Thermally Treated Oyster Shell Exacerbates Phosphorus Immobilization during Acidic Soil Amendment

Abstract Soil acidification constrains crop productivity, and oyster shell (OS) waste is increasingly used as a sustainable liming material. Phosphorus (P), one of the three essential macronutrients for crops, can react with calcium (Ca)-containing materials during liming and thereby affect its bioavailability. However, cooking-related thermal treatments alter the physicochemical properties of OS, and whether this alteration affects phosphorus bioavailability during liming remains unclear. We hypothesized that thermal loss of the OS organic matrix strengthens calcium (Ca2+)-phosphate (HnPO4(3–n)–) interactions, promotes calcium phosphate (CaP) nucleation, reshapes soil microbial P-cycling responses, and thereby reduces plant-available phosphorus despite effective acidity neutralization. To test this, we compared raw (ROS), steamed (SOS), and grilled (GOS) OSs via organic matrix characterization, binding energy analyses for interfacial mechanisms, and pot experiments with maize in acidic soil for phosphorus bioavailability. Thermal treatment progressively degraded the organic matrix, particularly carboxyl-rich proteins, and subsequently enhanced phosphate binding on CaCO3. These stronger interactions accelerated heterogeneous CaP nucleation and promoted the formation of apatite-like CaP phases on thermally treated surfaces. In pot experiments, ROS increased maize shoot biomass and phosphorus uptake relative to SOS and GOS, and maintained higher soil-available phosphorus with lower Ca-bound phosphorus fractions, despite all treatments raising soil pH to a similar extent. At the microbial level, thermally treated OS was associated with a predicted phosphorus-starvation response and reduced soil phosphatase activities, whereas ROS sustained a more active microbial phosphorus-mineralizing community. Together, the abiotic interfacial reactions and the coupled microbial responses acted in concert to reduce phosphorus bioavailability under thermally treated OS, while ROS preserved both the organic-rich surface and microbial phosphorus turnover. Collectively, these results show that thermal treatment of OS enhances phosphate binding and CaP immobilization, reducing phosphorus bioavailability despite comparable acidity neutralization, and identify raw oyster shell as the preferred amendment for preserving phosphorus availability in acidic soils.