Are mycorrhizal fungi essential for polyphosphate uptake?

Phosphorus (P) is a limited nutrient with high sorption reactivity, leading to excessive P-fertilizer use. Polyphosphate (Poly-P) fertilizer, comprised of two or more orthophosphate (ortho-P) units, has been introduced to enhance the P-fertilizer use efficiency due to its slow-release nature. Poly-P must be hydrolyzed to become available to plants and is generally associated with soil bacteria activity. Poly-P is a common pool in soil microorganisms and organic wastes, potential alternative P fertilizer sources. Arbuscular mycorrhiza fungi (AMF) form a symbiotic association with most plants and enhance the P uptake. The role of AMF in synthetic or natural poly-P acquisition is unknown. Our research aims to study the contribution of AMF to P uptake from natural (hydrochar and biofertilizer) and synthetic poly-P fertilizers. A Rhizobox experiment and a pot experiment were conducted under controlled conditions, using autoclaved soil with inoculated AMF. The results showed that AMF inoculation significantly enhanced tomato plant biomass, with AMF-inoculated plants showing 1.68-fold higher biomass than non-inoculated plants, regardless of the P fertilizer type. In the pot experiment, tomato plants were grown using algae-based biofertilizers and conventional fertilizers. Plants fertilized with biofertilizers exhibited the highest AMF colonization (47%) compared to poly P (35.6%), no P (32.1%), and ortho P (31.9%) fertilizers. Algae-based biofertilizer-treated plants also showed higher biomass and P uptake, suggesting that the increased organic matter in biofertilizers promoted fungal growth and enhanced P absorption. These findings highlight the critical role of AMF in improving poly P uptake and plant growth.