Assessing phosphorus risk in affected mangrove sediments using elemental ratios and extractable phosphorus

Abstract

This study investigates major elements (Al, Fe, Ca, Mg, N, C) and extractable phosphorus (P) in surface sediments from both human-affected and conserved areas within a mangrove forest in Ho Chi Minh City, Vietnam. The research aims to understand the roles of elemental ratios (C: N, Fe: P, C:P) and different phosphorus pools (Labile P, apatite P, non-apatite inorganic P) in controlling phosphorus release and assessing environmental risks. A total of twenty - eight sediment samples were analyzed for major elements including Fe, Al, Ca, Mg, and P after aqua regia digestion. Different phosphorus pools were determined based on chemical extraction methods. Organic carbon and total Kjeldahl nitrogen were also analyzed to assess the overall nutrient composition and potential for contamination.The sediments were acidic (average pH = 6.27), with average metal contents of Fe₂O₃ (5.69%), Al₂O₃ (5.73%), MgO (1.86%), and CaO (0.29%). The average phosphorus pool rankings were apatite P (204 mg/kg), non-apatite inorganic P (66 mg/kg), and Labile P (2 mg/kg). The results from the elemental ratios suggest that P is more tightly bound to Fe-P complexes and minerals in the sediments of affected areas, whereas it is more associated with organic P forms in the sediments of conserved areas. Consequently, mineralization in the conserved area may occur more slowly, resulting in a slow release of P from sediments in that region. Nutrient indices showed moderate pollution for P and severe for N across seasons and areas. Risk assessment (RI), calculated from bioavailable P, indicated a significant increase in RI (from 0.12 to 1.33) in affected areas under worst-case scenarios, suggesting that these areas are more susceptible to environmental changes regarding P compared to conserve area, which leads to a greater impact on the RI. These findings imply that the conserved area may be more effective in regulating changes in P concentrations in response to external factors and suggest that it plays a crucial role in mitigating P fluctuations, thereby enhancing ecosystem stability and reducing the risk of nutrient-driven environmental degradation.