In order to reveal the response of photosynthesis and water consumption characteristics in Tamarix chinensis leaves to the depth of groundwater table, and define the depth of groundwater table to maintain T. chinensis higher photosynthetic efficiency and suitable growth. By taking three-year-old T. chinensis seedlings as experimental materials, a total of 7 submersible depths of groundwater table (0 m, 0.3 m, 0.6 m, 0.9 m, 1.2 m, 1.5 m and 1.8 m) was designed, and the light response processes of gas exchange parameters and the daily dynamics of sap flow in T. chinensis leaves to different depths of groundwater table were analyzed. The results showed that different depths of groundwater table could significantly change the soil moisture condition, and affect the photosynthetic process and water consumption in T. chinensis leaves. The net photosynthetic rate (P_n), photosynthetic response parameters, water use efficiency (WUE) and sap flow rate in T. chinensis leaves had obvious response to the depth of groundwater table. (1) With the depth of groundwater table rising, the P_n, WUE, transpiration rate, and sap flow rate increased first and then decreased, which showed the maximum value at the depth of groundwater table of 1.2 m with relative soil water content of 40.51%. The stomatal conductance, stomatal limit value increased first and then decreased, while the intercellular CO₂ concentration was the opposite; From 1.2 m to 1.8 m depth of groundwater table or from 1.2 m to 0 m depth of groundwater table, the photosynthetic decrease was mainly due to the stomatal limitation. (2) With the depth of groundwater table rising, the maximum net photosynthetic rate, light saturation point, and apparent quantum efficiency in T. chinensis leaves increased first and then decreased, reaching the maximum value (21.15 μmol·m^-2·s^-1, 1513.4 μmol·m^-2·s^-1, and 0.06 μmol·mol^-1, respectively) at the 1.2 m depth of groundwater table, while the light compensation point was the opposite. (3) The 1.2 m depth of groundwater table was the turning point of photosynthesis from stomatal limitation to non-stomatal restriction. The stomatal regulation made T. chinensis maintain high photosynthetic characteristics under drought stress. The conclusion was that the T. chinensis showed photosynthetic water adaptability to drought-tolerant water and moisture, and the 1.2 m depth of groundwater table under freshwater conditions was suitable for the growth of T. chinensis.

A crucial question in restoration of coastal saltmarsh ecosystem is how to keep sustainable and evenly distributed seed banks, and approaches to enhance the seed settlement and germination. Thus, an urgent need is to develop a method by ameliorating the available conditions for seeds settlement and germination, thereby promoting the effectiveness of coastal vegetation restoration. Here, using an annual saltmarsh vegetation Suaeda salsa as a model species in the Yellow River Delta, we experimentally enveloped a thin layer outside each seed by three kinds of super absorbent materials, thereby testing their difference and providing an optimal selection for the restoration of Suaeda salsa. The materials we selected were polyacrylamide PAM, hydroxyethyl cellulose HEC, and hydroxypropyl methyl cellulose HPMC, which of each harbors the high ability in water absorption. Firstly, we coated the powder of each kind of absorbent material outside the seeds; secondly, we experimentally manipulated a gradient of water salinity to know the coated seed germination vary with the salinity; thirdly, we experimentally released 30 seeds for testing their mean suspension time and 20 seeds for the whole process of settlement; in the end, we investigated the seeds germination with different absorbent materials. Our results showed that, compared to the natural seeds dispersed to the bare ground, seeds that were enveloped by the water retaining materials (i.e. seed product) significantly harbored more water and maintained the water for a longer time. Our findings are that seed product coated by absorbent materials presented shorter suspension time and quick settlement within the sea water; the seed product significantly increased the germination rate and salt tolerance In addition, we observed that PAM, HPMC and HEC differed significantly in making contribution to the seed suspension, settlement, and seed germination, showing that the seed product with PAM is higher than HPMC, HEC and natural seeds, successively. Our study provides an available approach for governing the persistence of the coastal seed banks, with important management implication to use artificial seed products in the restoration of degraded saltmarsh ecosystems.

