Polyphosphate-enriched algae fertilizer as a slow-release phosphorus resource can improve plant growth and soil health

• Polyphosphate-enriched algae fertilizer (PEA) gradually releases phosphorus, ensuring a sustained and steady supply of phosphorus in the soil. • PEA enhances phosphorus solubilization and uptake by improving soil microbial activity and promoting beneficial microbial communities. • Algae fertilizers, especially PEA, support plant growth and soil health, offering a promising solution for sustainable phosphorus management in agriculture. Using phosphorus (P) fertilizers has historically increased agricultural productivity, yet the highly dissipative nature of phosphate rock and the low efficiency due to soil fixation and runoff raise sustainability concerns. Algae fertilizers have emerged as a promising eco-friendly alternative. However, the potential of algae fertilizers for providing sustained P availability and their impacts on plant growth, soil microbes, and nutrient cycling remains to be explored. In this study, we developed a polyphosphate-enriched algae fertilizer (PEA) and conducted comparative experiments with chemical P fertilizers (CP) through soil and solution cultures, as well as crop growth trials. Soil cultivation experiments showed that PEA released twice as much labile P as initially available in the soil, and it functioned as a slow-release P source. In contrast, soils treated with CP initially exhibited high levels of labile P, which was gradually converted to stable forms, but it dropped to 30% of the labile P level in PEA after three months. Further tests revealed that the slow release of P from PEA was linked to increased microbial activity, and the microbial biomass P (MBP) content was about eight times higher than in soils treated with CP after three months, resulting in a 75% decline in the microbial biomass carbon (MBC) to MBP ratio. Microbial diversity analysis showed that algae fertilizers could recruit more beneficial microbes than CP, like phosphorus-solubilizing bacteria, plant growth-promoting bacteria, and stress-resistant bacteria. Crop pot experiments, along with amplicon and metagenomic analysis of tomato root-associated microbes, revealed that algae fertilizers including PEA promoted plant growth comparable to CP, and enhanced soil P cycling and overall nutrient dynamics. These data showed that algae fertilizers, especially PEA, can stabilize soil P fertility and stimulate plant growth through their slow P release and the recruitment of beneficial microbes. Our study highlights the potential of PEA to foster sustainable agriculture by mitigating the P scarcity and soil P loss associated with chemical fertilizers and improving plant growth and soil health.