Transforming waste phosphorus into a nutrient-rich source for a greener future

Abstract

The resource constraints and rising costs of conventionally used mineral fertilizers have driven scientific research toward alternative pathways for fertilizer production. In this context, understanding how plants respond to renewable nutrient sources and also developing effective strategies for transferring these technologies to the field is essential to ensure food and nutritional security while reducing the environmental impacts associated with fertilizer management. This thesis was structured into two chapters addressing the challenge of meeting agricultural phosphorus (P) demand while reducing dependence on non-renewable phosphate rock reserves. In Chapter I, the focus was on the synthesis, characterization, and agronomic evaluation of calcium and iron-based (Ca/Fe) materials derived from Fe mining activity. The mining waste was heat-treated to produce a reference material, R900, and then modified by Ca impregnation, resulting in ReFert, in which the formation of Ca ferrite domains provides an affinity for phosphate. ReFert exhibited higher P removal capacity than its Ca-free precursor and highlighted its potential for P recovery from aqueous solution. The P-loaded material, ReFertP, was then evaluated using seed priming and coating techniques applied to maize (Zea mays L.) grown in acidic soil. These experiments demonstrated that the adsorbed P could be gradually released in the rhizosphere, sustaining germination, early plant growth, and P accumulation at levels comparable to those achieved with conventional soluble fertilizers. These results showed that recovered materials can be strategically positioned at the seed–soil interface, promoting efficient early P nutrition while simultaneously valorizing Fe-rich mining residues. In Chapter II, the investigation was focused on the behavior of these materials in alkaline soils, where P fixation by Ca and carbonates strongly limits fertilizer efficiency. A greenhouse pot experiment with ryegrass (Lolium multiflorum Lam.) compared four P supply treatments (control, triple superphosphate - TSP, ReFert, and ReFertP) for 126 days. ReFertP was a slow-release P source, promoting a gradual increase in rhizospheric P availability, sustaining biomass production, and resulting in high phosphorus use efficiency by the end of the experiment. ReFert actively influenced soil P dynamics by interacting with native soil P pools, adsorbing phosphate from the soil solution, and progressively making it available to plants through the rhizosphere. Together, these two chapters demonstrate that Ca-enriched Fe mining residues can be transformed into multifunctional materials would are capable of removing P from effluents, recycling this nutrient for agricultural use, and regulating its availability in both acidic and alkaline soils. These findings reinforce the potential of ReFert as a functional interface between P removal, recovery, and reuse, contributing to a more resilient and sustainable circular nutrient economy. Keywords: Plant nutrition; Nutrient flows; Nutrient use efficiency; Fertilizer; Circular bioeconomy.