Coupled patterns of natural and anthropogenic resources in typical ecosystems in coastal areas of China.

The coastal area of Yancheng, China, is one of the hotspots for ecological research. Under the coupling of human and natural ecosystems, the region has gradually evolved into a coexistence of aquatic, agricultural and mudflat ecosystems. What are the patterns of natural and artificial resource inputs and patterns of change in ecosystems? How can ecological flows be analyzed at a uniform scale? Here, we selected six typical local ecosystems, namely, rice‒wheat for enterprises (RWE), rice‒wheat for smallholder households (RWS), chrysanthemum‒wheat (CW), fish polyculture (FP), juvenile crab farming (JF) and clam polyculture (CP), and analyzed their energy flow flux and sustainability based on emergy theory. The results showed that anthropogenic resource inputs were higher than natural resource inputs in all ecosystems, and the inputs of aquatic ecosystems were greater than those of agroecosystems. The greatest total input was 2.0 E+17 seJ/ha/yr for FP, and the lowest was 1.9 E+16 seJ/ha/yr for RWE. The proportions of renewable and artificial inputs for RWE, RWS, CW, FP, JF and CP were 32.8% vs. 96.1%, 40.3% vs. 96.5%, 34.7% vs. 97.0%, 32.6% vs. 99.4%, 55.1% vs. 98.5%, and 62.5% vs. 98.6%, respectively. The highest input to agroecosystems was nitrogen fertilizer, while in JF and CP, it was water, and feed (63.3%) accounted for the highest percentage of input in FP. JF and CP had lower environmental loads and higher sustainability than other ecosystems, but this still represents a high input compared to agroecosystems. Human-led resource coupling profoundly affects ecosystem sustainability, and various thresholds of energy use and ecological sustainability need to be studied in depth. Continuous exploration of methods and mechanisms for the maintenance and evolution of ecosystems with low total inputs and low inputs of non-renewable resources can contribute to high-quality sustainable development of an area or region.

Shifts of active microbial community structure and functions in constructed wetlands responded to continuous decreasing temperature in winter.

Important functions of constructed wetland related to biogeochemical processes are mediated by soil microbes and low-temperature damage is the main limiting factor for microbes in winter. However, the response thresholds for active microbial community and enzyme activities to continuous decreases in temperature remain unclear. In this study, total 90 soil samples were collected every week over a 6-week period to track the dynamics of four enzymes involved in cycles of C, N, P and active bacterial community as field soil temperature decreased continuously from 6.62 °C to 0.55 °C. Enzyme activity changed suddenly when the temperature decreased to 4.83 °C, the nitrite reductase activity reduced by 36.2%, while alkaline phosphatase activity is increased by 396%. The cellulase and urease were only marginally influenced by cold stress. Decreased nitrite reductase activities corresponded with loss of nir-type denitrifiers important for nitrite reduction. For cold stress, N-related bacteria were sensitive species. Whereas increased alkaline phosphatase activity may be due to the fact that P-related bacteria were opportunistic species. Key functional taxa connected with degradation of cellulose promoted species coexistence and microbial network stability. The lower and upper temperature thresholds for community change were 4.85 °C and 6.30 °C, respectively. Collectively, these results revealed that microbial taxa involved in C, N and P cycling respond differently to continuous decreases in temperature and higher than 4.85 °C is an ideal environment to prevent loss of microbial diversity and functions in winter, providing a scientific reference for the targeted isolation and cultivation of key microbial taxa in rhizosphere soil and adjusting temperature range to improve the purification capacity of wetlands during low temperature periods.

Effect of freshwater on plant species diversity and interspecific associations in coastal wetlands invaded by Spartina alterniflora.

