Long-term organic fertilization reduces cadmium bioavailability via enhanced binding to organic fractions and Fe-bearing minerals

Fertilizers provide essential plant nutrients but may introduce toxic cadmium (Cd), posing soil ecosystem risks. However, mechanisms governing Cd mobility and stabilization under different fertilization regimes remain unclear. In this study, soils from a 27-year field experiment were investigated under three treatments: inorganic fertilizer only (NPK), inorganic fertilizer with pig manure (NPKM), and no fertilizer (CK). Total and bioavailable Cd were quantified using chemical extraction and the diffusive gradients in thin films technique. Results indicated that NPKM increased total soil Cd 5-fold relative to NPK, but the proportion of bioavailable Cd (bioavailable Cd/total Cd) was 4-fold lower. Dissolved organic matter incubation coupled with Fourier transform infrared (FTIR) spectroscopy, two-dimensional correlation spectroscopy, and Fe K-edge X-ray absorption fine structure (Fe-XAFS) revealed that NPKM promoted the formation of active soil minerals, particularly reactive Fe-bearing minerals, and altered Cd-binding sequences in soil functional groups. In NPKM soils, wavelet analysis indicated atomic-scale Fe-Cd coordination (R = ~1.8 Å, k = ~12 Å⁻¹), suggesting that Fe–bearing minerals served as nucleation sites for Fe-Cd cluster formation. Synchrotron radiation-based FTIR spectroscopy (SR-FTIR) further demonstrated strong correlations (p＜0.0001, r² = 0.34–0.85) between organic and inorganic functional groups, indicating that Fe–Cd clusters co-assembled with organic matter into stable organo-mineral complexes. These complexes effectively immobilized Cd, thereby reducing its mobility. This study established a novel multi-spectroscopy approach for elucidating Cd-binding mechanisms in soils, providing new insights into sustainable agriculture and soil remediation.