Multi-level evaluation of sedimentary phosphorus release risk in freshwater water bodies based on dilute-acid...

Elevated internal loading of bioavailable phosphorus (P) from sediments sustains aquatic eutrophication globally, impeding restoration of diverse water bodies. Establishing a unified method to assess sediment P multi-level risk is critical for effective aquatic ecosystem management. This study examined the feasibility of using 0.1 M HCl-extracted sediment P (dHCl-P) as a novel indicator for assessing internal P release risk. Sediment samples were collected from four water bodies: Honghu Lake (HL, n = 6), Xiashan Reservoir (XR, n = 18), Zhanghe Reservoir (ZR, n = 5), and Yudong Reservoir (YR, n = 30) in China. The dependences of dHCl-P on reductive sediment types (RedOr-Fe(II) sediment and RedOr-Sⁿ sediment; RedOr: reducing organic substances), sediment matrix types (Ca²⁺/Mg²⁺-dominant and Fe/Al-dominant), total P (TP), and P fractions were evaluated. Statistical relationships between dHCl-P and both total dissolved P (TDP) and soluble reactive P (SRP) in overlying water were characterized. The results showed that dHCl-P content was primarily controlled by reductive sediment types rather than sediment matrix types and TP. RedOr-Sⁿ sediments (HL, XR; mean TP of 574 ± 183 mg kg^-1DW) exhibited significantly higher dHCl-P (mean 314 ± 147 mg kg^-1DW) than RedOr-Fe(II) sediments (ZR, YR; mean TP of 1260 ± 354 mg kg^-1DW) with an average value of 165 ± 182 mg kg^-1DW (p < 0.01). The proportion of dHCl-P in TP (dHCl-P/TP) increased linearly with the reduction degree of iron oxides (ratio of Fe(II) to total Fe, Fe(II)/TFe). P fraction components of dHCl-P consisted apparently of exchangeable P (Ex-P), P bound to Fe(II) (Fe(II)-P), and P bound to Ca²⁺ (Ca-P). Sediment reduction promoted the transformation of P bound to Al oxides (Al-P), P bound to Fe oxides (CDB-P), and organic P (O-P) to dHCl-P, resulting in its enrichment. Statistical analysis confirmed the significant linear correlations between dHCl-P in sediments, and TDP and SRP in overlying water (p < 0.01). Based on reductive sediment types and the relationship between dHCl-P/TP and Fe(II)/TFe, our findings indicate that dHCl-P can serve as an effective indicator for establishing an operational framework, enabling four-level assessment of internal P risks across different water bodies.