The levels of metals in sediments of urban river ecosystems are crucial for aquatic environmental health and pollution assessment. Yet little is known about the interaction of nutrients with metals for environmental risks under contamination accumulation. Here, we combined hierarchical cluster, correlation, and principal component analysis with structural equation model (SEM) to investigate the pollution level, source, toxicity risk, and interaction associated with metals and nutrients in the sediments of a river network in a city area of East China. The results showed that the pollution associated with metals in sediments was rated as moderate degree of contamination load and medium-high toxicity risk in the middle and downstream of urban rivers based on contamination factor, pollution load index, and environmental toxicity quotient. The concentration of mercury (Hg) and zinc (Zn) showed a significant correlation with toxic risks, which had more contribution to toxicity than other metals in the study area. Organic nitrogen and organic pollution index showed heavily polluted sediments in south of the study area. Though correlation analysis indicated that nutrients and metals had different input zones from anthropogenic sources in the urban river network, SEM suggested that nutrient accumulation indirectly intensified toxicity risk of metals by 13.6% in sediments. Therefore, we suggested the combined consideration of metal toxicity risk with nutrient accumulation, which may provide a comprehensive understanding to identify sediment pollution.
Toxicity rate of metals in sediments from urban river network indirectly intensified by nutrients accumulation
Due to the important roles of carbonyl sulfide (COS) and carbon disulfide (CS2) in atmospheric chemistry, this study was designed to determine different proportions of COS and CS2 fluxes contributed from different sources, i.e., vegetation, soil and roots, at monthly and hourly timescales in the arid area in Xinjiang, China. Results indicated that the seasonal net uptake of COS by vegetation was predominant in the growing season. The CS2 fluxes from vegetation and soils had no significant seasonal variations compared with COS. The exchange rates of COS and CS2 have been found to be stimulated by the addition of nutrients in the form of urea fertilizer. Compared with the results of plots that were treated only with nitrogen, the treatments with both nitrogen and sulfur displayed no significant difference in the exchange fluxes. The results of compartment experiments indicated that the aboveground plants had the highest uptake of COS and had a vital role in the uptake of COS during the main growth period. The shares of COS emissions from the soil and roots increased to 6–17% and 55–58%, respectively, in the total COS fluxes when conditions, such as drought and senescence, were unfavorable for the developmental of vegetation. Observations of the preliminary diurnal fluxes indicated that the fluxes that occurred at night, with contributions from soils and plants, accounted for 27% of the total daily uptake of COS uptake. These quantitative results may be reasonably accounted for the use of COS as a promising tracer to obtain independent constraints on terrestrial carbon exchange at regional to global scales for their response to special environmental conditions in semiarid area.
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