the crops was considered for the analysis , all differences will be related to the use of struvite instead of monopotassium phosphate  in the form of KPO4H2, leaving out the laboratory infrastructure and auxiliary equipment, as well as the end-of-life processes. The obtained results for six out of seven impact categories show that fertilization with struvite has lower impacts than the control, and for the cases of ET, MRS, FRS and GW, impacts are also reduced as we increase the amount of struvite applied. In terms of eutrophication, FE, which is directly related to the emissions to water, had the greatest impact on the control irrigated with mineral P, followed by 20LE, which was the treatment with the highest quantity of struvite per plant. ME, although related to the emissions to water, also does not decrease substantially for the struvite-treated crops due to its relation to nitrogen emissions, which are sustained for all treatments. Furthermore, we can observe that although a reduction of the impacts is occurring for the 20LE treatment, this reduction is most likely not a consequence of a reduced N emission to water but due to greater yields obtained; on the other hand, treatment 5LE is overshadowed by the lower yields generated and a proportionally greater N emission due to the smaller plant growth. The results obtained for the pepper crop indicate a considerably abrupt decrease in the emissions in all impact categories for treatments 10P and 20P in comparison to the CP treatment. In comparison to the lettuce crop, the ME was severely reduced for these two treatments. The 5P treatment with lower production rates and therefore lower FU experiences much greater values for all impact categories except FE and MRS, which are slightly below the control treatment CP in the latter.