Phosphorus and Carbon Interactions in Tropical Soils: Effects of Customized P-Based Fertilizers

As a solution to tackle the high level of P fixation, which restricts crop growth and puts long-term soil health at risk in tropical soils, we customized next-generation P-based fertilizers designed to enhance the conventional TSP (triple superphosphate fertilizer). We developed two TSP-MgSiO₃-based fertilizers: (1) mixed formulation and (2) coated formulation to reduce the P binding to iron and aluminum oxides, enhancing phosphorus use efficiency (PUE) and supporting soil carbon preservation. Using a novel microcosm setup (2.26 cm³ soil cups) to simulate rhizosphere–granulosphere interactions, we hypothesized that enriching TSP with magnesium silicates will help reduce P-fixation (P-binding to Al and Fe oxides) and increase P availability in the granulosphere-rhizosphere while maintaining the carbon stock because silicon will form stable Fe-Si and Al-Si complexes, occupying reactive sorption sites on iron and aluminum oxides and preventing phosphorus binding with Al and Fe oxides. As We assessed carbon stabilization through FTIR spectroscopy, CHNS analysis, P speciation via XANES, and elemental mapping by high-resolution XRF. In contrast to TSP, the MgSiO₃-enriched TSP fertilizers both (1) and (2) significantly increased P availability at the short term (1 month incubation) and long term as well (1 year incubation) and reduced the rapid P diffusion (p < 0.05) as it boosted root density significantly (p < 0.05).While repeatedly showing higher carbon levels and an improved C/N ratio (P < 0.05).By promoting simultaneous dissolution and broader phosphorus diffusion within the granulosphere. The homogeneous incorporation of MgSiO₃ in mixed TSP (1) allowed for the formation of Fe-Si and Al-Si complexes that occupied reactive sorption sites. At 20 µm away, it binds with only 33% Boehmite (Al-P), and while at 20-100 µm, it binds with 40% Boehmite and only 1% Hematite (Fe-P). Although total carbon did not differ statistically, these treatments consistently showed more favorable C/N ratios, indicating reduced mineralization risk. Sharper C–O and Si–O bands in FTIR spectra indicated stronger organo-mineral interactions, indicating improved carbon stability. These results highlight that mixed TSP with 18% Magnesium silicates is the suitable ecofriendly P source the farmer can adopt due to its upgraded PUE.