The impact of alternative recycled and synthetic phosphorus sources on plant growth and responses, soil interactions and sustainable agriculture - lettuce (Lactuca sativa) as a case model.

This research assesses the efficacy of two phosphorus (P) adsorbents as alternative fertilizers in promoting lettuce growth. A synthetic Mg/Al-layered double hydroxide (LDH) and an iron-based recycled water treatment residual (Fe-WTR), both enriched with P from dairy wastewater and added at three dosage levels. We hypothesized that the adsorbents' physicochemical nature will overshadow the biological efforts in the plant ecosystem to increase P solubility, impacting plant growth, nutritional composition, and metabolite profiles. Fe-WTR significantly enhanced lettuce biomass compared to LDH. Yet, elemental analysis revealed higher or equal P concentrations in the low-biomass LDH plants relative to other treatments. Phosphorus uptake appears to influence the assimilation of other nutrients that divided into two groups: calcium, magnesium, zinc, and copper with notable correlations to P and nitrogen, iron, aluminum, vanadium and manganese with low correlations to P. Conversely, P retained poor correlation with most metabolites whereas iron showed a higher correlation with numerous metabolites. Analysis of metabolites, encompassing carbohydrates, the Krebs cycle, amino acids, nucleic acids, and stress and regulatory pathways, revealed diminished levels in the LDH treatments. Overall, carbon assimilation (plant growth) was more effectively predicted by soil P availability (adsorbent type and dose) rather than by cellular P concentration, suggesting root signaling was at play, influencing carbohydrate translocation to the roots. Diminished levels of cellular sugars further affect metabolic pathways and iron uptake, thus restricting photosynthesis. The results illustrate the substantial influence of the P source on the plant's metabolic processes and soil biogeochemistry. The synthetic LDH adsorbent with high sorption capacity, tightly binds its substantial P pool, rendering it inaccessible and potentially disrupting rhizosphere biogeochemical interactions. In contrast, the chemical nature of Fe-WTR enabled efficient nutrients acquisition bioactivity. The study highlights Fe-WTR as a promising sustainable alternative to conventional fertilizers, emphasizing its potential scalability and adaptability in agricultural contexts.