Phosphorus fractions and sorption-release characteristics of Huixian Karst Wetland soils under different land uses

Abstract

Phosphorus (P) is a key limiting factor in the eutrophication of water bodies. However, the regulatory mechanisms by which the unique high-Ca2+/HCO3− hydrochemical conditions in karst regions influence soil-sediment P adsorption–release behavior remain poorly understood. In this study, soils from five land use types and lake margin sediments (SED) in the Huixian Karst Wetland, Guangxi, China, were analyzed through sequential P fractionation, adsorption kinetics experiments, and long-term overlying water–sediment incubation to elucidate the effects of karst hydrochemistry on P adsorption and release dynamics. The results showed that: (1) Total P ranged from 301.97 to 1119.33 mg·kg−1 across soil types, with inorganic P dominating and iron-bound phosphorus (Fe–P) accounting for 27–54%, indicating a high potential for P release. Land use regulates key soil properties (total organic matter (OM), total calcium, and exchangeable Ca/Mg), thereby controlling P mobility. (2) P adsorption in karst paddy soil (PS) followed pseudo-second-order kinetics, indicating that chemical adsorption was the dominant mechanism. In contrast, non-karst grassland soil (GS) reached equilibrium within 10 min with a higher maximum adsorption capacity. Ca2+ linearly promoted PS adsorption (R2 = 0.823), whereas high HCO3− concentrations suppressed it, and GS showed negligible sensitivity to both ions. (3) Overlying water P concentrations followed a rise-then-decline pattern, with the release capacity ranked as Sediment > PS > Ditch soil (DS) > GS. Elevated Ca2+ and HCO3− concentrations inhibited P release from SED but promoted it from PS. A sharp Fe–P decline during days 50–100 corresponded to elevated summer temperatures (26 ~ 33℃), accelerating internal P loading. These findings clarify that Ca2+ and HCO3− concentrations exert substrate-dependent controls on P cycling in karst wetlands: elevated Ca2+ and HCO3− suppress P release from lake SED while promoting it from PS, with temperature further modulating Fe–P dynamics and internal loading. These results provide a theoretical basis for source-specific management of eutrophication in karst regions.