Tamarix chinensis Lour. is a constructive species of the saline-alkali wetland in the Yellow River Delta, and shallow groundwater is the main water source for the growth of T. chinensis in this area. To explore the response mechanism of T. chinensis to groundwater salinity in shallow groundwater area in the Yellow River Delta, parameters of photosynthesis and water consumption were determined under 4 groundwater salinity gradients of fresh (0 g·L^-1), brackish (3 g·L^-1), saline (8 g·L^-1) and salt (20 g·L^-1) water at simulated groundwater level of 0.9 m via leaf gas-exchange and stem sap flow (SF) techniques. The results showed that: (1) With increasing groundwater salinity, soil water content, soil salt content and absolute concentration of soil solution increased gradually; (2) Parameters of the max net photosynthetic rate (P_nmax), apparent quantum yield (AQY), light saturation point (LSP), stomatal conductivity (G_s), transpiration rate (T_r), intercellular CO₂ concentration (C_i) and stem SF rate increased first and then decreased with the increase of groundwater salinities, reaching the maximum values respectively under brackish groundwater (G_s, AQY, T_r, C_i, and SF rate) and saline groundwater (P_nmax and LSP) treatments, while all values of the above parameters reached the minimum values under salt groundwater treatment; (3) With the increase of groundwater salinity, the water use efficiency (WUE) and stomatal limitation value (L_s) of T. chinensis decreased before increasing, achieving the lowest level under brackish groundwater and the highest under salt groundwater. Under the condition of groundwater depth of 0.9 m, the salinity of groundwater significantly affects the characteristics of soil water and salt contents. The increase of soil water and salt contents, especially the absolute concentration of soil solution, further affects the photosynthetic efficiency and water use strategy of T. chinensis. Moderate groundwater salinity (brackish and saline groundwater) could improve photosynthetic capacity and reduce WUE of T. chinensis, while excessive high groundwater salinity (salt groundwater) would inhibit photosynthesis severely and improve WUE. T. chinensis has high photosynthetic capacity, wide light ecological amplitude and high WUE under saline groundwater conditions at the groundwater level of 0.9 m. This research can be used for the further study of the relationship between plant photosynthetic processes and soil water and salt contents, and can provide theoretical reference for the restoration and reconstruction of soil and water conservation shelterbelts in the Yellow River Delta.

Phragmites auatralis wetlands affected by different flooding conditions (TW: tidal flooding wetlands; FW: fresh water restoration wetland; SW: seasonal flooding wetlands) were selected in the Yellow River Estuary to analyze the adsorption kinetics characteristics and influencing factors of phosphorus on surface wetland soils. Our results suggested that the adsorption of phosphorus in the overlying water on reed wetland soils with different water and salt conditions showed an order of FW>TW>SW, which indicated that the phosphorus adsorption capacity of FW soils was the largest. The phosphorus adsorption of soils from these three types of wetlands exhibited the fastest initial stage (0-3 h), and then gradual slowing (3-24 h), after that a slow and balanced state (after 24 h). However, the FW soils had the fastest absorption rate to phosphorus, while the SW soils showed the slowest absorption rate. The difference in water and salt conditions and physicochemical properties of wetland soils (such as pH, salinity, soil texture, Al₀, Fe₀ and Ca₀) were the main factors leading to differences in phosphorus adsorption amount and rate among the three types of wetland soils. The Simple Elovich model and the Power Function model were more suitable to simulate the dynamic characteristics of phosphorus adsorption by Phragmites auatralis wetland soils in the Yellow River Estuary. The freshwater restoration project of degraded wetlands in the Yellow River Estuary can promote the adsorption of phosphorus on wetland soils to a certain extent, and thus reduce the eutrophic risk of the overlaying water of coastal wetlands.

Salt marshes are increasingly valued for their function of coastal protection. Affected by periodic tidal events, salt marshes are highly dynamic ecosystems, especially for the hydrological characteristics. The dynamic hydrological characteristics have significantly affected the recruitment of plant, resulting in a profound influence on the distribution of salt marsh plants. To reveal the effect of hydrological characteristics on plant recruitment in salt marshes, this study conducted a field experiment on a bare patch of Yellow River Delta coastal salt marshes. A typical tidal inundation gradient was set, by choosing six transects from a main tidal creek to high areas on a bare patch in mid to high salt marshes. And the measurement of hydrological and soil characteristics, seed emergence and seedling survival experiments were conducted on the tidal gradient. Our results showed that: (1) Elevation, tidal hydrodynamics, and the inundation gradient had significant effects along the gradient from the creek to the salt marsh: The elevation of sampling points increased with the distance from the creek; on the temporal scale, there were wet and dry periods of hydrological process, the inundation frequency, water table and cumulative inundation period were lower during November to April, and they started to increase since May; on the spatial scale, the inundation frequency and cumulative inundation period decreased with the increase of elevation, and became stable after the average high water level; the relationship between the maximum water depth and elevation was revealed as a quadratic curve, and the maximum water depth reached the highest values in the elevation of 0.9 to 1.1 m. (2) Soil salinity and moisture content were not correlated with the hydrological gradient, but their changing trends were significant: with the increase of the distance to the tidal creek, soil salinity was increasing, while soil moisture content was decreasing. (3) The captured Suaeda salsa seeds (by seed traps), seed emergence rate and survival rate decreased with the increase of the distance to the tidal creek, which revealed that the recruitment of Suaeda salsa was significantly affected by hydrological characteristics. At the dry end of hydrological gradient (the higher area), the lower inundation frequency went against the dispersal of Suaeda salsa seeds, seeds emergence and seedling growth, which reduced the recruitment success of Suaeda salsa.

To explore the relationship between hydrological connectivity of wetlands and the characteristics of stable carbon and nitrogen isotopes in wetland components (plant, soil and water), hydrological connectivity of typical sites and stable carbon and nitrogen isotopes of plant, soil and water suspended particulate (TSS) samples from the Yellow River Delta were analyzed by field investigation and laboratory analysis. There were significant differences of δ¹³C between the aboveground and underground parts of the studied three plants (Phragmite australis, Typha orientalis and Suaeda salsa) in restored wetlands. The δ¹³C in TSS of restored wetlands was lower than that in tidal wetlands. The hydrological connectivity degree of restored wetlands and tidal wetlands was 0.0520 and 0.0484, respectively. The hydrological connectivity degree among sites was quite different. There was a significant negative correlation between the hydrological connectivity degree and the content of δ¹³C in TSS and aboveground part of plants, which indicated the probable effect of hydrological connectivity on the source of suspended particulate matter in water bodies, as well as the carbon metabolism in plant and plant growth.

