Fertilizer source governs biostimulant efficacy for phosphorus acquisition in maize: comparison of recycled phosphorus fertilizers and soluble phosphate

Phosphorus (P) recovered from secondary waste streams may substantially reduce dependence on mined phosphate rock; however, the effectiveness of recycled P fertilizers remains highly variable and is often lower than that of mined mineral fertilizers. This study investigated whether the efficacy of three non-microbial biostimulants (BSs), namely humic substances (HSs), seaweed extract and chitosan, depends on the P fertilizer with which they are paired. Maize was grown for 42 days in rhizoboxes containing a P-deficient soil that was either unfertilized or fertilized with monocalcium phosphate (Ca-phosphate), struvite or a Na-treated sewage sludge ash produced by the ASH-DEC process (Na-SSA), in a fully factorial design with the three BSs and an untreated control, arranged in four randomized complete blocks. Plant biomass, P uptake, P-use-efficiency indices, rhizosphere pH, acid phosphatase activity, calcium-acetate-lactate-extractable P (CAL-P), root length by diameter class and shoot nutrient uptake were determined at harvest. A parallel four-month plant-free soil incubation with the same treatments was conducted to characterize the temporal dynamics of CAL-P in the absence of plant-mediated processes. The response to BSs depended strongly on the fertilizer source. With Ca-phosphate and struvite, HSs and chitosan increased plant biomass by 21–34%, total P uptake by 23–35%, P partial factor productivity (PFPP) by 21–34% and agronomic P-use efficiency (APE) by 63–77%. With Na-SSA, none of the BSs produced detectable gains within the six-week rhizobox period. The plant-free incubation, nonetheless, revealed that BSs progressively increased CAL-P with all three fertilizers, including Na-SSA, with statistically robust effects emerging between days 90 and 117. HSs were associated with rhizosphere acidification (ΔpH = −0.12 to −0.40) and 42–48% fine-root proliferation, and they imposed a pronounced K-up/Mg-down shoot ionome signature, with the K/Mg ratio is rising about fivefold. Pairing struvite with HSs may thus represent a practical strategy to increase P-use efficiency in maize under circular fertilization regimes, with Ca-phosphate + HSs serving as a soluble reference benchmark, whereas Na-SSA-based systems may require longer time periods or alternative agronomic management.