Retention of wastewater-borne nitrogen and phosphorus in mangrove soils

Two different surface mangrove soils were collected from Hong Kong (Sai Keng) and the People's Republic of China (Shenzhen), and their capacity for retention of nutrients was evaluated by column leaching experiments. These soils were irrigated three times a week with strong synthetic wastewater over a period of five months. At the end of the experiment, both types of mangrove soils had significantly higher concentrations of total salts, organic matter, inorganic N (NH₄ ⁺- and NO₃ ^.-N), Kjeldahl N, extractable and total P, and extractable K than the control (heated with artificial seawater), indicating that nutrients from wastewater were retained and accumulated in the mangrove soils. The wastewater-borne N and P were mostly concentrated in the upper 1 cm of the soil masses, with very little downwards migration, suggesting that the capacity of the mangrove soils in immobilizing nutrients was very high. In both treated and control columns, Sai Keng soils had significantly lower concentrations of organic matter and nutrients except nitrate than those from Shenzhen. However, the enrichment factors of Sai Keng soils were higher than those of Shenzhen, which might be related to the fact that Sai Keng soils had lower background values of organic matter and nutrients. These results indicate that the capacity of mangrove soils to trap organic matter and nutrients varied between soil types, affected by the chemical composition of the parent soil.