Coastal wetland is the transitional area between land and sea, with high habitat complexity, species biodiversity and ecological service functions. However, due to the influence of human activity and climate change, the areas of the wetlands are gradually reduced, and the structure and functions of wetlands such as hydrological connectivity are greatly influenced. In the present work, we selected three typical tidal channels with obvious differences in hydrological connectivity, investigated the macrobenthos community between different tidal channels and analyzed the influence of longitudinal hydrological connectivity on the distribution and diversity (density, biomass and biodiversity) of macrobenthos. A total of 52 species were documented, and the numerically abundant taxonomic groups were Polychaeta and Mollusca; Overall, with the increase of hydrological connectivity, the total density of macrobenthos showed an upward trend; the areas with moderate hydrological connectivity tended to have a higher biodiversity; different groups of macrobenthos showed different density and distribution responding to environmental factors caused by hydrological connectivity; Polychaeta and Oligochaeta were dominant in areas with low hydrological connectivity and Mollusca were dominant in areas with high hydrological connectivity and abundant nutrients; Crustacea were dominant in areas close to the channel source with the highest hydrological connectivity. This study is helpful to understand the mechanism of wetlands hydrological connectivity on biological connectivity, and provide an important reference for the protection and restoration of coastal wetlands.

Coastal saline-alkaline soil plays important roles in global carbon and nitrogen cycling and climate change regulation. Biochar (BC), as an eco-friendly soil amendment, shows a promising prospect in terms of alleviating climate change and promoting sustainable agricultural development. However, most of the previous studies focused on the influence of BC application on greenhouse gas emissions and nitrogen availability and loss in coastal saline-alkaline soil, but little information that comprehensively summarized the effect of BC on the soil carbon and nitrogen cycling is available. As a result, the objective of this review is to comprehensively summarize that: (1) The influences of BC on soil carbon pools (vegetation and soil organic carbon) and soil organic carbon mineralization in the coastal saline-alkaline soil; (2) The influences of BC on nitrogen cycling, including biological nitrogen fixation, nitrification, denitrification, nitrogen mineralization, and ammonia volatilization; (3) The underlying mechanisms responsible for the BC-regulated carbon and nitrogen cycling in the soil. At last, we also point out that more efforts should be paid to the investigation of long-term experiments in field circumstances in future, and the explanation of the microbial mechanisms underlying soil carbon and nitrogen cycling affected by BC application using modern molecular biotechnology (e.g., metagenomics). This review would provide useful information for maintaining health and function of the coastal soil ecosystem by incorporation of BC.

We developed a four-step method to quantify trade-offs among ecosystem services: evaluate the targeted services at different spatial-temporal scales; identify whether a trade-off exists between pairs of ecosystem services; graph the production-possibility frontier curve; and use that curve to calculate the trade-off intensity between services. Using China's Yellow River Delta wetlands as a case study, we examined the potential trade-offs between material production, carbon storage, and habitat quality under three land-use scenarios (business-as-usual, protection, and exploitation). We found significant correlations between all pairs of the three ecosystem services. Trade-offs existed between material production and habitat quality in all scenarios, with the following order of intensity: protection (6.4) < 2015 status quo (21.8) < business as usual (22.5) < exploitation (24.3). Synergies always existed between habitat quality and carbon storage. The material production and carbon storage services were synergistic in the protection and business as usual scenarios, but they exhibited a trade-off in the exploitation scenario, with a trade-off intensity (59.9) comparable to that (60.3) in the 2015 status quo. The methodology can be flexibly used to analyze trade-offs and compare alternative management plans, thereby revealing the optimal management, which provides a scientific basis for achieving sustainable regional development and resource management.

Tidal channels play important roles in governing landscape succession in the estuary areas. In the present work, tidal channels and landscapes types of the Yellow River Delta (YRD) were extracted by visual interpretation based on remote sensing images combined with field investigation during 1989-2016, aiming to decipher effects of tidal channel development on Phragmites australis, Suaeda salsa and mudflats in the last 20 years through Redundancy Analysis (RDA). Results indicated that area ratio of mudflats was in the fluctuation of increase and decrease during the past 20 years, and the change trend is opposite to that of Phragmites australis and Suaeda salsa. Patch density was larger in the Northern YRD than that in the Southern YRD, and fragmentation is relatively high. RDA analysis showed that the first two sequence axes could explain about 85.7% variations of landscape indexes, which better reflected the correlation between the two. Length, number, network connectivity and curvature had greater effects on landscape features; the effect of fractal dimension was the smallest. Length, number, and fractal dimension were negatively correlated with the area ratio and patch density of Phragmites australis and Suaeda salsa while they were positively correlated with the area ratio of mudflats.