How Does the Microstructure of Si-Enriched Triple Super Phosphate Granule Govern Microscale Phosphorus Release and Dynamics in the Granulosphere of a Tropical Soil?

The low P use efficiency of soluble triple superphosphate (TSP) in highly weathered soil is largely due to rapid fixation by Fe and Al oxyhydroxides, highlighting the need for an alternative P source that synchronizes efficiently nutrient release with plant demand. This study evaluated the potential effects of the fertilizer’s granule architecture on phosphorus (P) dynamics in tropical soils. Where our work combines µ-XRF mapping and P XANES together with chemical P extraction to evaluate the effect of the enriched TSP-MgSiO3 granules on the P diffusion and binding behaviors and availability in the granulosphere. Two enrichment methods were tested: coating and cogranulation. We conducted a short-term (1 month, maize in rhizoboxes) and long-term (1 year, without plant). Although Mixed and Coated TSP shared the same chemical composition, the mixed TSP strongly reduced early P fixation and increased labile P in the proximal zone (up to 100 µm), as shown by its relatively lower fixation index (FI = 0.36-0.46) compared to coated TSP (FI = 0.63-0.76). Meanwhile, coated TSP improved the spatial nutrient diffusion zone, meaning that the granule micro-structure controls the fate of P diffusion and P binding, highlighting the complementarity of Si-addition and granule formulation in enhancing P availability in highly weathered tropical soils.