Comparative assessment of mobility, potential availability, and speciation of reclaimed struvite and conventional phosphorus fertilizers in mildly calcareous, neutral, and acidic soils

Phosphorus (P) is a vital nutrient for crops. Recycling P as reclaimed products, such as wastewater-derived struvite (NH₄MgPO₄·6H₂O), offers a promising potential as an alternative P source. This study aimed to evaluate the effectiveness of reclaimed struvite as a P fertilizer under mildly calcareous, neutral, and acidic soil conditions compared to conventional fertilizers: monoammonium phosphate (MAP), diammonium phosphate (DAP), and ammonium polyphosphate (APP). A short-term incubation study was conducted in Petri dishes with four fertilizers (struvite, MAP, DAP, and APP) and control treatments across three soil types, incubated for 1, 2, 5, 9, and 12 weeks. Post-incubation, soil samples were sectioned and analyzed for pH, resin-extractable P, total P diffusion from the point of application, and X-ray absorption near-edge structure spectroscopy. Struvite demonstrated the highest P availability in neutral and acidic soils, performing comparably to conventional fertilizers after 5 weeks. In mildly calcareous soil, the diffusion of struvite-P was limited (only ∼5% into further sections) compared to MAP (13%), DAP (7%), and APP (25%). Overall, resin P extractability was higher for struvite in acidic soil (67%) and neutral soil (40%) than in calcareous soil (9%) after 12 weeks. The XANES speciation revealed that, in calcareous soil, Ca phosphate precipitation and sorption of P by CaCO₃ limited P mobility and extractability in P fertilizers, while Fe/Al-bound and clay-adsorbed P dominated in neutral and acidic soils. In conclusion, struvite emerges as a promising P source in neutral and acidic soils, performing comparably to conventional fertilizers; however, it may not be a viable P fertilizer in calcareous soils.