Nitrate contamination of water is a worldwide environmental problem. Recent studies have demonstrated that the nitrogen (N) and oxygen (O) isotopes of nitrate (NO3(-)) can be used to trace nitrogen dynamics including identifying nitrate sources and nitrogen transformation processes. This paper analyzes the current state of identifying nitrate sources and nitrogen transformation processes using N and O isotopes of nitrate. With regard to nitrate sources, δ(15)N-NO3(-) and δ(18)O-NO3(-) values typically vary between sources, allowing the sources to be isotopically fingerprinted. δ(15)N-NO3(-) is often effective at tracing NO(-)3 sources from areas with different land use. δ(18)O-NO3(-) is more useful to identify NO3(-) from atmospheric sources. Isotopic data can be combined with statistical mixing models to quantify the relative contributions of NO3(-) from multiple delineated sources. With regard to N transformation processes, N and O isotopes of nitrate can be used to decipher the degree of nitrogen transformation by such processes as nitrification, assimilation, and denitrification. In some cases, however, isotopic fractionation may alter the isotopic fingerprint associated with the delineated NO3(-) source(s). This problem may be addressed by combining the N and O isotopic data with other types of, including the concentration of selected conservative elements, e.g., chloride (Cl(-)), boron isotope (δ(11)B), and sulfur isotope (δ(35)S) data. Future studies should focus on improving stable isotope mixing models and furthering our understanding of isotopic fractionation by conducting laboratory and field experiments in different environments.
Reclamation along coastal zones is a method that has been used to relieve the problems of strained resources and land. Aquaculture, as one of the major man-made activities in reclamation areas, has an environmental impact on coastal waters. The effluents from aquaculture ponds are known to enrich the levels of nutrients such as nitrogen and phosphate. The goals of the present study are to evaluate the environmental impact of mariculture on coastal waters in the east coast of Laizhou Bay, China, and to identify the nitrate sources. Monitoring the concentrations of dissolved nitrogen and phosphate was used to assess their impact on the water quality of coastal waters. A dual isotope (δ(15)N-NO3(-) and δ(18)O-NO3(-)) approach was used to identify the nitrate sources. Higher dissolved nitrogen concentrations (NH4(+) and NO3(-)) than PO4(3-) concentrations associated with enriched δ(15)N-NO3(-) values were observed in the drainage channels, sea cucumber ponds, and groundwater, which indicated that aquaculture activity has more influence on nitrogen nutrients than on phosphate nutrients. In this coastal area with seawater intrusion, nitrogen released from sea cucumber ponds accumulated in nearshore water and migrated in the offshore direction in groundwater currents. This behavior results in nitrogen enrichment in groundwater within the hinterland. Isotopic data indicate that mixing of multiple nitrate sources exists in groundwater, and nitrogen from mariculture is the main source.
Underground cut-off walls in coastal regions are mainly used to prevent saltwater intrusion, but their impact on nutrient dynamics in groundwater is not clear. In this study, a combined analysis of multiple isotopes ([Formula: see text]) and nitrogen and phosphorus concentrations is used in order to assess the impact of the underground cut-off walls on the nutrient dynamics in groundwater in the lower Wang River watershed, China. Compared with the nitrogen and phosphorus concentrations in groundwater downstream of the underground cut-off walls, high [Formula: see text] and total dissolved nitrogen concentrations and similar concentration levels of [Formula: see text] and total dissolved phosphorus are found in groundwater upstream of the underground cut-off walls. The isotopic data indicated the probable occurrence of denitrification and nitrification processes in groundwater upstream, whereas the fingerprint of these processes was not shown in groundwater downstream. The management of fertilizer application is critical to control nitrogen concentrations in groundwater restricted by the underground cut-off walls.
The assessment of habitat quality plays an important role in the effective conservation of wetland biodiversity. The Yellow River Delta Wetland is located in the intertwining zone of sea, terrestrial, and river ecosystems, increasing human activities and climate change posed a great threat to wetland biodiversity. This study first analyzed the spatial and temporal evolution characteristics of habitat quality in the Yellow River Delta Wetland under the evolution of the shoreline after runoff-sediment variability (1986–2020) using the InVEST-habitat quality model and then identified the dominant influence factors on habitat quality based on Geographical Detector. Finally, elasticity index was introduced to analyze the impacts of different reclamation activities on habitat quality. Results showed that the habitat quality decreased from 0.4798 in 1986 to 0.4078 in 2020, with high values of habitat quality concentrated in mudflat wetlands and low values of habitat quality concentrated in construction land and salt pans. The results of the Geographical Detector analysis showed that the influence of human activities, especially reclamation activities, had stronger effects on habitat quality than climatic factors. The elasticity index analysis showed that the elasticity of all three types of reclamation activities, namely, culture ponds, construction land, and salt pans, were negative from 1986 to 2005, 2005 to 2020, and 1986 to 2020. The reclamation activities had a negative impact on habitat quality. The habitat quality of the Yellow River Delta Wetland was most sensitive to the change in reclamation intensity of construction land from 1986 to 2020, and the sensitivity of the change of habitat quality to the change of reclamation intensity of culture ponds and salt pans was strengthening. This study explicitly revealed the effect of climate change and human activities on the habitat quality of the Yellow River Delta Wetland and proposed to analyze the response intensity of habitat quality to different reclamation activities by using the elasticity index, thus providing a scientific basis for mitigating the tradeoff between biodiversity conservation and rapid social development in the Yellow River Delta Wetland in the future.
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