Brown Algal Fermentation Products Improve Phosphorus Availability and Maize Growth in Low Phosphorus Soil

Brown algal fermentation products (BA) are promising biostimulants, but the mechanisms by which they improve phosphorus (P) availability and plant performance in P-deficient soils remain unclear. This study investigated whether BA enhances P use efficiency in maize grown in calcareous soil under contrasting P supply by altering soil P availability and aggregate structure, thereby improving root foraging, photosynthetic performance, and plant P uptake. Maize was grown under low-P (LP) and high-P (HP) conditions with or without BA application, and soil P availability, inorganic phosphate (Pi) fractions, water-stable aggregates, root traits, leaf gas exchange, biomass, and plant P uptake were assessed. BA produced clear positive effects under LP but had limited effects under HP. Under LP, BA increased Olsen-P and shifted Pi from the less labile Ca₈-P fraction toward the more labile Ca₂-P fraction, with little effect on Al-P, Fe-P, or occluded P. BA also moderately restructured water-stable aggregates in maize-planted soil, increasing the proportions of the 0.25–2.00 mm and 0.106–0.25 mm fractions. These soil changes were accompanied by greater root foraging capacity, reflected in higher total root length, specific root length, and fine-root proportion. BA also enhanced leaf gas exchange, particularly photosynthetic rate and stomatal conductance, and ultimately increased biomass and plant P uptake. Overall, BA improved P acquisition under LP through a coordinated soil–root–leaf cascade involving enhanced Pi availability, altered aggregate structure, improved root architecture, and greater photosynthetic performance. These findings provide mechanistic insight into the context-dependent efficacy of algal-derived biostimulants and support their potential to improve P use efficiency in P-limited calcareous soil.