| The irrational use of wetland resources, such as massive reclamation of wetlands, excessive discharge of pollutants have caused structural damage and functional decline to wetland ecosystems, for instance, loss of biodiversity and severe soil contamination. Therefore, wetland restoration and protection has become a hot issue. Currently, the main method to restore wetland ecosystem is using appropriate ecological engineering for degradation wetlands to help gradually repair or reconstruction; wetland ecosystem structures and functions is restored, and ultimately to reach self-sustaining state of ecosystem. However, in the process of wetland restoration, the relationship between ecosystem properties and functions is neglected, and scientific assessment of ecosystem function and service value of wetlands ecosystem after restoration remains controversial.Nansi Lake is the important channel for water regulation and storage on the east route of China’s South-to-North Water Transfer Project; therefore, the ecosystem health of Nansi Lake wetland is an important guarantee of water quality to ensure the implementation of the water diversion project. To improve the water quality and restore biodiversity of Nansi Lake wetland ecosystem, a large-scale wetland restoration project was conducted during the national "Eleventh Five-Year" period. In our study, we use Xinxue River constructed wetland as a case study, which is a demonstration project for wetland restoration in Nansi Lake. We examined the four components of diversity (dominance, richness, evenness and divergence) in the restored wetland, furthermore, combined with ecosystem aboveground biomass, soil properties and nutritional status of the studied wetland, we analyzed the effects of wetland plant diversity and ecosystem properties. Meanwhile, a three-step extraction procedure was applied to analyze the content of three mercury fractions (mobile Hg, semi-mobile Hg and non-mobile Hg) in studied wetland soils. Combined with the soil basic physical-chemical properties, metal oxides, nutritional status and hydraulic conditions, soil mercury transformation mechanisms were discussed as well as the response of different mercury fractions for different hydraulic conditions and nutritional factors. In addition, Emergy analysis was applied to assess the sustainability of the restored wetland. The material and energy flows were calculated, and six emergy indicators including emergy yield ratio (EYR), environment loading ratio (ELR), emergy self-support ratio (ESR), emergy investment ratio (EIR), and emergy sustainability index (ESI), were adopted. Ecosystem services and natural capital after wetlands restoration were then evaluated by emergy transformation.Data from field investigation showed that31families,61genera and79species were recorded in the studied constructed wetland. Both plant species composition and aboveground biomass varied greatly in different communities, each of them is dominant with one to three wetland species. The results showed that richness and divergence explained more variation in ecosystem processing than evenness did, and traits-based functional diversity indices better explained the variation in most of the ecosystem processes examined than non-trait taxonomic diversity did. Dominance in terms of mean trait value was best at explaining variation in ecosystem processing and was of primary importance in determining the effects of diversity on ecosystem processes and properties, and species richness and functional divergence also played important roles in some instances. The results indicated that selection effect is the leading mechanism during the restoration of engineered wetland. For maintaining the wetland ecosystem processing, especially in the conservation case, community dominance would be the primary consideration, while richness and divergence also deserve more attention than evenness.Analysis results of soil three mercury fractions content showed that toxic mobile and semi-mobile mercury fraction took a great portion of total mercury (50%～67%). Higher soil organic matter content reduced the production of non-mobile mercury, while aluminum, iron and manganese show positive effects in increasing the toxicity of mercury. Our study suggests that higher pH and free alumina content, lower organic matter and manganese content would promote wetlands functioning as a "sink" for mercury sequestration. Seasonal flooding wetland would reduce the production of toxic mobile mercury more than perennial flooding wetland. Ammonia nitrogen in total nitrogen was the principal nitrogen component in influencing the mobility of mercury; ammonium nitrogen showed significantly negative correlation with mobile toxic mercury while nitrate nitrogen showed the opposite trend. Carbon nitrogen ratio and pH also presented correlation with mercury mobility in studied wetland.Emergy accounting was conducted to investigate the energy and resource flows in constructed wetlands during the restoration process. Emergy-based indexes were adopted to evaluate the sustainability of the studied constructed wetland. Emergy and emdollar values for ecosystem services and natural capital were also calculated. The results showed that when outflow was considered as the product, the studied large-scale constructed wetland was more self-supporting and could be operated with lesser financial investment, although the waste treatment efficiency and the sustainability index was lower than conventional small-scale treatment constructed wetlands. Compared with natural wetlands, more visits from tourists and lesser financial investment coming in as feedback into the wetland would reduce system environment loading and promote system self-support ability, ultimately generating sustainability. In addition, the studied large-scale constructed wetland can effectively simulate energy and resource flows of natural wetland ecosystem and contribute a roughly equal value of ecosystem services in term of gross primary production. The studied large-scale constructed wetland can successfully achieve ecosystem functions as replacement for natural wetland and hasten the restoration process, although the restoration effectiveness of ecosystem structures in terms of living biomass and water using emergy-value accounting is still inconclusive. In general, the studied constructed wetland is an ecological-economic system with sustainability in a long term, and is playing an important role in wetland ecosystem recovery and ensuring the ecological safety during the water transfer.To make a summary, our study found that ecosystem properties are significantly related to certain traits of the dominant species, which indicates selection effect dominantly exists during wetland restoration. Richness and divergence also had correlations with ecosystem properties in some instances. Thus, in order to enhance the ecosystem service of wetland, dominant species and functional traits should be considered the priority. Moreover, during the water transfer process, the alternation of water level and nitrogen inputs might promote the mercury mobility and bioavailability in the watershed of Nansi Lake, and raise the potential mercury hazard in the watershed. In general, the constructed wetland in Nansi Lake have better self-support ability and is eco-sustainable in a long term, the constructed wetland is playing an important role in maintaining ecosystem health of Nansi Lake and ensuring the ecological safety during the water transfer. |
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