| It has been widely confirmed that wetland can serve as the natural barrier for nutrient pollutant.To explore the transportation and transformation process of nutrient in the natural wetland,and to identify the key controlling factors and processes has important practical significance for the application of appropriate management strategies for using wetland to control nutrient pollutants.Luoshijiang estuarine wetland,which was located on the upstream of Erhai Lake,was selected as the research object and field test which hold the period of one year was carried out.The paper intended to explore the removal efficiencies and key influence factors of nitrogen and phosphorus nutrient retained by estuarine wetland,make clear the spatial and temporal change rules of distribution of nitrogen and phosphorus in the water-sediment-wetland plants,three-phase environmental medium,evaluate the nitrogen and phosphorus pollution risk of the important sink of wetland,and finally focus on the spatial and temporal diffusion flux changes of nitrogen and phosphorus at the interface of water-sediment.The results could provide basic data and scientific proof for strengthening the estuarine wetland purification function to water pollution load,reducing the nutrient release,and protection of water environment of Erhai Lake.The following conclusions were conducted based on the monitoring data:(1)Obvious seasonal and spatial heterogeneity was showed for the removal efficiencies of different forms of nitrogen and phosphorus by estuarine wetland.The removal rates of TN and NN of channel I was higher than that of channel II,and that in dry season was better than that in wet season.On the other hand,the removal rates of AN showed the opposite trend and it did exist the release of nitrogen to wetland water in some months.The removal effect for phosphorus is generally worse than nitrogen,and that of TP and Ortho-P of channel II is higher than that of channel I,while that of DTP showed the contrary trend.Removal effect of TP in dry season is greater than that in wet season,and the phenomenon of phosphorus release into the water existed in most of the month of wet season.(2)The main physical and chemical parameters of water which affected the removal of nitrogen and phosphorus in various form by estuarine wetland were pH,SS and Ch-a,which may change the sedimentation of particles rich in nitrogen and phosphorus and also the microbial activity related to nitrogen and phosphorus transformation.The main factors which differed the removal rates between channel I and II were ORP,EC and T,while that differed the removal rates between wet and dry season were SS,Ch-a and ORP.The effects of sediment physical and chemical properties on nutrient removal rates were not significant.The main wetland structure factors affecting the nutrient removal efficiencies were depth,thickness of sediment and nitrogen accumulation by plant,which may affect the oxidation-reduction environment,sediment phosphorus enrichment capability and uptake of ammonia nitrogen by plants.Water depth was main factor differed the removal efficiencies between channel I and II,and depth and thickness of sediment were the main factors differed that in dry and wet season.A single wetland plant species(Zizania Caduciflora)showed the most significant contribution to the nutrient removal efficiency.(3)Sediment was the most important medium for nitrogen and phosphorus enrichment among three-phase mediums as water,plant and sediment in estuarine wetland,and contributed more than 99% to the total.At the same time,the amounts of nitrogen and phosphorus enrichment in three-phase medium were affected by channels and seasonal changes.The amounts of TN and TP in the sediment and plant both showed that channel II was greater than channel I,and no obvious trend showed in water.The amounts of TN in water and sediment showed that dry season was greater than wet season,and plant showed the opposite trend.The amounts of TP in sediment showed higher values in dry season than that in wet season,which was opposite in plant,and no obvious seasonal trend in water.The differences of exogenous input,channels,seasonal change and run off rules for nitrogen and phosphorus may be the dominant factors affecting the enrichment processes.(4)NN,TN and DTP were the main factors which controlled the distribution of nitrogen and phosphorus in wetland system if only the physical and chemical properties of water were considered.It also indicated that NN and DTP were the main form of nitrogen and phosphorus in water,which could be easily absorbed by sediments and fixed by plants.TN and HN were the main influence factors of surface sediment,and HN was the main form which had the release potential but could not be easily used by plants.The plant configuration type and BM were the dominant factors of wetland structure for nitrogen and phosphorus distribution,and the plots with abundant biomass and various types of plants showed significant nitrogen and phosphorus enrichment.(5)The amounts of nitrogen,phosphorus and organic matter in sediments all showed significant enrichment in the surface layer of sediment,and decreased with the increasing depth.The concentrations of nitrogen and phosphorus in dry season were higher than that in wet season,while no obvious difference for organic matter.The pollution level of nitrogen,phosphorus and organic matter in surface layer sediment was in moderate to severe level,and that in middle layer and below was in mild to moderate level.The pollution level in dry season was higher than that in wet season.Sediment showed obvious trend of releasing nitrogen and phosphorus to water body in wet season.The nitrogen and phosphorus in sediments mainly came from the exogenous source while organic matter from the endogenous,at the same time,the contribution of endogenous nitrogen and phosphorus increased significantly in the dry season.(6)The diffusion flux of different forms of nitrogen and phosphorus on the interface of water-sediment in estuarine wetland showed obviously spatial and seasonal changing trends.The diffusion fluxes of AN in the whole year all showed releasing trend from sediment to water,and the largest releasing amount appeared in dry season,while channel II released more than.The diffusion fluxes of NN from Jan.to Mar.showed sediment accumulation,while the rest month of the year showed release from sediment to water,and channel II had the higher releasing amount.The diffusion fluxes of Ortho-P from Jan.to Mar.showed the trend of releasing to water,while channel II was greater,and for the rest month of the year,both the release and accumulation phenomenon existed.The diffusion fluxes of nitrogen and phosphorus in each plots varied significantly,while Ortho-P had the highest changing magnitude,followed by NN,and AN had the lowest.(7)The following properties including PW-AN,PW-NN,PW-Ortho-P,TP,TN and Ch-a showed significantly influence to the diffusion fluxes on the interface of water-sediment considering all the parameters in overlying water,pore water and surface sediment.And the accumulative explanation rate nearly reached to 100.00%.Among these factors,the concentrations of different forms of nitrogen and phosphorus in pore water showed positively relation with the corresponding diffusion flux,while the concentration of nitrogen,phosphorus and chlorophyll a in overlying water significantly affected the diffusion fluxes of nitrogen and phosphorus. |
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