Soil Legacy Phosphorus Reshapes the Soil–Plant Nutrient Continuum: Evidence from 77 Taiwanese Rice Paddies

Legacy phosphorus (P) from long-term fertilization persists in farmland soils due to strong soil P fixation. Because P mobility and accumulation are linked to reactions with other elements, excessive soil P accumulation may alter nutrient translocation across the soil-rhizosphere-plant continuum, potentially disrupting crop nutrient homeostasis. To investigate these effects, this study collected samples from 77 rice paddies across Taiwan, which spanned a wide range of soil P levels. Element concentrations in soils and in rice (Oryza sativa L.) roots, shoots, and grains were analyzed using ICP-OES and ICP-MS after microwave digestion. Accumulation factors and translocation factors were subsequently calculated and compared with data from previous studies. The results showed that increasing soil P led to a significant increase in P concentration only in roots, but no corresponding increase in P concentrations in shoots or grains, indicating strong retention of excess P in roots. Magnesium (Mg) and zinc (Zn) concentrations in rice grains were lower than literature benchmarks, with Mg ranging from 865.0 to 1344.4 mg·kg⁻¹ (≈1400 mg·kg⁻¹ in previous studies) and Zn averaging 23.3 mg·kg⁻¹ (36.1 mg·kg⁻¹ in previous studies). As root P concentrations increased, the root-to-shoot translocation of both Mg and Zn decreased, suggesting that phosphate-driven binding and/or precipitation within the root system. Selenium (Se) concentrations in rice grains also showed a declining trend (averaging 0.01 mg·kg⁻¹) relative to previous soil-based studies (≈0.07 mg·kg⁻¹). Furthermore, Se accumulation in roots decreased with increasing soil phosphorus levels, suggesting competition between selenite (SeO₃²⁻) and phosphate (PO₄³⁻) during plant uptake and translocation. Manganese (Mn) in shoots averaged 303.6 mg·kg⁻¹, lower than the 560 mg·kg⁻¹ reported previously, and root Mn concentrations decreased with increasing soil P concentrations, suggesting that elevated P may reduce Mn availability through precipitation or adsorption processes under high P conditions. Overall, these results suggest that soil legacy P can alter the uptake and internal partitioning of multiple micronutrients in rice, and may reduce some micronutrients in grains. Mechanistic confirmation (e.g., root-phase speciation and transporter-level evidence) is needed to resolve the processes underlying these patterns.