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In the coastal wetland, nitrogen is a limiting element for plant growth and reproduction. However, nitrogen inputs increase annually due to the rise in nitrogen emissions from human activity in coastal wetlands. Nitrogen additions may alter the coastal wetlands’ soil properties, bacterial compositions, and plant growth. The majority of nitrogen addition studies, however, are conducted in grasslands and forests, and the relationship between soil properties, bacterial compositions, and plant growth driven by nitrogen addition is poorly understood in coastal marshes. We conducted an experiment involving nitrogen addition in the Phragmites australis population of the tidal marsh of the Yellow River Delta. Since 2017, four nitrogen addition levels (N0:0 g • m-2 • year-1, N1:5 g • m-2 • year-1, N2:20 g • m-2 • year-1, N3:50 g • m-2 • year-1) have been established in the experiment. From 2017 to 2020, we examined soil properties and plant traits. In 2018, we also measured soil bacterial composition. We analyzed the effect of nitrogen addition on soil properties, plant growth, reproduction, and plant nutrients using linear mixed-effect models. Moreover, structural equation modeling (SEM) was utilized to determine the direct and indirect effects of nitrogen addition, soil properties, and bacterial diversity on plant growth. The results demonstrated that nitrogen addition significantly affected plant traits of P. australis. N1 and N2 levels generally resulted in higher plant height, diameter, leaf length, leaf breadth, and leaf TC than N0 and N3 levels. Nitrogen addition had significantly impacted soil properties, including pH, salinity, soil TC, and soil TS. The SEM revealed that nitrogen addition had a direct and positive influence on plant height. By modifying soil bacterial diversity, nitrogen addition also had an small indirect and positive impact on plant height. However, nitrogen addition had a great negative indirect impact on plant height through altering soil properties. Thus, nitrogen inputs may directly enhance the growth of P. australis at N1 and N2 levels. Nonetheless, the maximum nitrogen addition (N3) may impede P. australis growth by reducing soil pH. Therefore, to conserve the coastal tidal marsh, it is recommended that an excess of nitrogen input be regulated.

Water-use characteristics are important aspects of plant adaptation to environmental factors. While water-use efficiency (WUE) and water sources used by plants in arid habitats are commonly studied, water-use strategies of plants in coastal wetlands are poorly studied. Although ample water is available in coastal wetlands, water conditions vary among seasons, suggesting that the plant in coastal wetlands may need to adapt to seasonal changes in water availability.To uncover temporal variation in plant water-use strategies in the coastal wetland, we investigated WUE and water sources used by Phragmites australis in the tidal marsh of the Yellow River Delta employing stable isotopic composition indicators. Temporal variation of δ13C and δ18O in leaf organic matter were measured in two years (2018 and 2019) along with plant traits and environmental factors, to determine the factors affecting δ13C and δ18O in leaf organic matter. Water sources (rainfall, soil water in different depths, tidal water, and groundwater) were sampled for δ18O composition in the early growth season (2019 May, June, and July) to determine water sources using the Bayesian mixing model MixSIAR.We found that P. australis had high WUE in spring and autumn, but WUE declined during summer. Changes in WUE were explained by variation in plant height, leaf nutrient concentrations (C: N, total N, total S), leaf traits (leaf length and leaf number), soil electronic conductivity (EC), soil pH, soil nutrients (NO3-N at 0–30 cm soil depth, total S at 0–30 cm soil depth, available P at the soil depths of 0–10 cm and 40–50 cm, and total C at the 30–40 cm soil depth). Spring drought may enhance WUE through increasing soil salinity (i.e., EC) and lowering pH. Water uptake in the early growth season was composed mainly of rainfall, soil water, and groundwater. However, tidal water (i.e., ocean water) was not a main water source. To sum up, leaf WUE varied with seasons in the coastal wetland, which may be related to plant traits and soil factors, particularly salinity and pH. The plant in the tidal marsh may not take up ocean water, even though it is really abundant. Precipitation is still an important water source promoting growth of these plants in the coastal wetland, therefore, to conserve them, supplementing fresh water in the drought season is suggested.

Land reclamation can significantly influence spatial distribution of heavy metals in inshore sediments. In this study, the distribution and contamination of heavy metals (Cd, Cr, Cu, Ni, Pb, and Zn) in inshore sediments of Bohai Bay were investigated after the land reclamation of Caofeidian. The results showed that the concentrations of Cd, Cr, Cu, Ni, Pb, and Zn in the sediments were 0.20–0.65, 27.16–115.70, 11.14–39.00, 17.37–65.90, 15.08–24.06, and 41.64–139.56 mg/kg, respectively. These metal concentrations were generally higher in the area of Caofeidian than in other Chinese bays and estuaries. Spatially, the concentrations of Cd, Cr, Cu, Ni, and Zn were markedly lower in the sediments close to Caofeidian compared with other regions, whereas the concentrations of Pb showed an opposite case. Hydrodynamic conditions after the land reclamation were the major factor influencing the distribution of heavy metals in the sediments. Grain sizes dominated the distribution of Cu and Zn, and organic matters and Fe/Mn oxides/hydroxides also determined the distribution of the heavy metals. Multiple contamination indices showed that the inshore sediments were moderately to highly contaminated by Cd and slightly contaminated by other heavy metals. Similarly, Cd showed a high potential ecorisk in the sediments, and other metals were in the low level. Chromium contributed to higher exposure toxicity than other metals by the toxicity unit and toxic risk index. The results of this study indicate that after the land reclamation of Caofeidian the contamination and ecorisk of heavy metals in the sediments markedly decreased in the stronger hydrodynamic areas.

Aims The mechanisms of plant community assembly are hypothesized to vary at different stages of succession. Here, we explore the local assemblage structure of a herbaceous plant community at its early stage of succession in a supratidal wetland. Specifically, we assess the role of Chinese saltcedar (Tamarix chinensis), the lone dominant shrub species, in shaping the spatial structure and species composition in the local plant community, after landscape alteration. Methods We used the multivariate trend-surface analysis for analyzing the spatial structure of the community composition. A null model was also used to detect potential biotic interactions between species. Statistical significance was derived from a permutation test by randomizing the presence-absence matrix and functional traits independently. Sensitivity analysis by randomly selecting 50 subplots and repeating the null model tests was also done. Finally, rank correlation analysis was used to study the relationship between effect sizes and distance to nearest T. chinensis individuals. Important Findings The herbaceous plant community was highly structured and shaped by the presence of T. chinensis. At local scale, two functional traits, plant height and leaf area, were found to be significantly convergent. Dispersal, environmental stress and interspecific competition played a trivial effect on the local community assembly. The facilitating effect of T. chinensis on the pioneering herbaceous plants, through acting as a wind shelter, was put forward as the dominant community assembly process. [ABSTRACT FROM AUTHOR]