Novel Insights into Driving Forces of Carbon Burial, Nutrient Heterogeneity, and Pollutant Release Pathways in Urban Lake Sediments

Characterizing the variation of pollutants in lake sediment and clarifying their release into overlying water are essential for remediating contaminated sediment. In this study, the total carbon in urban lake sediments significantly decreased with increasing depth, indicating a gradual reduction in carbon burial capacity. Initially, carbon and nitrogen were sequestered by the lake sediments, but they gradually diffused into the water or escaped into the atmosphere over time. The C/N ratios and carbon isotope ratios indicated a shift from algae to C₄ or crassulacean acid metabolism plants as the major organic matter with decreasing sediment depth, revealing a reduction in lake water eutrophication over the past decades. Stronger pollution and higher temperatures promoted the release of pollutants, with nitrogen primarily existing in the form of NH₄⁺-N. The maximum release concentration of COD and total phosphorus (TP) occurred 8 h later than NH₄⁺-N and NO₃^--N, indicating differing affinities for the solid phase due to disturbance. Aeration facilitated pollutants release, but organic compounds diffusing from solid sediment to pore water and then releasing into the overlying water lagged behind that of TP. The addition of biochar was effective in limiting the release of NH₄⁺-N but did not significantly inhibit the release of COD and NO₃^--N. These findings contribute to a better understanding of pollutants variations, the mechanisms of carbon burial, and the release of pollutants into overlying water from urban lake sediment, in addition to providing new insight for understanding pollutants characteristics in urban lakes sediments.

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