| The occurrence mode, distribution, transformation and migration of nitrogen and phosphorus in the sediments play an important role in ecological function and eutrophication of lakes. As the largest freshwater lake of north China, Nansi Lake functions as an important water channel and regulation lake of South-to-North Water Diversion Project’s east route, and its eutrophication and ecological risk threaten the safety of water. Based on the study of Nansi Lake, the surface sediments of Nansi Lake, occurrence mode, distribution, transformation and migration and bioavailability are explored in this paper.By adopting the soil analiysis method and improved chemical sequential extraction method, distribution of occurrence mode of nitrogen and phosphorus on overlying water and different depth of sediments (0-50cm), as well as the correlation among different occurrence modes of nitrogen and phosphorus are analyzed. The result shows that the total amount of TN ranges from 373.72 to 745.86 mg·kg-1, among which NH4+-N,being the main component, accounted for 65.3% to 76.0%, while NO2--N being the least component, and constitutes for less than 1%. NO2--N increases with the increasement of sediments, while the distribution of NO3--N remain stable in different depth. The distribution characteristics of NH4+-N in different depth are highly correlated with the distribution of TN, which leads to a correlation coefficient of 0.9. The content of TN in estuary is 40% higher than it in centre of lake.The total amount of TP in the surface sediments ranges from 190.96 to 799.39 mg·kg-1. The total amount of TP presents periodic changes during the process of growth of aquatic plants and animals. The main source of exchangeable phosphorus (Ex-P), part of TP, is the degradation of the cells of aquatic animals and plant and transformation of calcium-binding phosphorus (Ca-P). The concentration level of Ex-P and the growth cycle of aquatic plants and animals are closely connected, accounting for 1.48% to 18.4% of TP, with the average amount constitute for 4.52%. Mass concentration of aluminum bound phosphorus (Al-P) in the sediment is related to geological environment and pH value of the lake, and is slowly converted into iron-bound phosphorus. Al-P constitutes the minimum content in surface sediments of the lake, accounting for only 0.4 to 1.3% of TP. Fe-P, originating mainly from neighboring sewage and industrial wastewater, accounts for 4.25% to 49% in the TP. Under aerobic conditions, Fe3+ easily combines with phosphorus, and forms phosphate precipitation, which become potential sources of phosphorus in sediments. Under hypoxic conditions, insoluble Fe(OH)3 is converted to soluble Fe(OH)2, causing huge amount of Fe-P go into the lakes, and accelerate the lake eutrophication. As a kind of phosphorus hardly absorbed by living organisms, occluded phosphorus (Oc-P) accounts for 3.9% to 26.0% in the TP. On the one hand, the concentration is related to the nature of sediments and diagenesis; on the other hand, it is affected by the remains of the animal and plant decomposed by soil microorganisms and absorption.Debris phosphorus (De-P) comes from early diagenesis in lake sediments, e.g. some insoluble mineralization with calcium phosphate and other products, maintains high stability, and accounts for 10.8% to 45.3% in the TP content. Ca-P is the main chemical speciation of inorganic phosphorus in the Nansi lake sediments, accounting for 10.0% to 47.6%. Most Ca-P exists in the form of calcium phosphate. Like Fe-P, Ca-P is also a potential phosphorus source in the lake sediments. Organophosphorus (Or-P) includes recalcitrance organphosphorus discharged by sewage, agricultural pollution and industrial wastewater and organic organphosphorus from residues of aquatic plants and animals. The amount of Or-P is low, accounting for 10.0% to 47.6% of TP.In the study of adsorption and desorption of nitrogen and phosphorus, the captive test showed that surface sediments of Nansi Lake absorbing nitrogen and phosphorus follows the quasi-second kinetic equation, and the release kinetics can be described by first-order kinetic reaction equation. At the same time, good linear correlation between nitrogen and phosphorus adsorption isotherm can be observed. The nitrogen and phosphorus adsorption increases significantly with temperature increasing, the adsorption of NH4+-N and N03--N in 25℃ is five times the adsorption in 5℃; in weakly alkaline environment, the release of Ex-P, Al-P and Fe-P is significantly higher than that of neutral or weak acid environment; hydraulic disturbances promotes the release of the Ca-P, De-P and other physical and chemical properties of a stable form of phosphorus.The release of Or-P and Oc-P is affected by temperature significantly. When the sediment releases the phosphorus, Ex-P, Al-P, Fe-P and Ca-P and other active or potentially active chemical speciation is the source of release; and when the sediment absorbs the phosphorus, phosphorus exists in the form of Ex-P, Al-P and Fe-P. With their stability nature, Oc-P, De-P and Or-P will go into the overlying water under certain conditions or converted into other forms of phosphorus attached to the sediment.In the upper surface of the sediment, Ex-P goes into the overlying water with the sediment interstitial water through diffusion. When the depth increases significantly, the diffusion in sediments is weakened.In the sediments, Ex-P is mainly transformed into AI-P, Fe-P even Oc-P and other forms absorbed on sediment particles.Experiment focuses on the migration and transformation of phosphorus in the sediments due to the occurrence mode of phosphorus is more and the main limiting of eutrophication in freshwater lakes.The release of Ex-P in upper surface sediments of the overlying water, as the most important form of the sediment phosphorus, plays a positive role in promoting the concentration of TP. As the most complex form of phosphorus, Fe-P is related with the migration and transformation of surface sediments, and plays a crucial role in the migration and transformation of chemical speciation of phosphorus. Fe-P can be released by converted to Ex-P and AI-P, as well as more stable Oc-P and Ca-P under the influence of certain environment or biological conditions. Through its own redox, Fe-P is either released or subsided.Ex-P, Al-P, Fe-P and Ca-P all function as important sources of biological phosphorus in the overlying water. Among them, Ex-P in dissolved form and Weak-adsorption state, particulate Al-P, absorbed and particulate Fe-P in dissolved form and particulate are the phosphorus forms with biological effectiveness, which can be directly used by aquatic plants, while the particulate Ca-P has the validity after being absorbed by aquatic plants or microbial conversion. With the control and reduction of exterior phosphorus into Nansi Lake, the sediment becomes the main source of phosphorus in the lake. Ex-P and Fe-P are the major biological phosphorus in the lake sediments, playing a crucial role to the content of FeO-P in the lake. The reduction and control of internal phosphorus in the surface sediments (especially biological phosphorus) is an efficient way to control the concentration of nutrients and eutrophication. |
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