Eggshell-derived calcium redistributes legacy phosphorus toward less labile operational pools while sustaining pak...

Phosphorus-enriched greenhouse vegetable soils can sustain crop nutrition while remaining vulnerable to phosphorus (P) mobilization because much of their legacy P persists in soluble or weakly buffered pools. We tested whether eggshell-derived calcium (Ca) could reduce indicators of legacy P mobility without compromising crop performance. A cross-scale framework integrating incubation, P fractionation, sorption-desorption, pot, and field experiments was used to compare washed eggshell (WES), unwashed eggshell (ES), and reagent-grade calcium carbonate (CC). Ca amendments increased soil pH by 0.9-2.3 units and reduced available P (AP) by 10-35% during incubation. After six weeks, H₂O-P and NaHCO₃-Pi declined, whereas HCl-extractable P fractions increased, indicating an operational redistribution of legacy P toward less labile pools. P retention was also strengthened, with markedly higher Langmuir sorption capacity and lower desorption at high solution P concentrations. In both pot and field experiments, lower bulk-soil AP did not reduce pak choi growth or P uptake. Under field conditions, WES applied at 4.8 t/ha decreased AP and water-soluble P but increased pak choi P uptake and fresh yield by 27.9% and 11.2%, respectively. These results indicate that eggshell-derived Ca can reduce indirect indicators of potential P mobility while maintaining agronomically effective P supply. Because Ca-P mineral phases and runoff or leaching P losses were not directly measured, the environmental implication should be interpreted as reduced potential P mobility rather than confirmed reductions in off-site P export.