Soil pH Modulates Cadmium Uptake and Translocation in Amaranthus hypochondriacus L. Under Divergent Phosphorus Fertilization Strategies

Soil pH is a key regulator of cadmium (Cd) mobility, uptake, and translocation under phosphorus (P) fertilization. We hypothesized that P fertilization would enhance Cd phytoextraction by improving photosynthesis and biomass, that foliar KH2PO4 would better maintain Cd availability than soil-applied P, and that NH4H2PO4 would promote Cd uptake through acidification, whereas CaHPO4 would reduce Cd availability by increasing soil pH. A greenhouse pot experiment compared soil-applied KH2PO4, CaHPO4, and NH4H2PO4 with foliar KH2PO4 at 0.2%, 0.4%, and 0.8% in grain amaranth grown in Cd-contaminated acidic soil, with four replicates. Soil-applied P increased P availability, photosynthesis, biomass, and plant Cd accumulation by 69.8–139.5%, primarily through biomass promotion rather than greater soil Cd availability; foliar P was less effective. CaHPO4 increased soil pH, whereas NH4H2PO4 decreased it. Pot-level analyses showed negative relationships between soil pH and CaCl2-extractable Cd and Cd concentrations in roots, stems, and leaves. NH4H2PO4 produced the highest tissue Cd concentrations, translocation factor, and total Cd accumulation, likely because acidification promoted Cd uptake and root-to-shoot translocation. Overall, the biomass-promoting and acidification hypotheses were supported, whereas the advantage of foliar P was not. However, repeated NH4H2PO4 application may intensify soil acidification and increase heavy-metal mobility, posing ecological risks.