Plant invasions in coastal wetlands lead to the degradation of native vegetation; the introduction of freshwater in coastal wetlands would prevent the spread of invasive plants and facilitate the restoration of native vegetation. In this study, we evaluated the effects of freshwater on plant communities in the coastal wetlands of Yancheng, China, invaded by Spartina alterniflora Loisel. Two field investigations were conducted in 2008 and 2018 before and after the introduction of freshwater (started in 2011). The characteristics of plant communities were subjected to hierarchical cluster analysis and compared using several diversity indices. In addition, differences in habitat community composition and interspecific relationships of dominant species were analyzed. The results showed that S. alterniflora reduced the overall species diversity in the region. Plant species diversity increased after freshwater was introduced into the study site when compared to the areas without freshwater introduction. The introduction of freshwater caused a shift often changes in the interspecific relationships between Suaeda salsa (L.) Pall. and other species. The intensified invasion of S. alterniflora changed the interspecific relationship of native halophytes from negative to positive. Although freshwater effectively inhibited further invasion of S. alterniflora, it also increased the risk of expansion of the glycophytes in the community. The results of this study highlight the need for early intervention for restoration of coastal wetlands, preservation of biodiversity, and management of plant resources.

Fungi drive soil multifunctionality in the coastal salt marsh ecosystem.

Salt marshes are highly productive intertidal wetlands located in temperate climatic zones, in which marine-to-terrestrial transition significantly influences microbial life. Numerous studies revealed the important coupling relationship between microbial diversity and ecosystem functions in terrestrial ecosystems, however, the importance of microbial diversity in maintaining soil functions in coastal ecosystems remains poorly understood. Here, we studied the shifts of microbial communities and soil multifunctionality (SMF; nine functions related with C, N and P cycling) along a vegetation gradient in a salt marsh ecosystem and investigated the microbial diversity - ecosystem function relationship. The aboveground vegetation shifted from mud flat (MF) to Scirpus triqueter (SM) and then Phragmites australis (PA) with increasing distance away from the sea. Average approach showed that the SMF was much higher in halophytes covered zones including SM and PA than in MF. Structural equation model (SEM) analysis confirmed that vegetation was an important predictor on SMF besides moisture and organic carbon. Linear regression and multiple threshold methods showed that in MF and SM zones, fungal rather than bacterial richness was significantly and positively correlated with SMF, while in the PA zone microbial diversity did not relate with SMF. Random forest analysis identified several Ascomycota taxa with preference over marine environment as strong predictors of SMF. Taken together, our study lays the basis for a better understanding on the relationships between belowground microbial diversity and soil functions in coastal ecosystems.

Long-term performance of nutrient removal in an integrated constructed wetland.

Constructed wetlands (CWs) have been regarded as efficient technologies for both wastewater treatment and reuse of water resources. Most studies on CW treatment efficiency are limited to a short-term perspective, and there are still many unknowns about the long-term performance of CWs. Here we evaluated the performance of an integrated CW that has been in operation for more than ten years. The average removal rates of TN and TP were maintained at 53.6% and 67.3% over 10 years, respectively. The annual mass reductions in TN and TP reached 937.5 kg ha-1 yr-1 and 303.2 kg ha-1 yr-1, respectively. In addition, TN removal rate was significantly higher in summer and autumn than those in spring, yet there was no seasonal difference in TP removal. The bacterial richness and diversity in summer and autumn were higher than those in spring. TN and TOC not only determine the bacterial community structure, but also affect the removal efficiency of CW. Denitrification and dephosphorization microorganisms were enriched and accounted for a considerable proportion (21.14-52.85%) in the bacterial community. In addition, the relative abundance of Pseudomonas was significantly positively related with the rate of TN and TP removal.

Spartina alterniflora Leaf and Soil Eco-Stoichiometry in the Yancheng Coastal Wetland.

Carbon, nitrogen, and phosphorus-nutrient and restrictive elements for plant growth and important components of the plant body-are mainly transferred and exchanged between plants and the soil environment. Changes in the carbon, nitrogen, and phosphorus eco-stoichiometry greatly impact the growth and expansion of Spartina alterniflora, and understanding these changes can reveal the nutrient coordination mechanism among ecosystem components. To explore the relationship between leaf and soil eco-stoichiometry and determine the key soil factors that affect leaf eco-stoichiometry, we collected leaf and soil samples of S. alterniflora at different tidal levels (i.e., 1, 3, and 5 km away from the coastline) in a coastal wetland in the Yancheng Elk Nature Reserve, Jiangsu province. We measured the leaf and soil carbon, nitrogen, and phosphorus contents and ratios, as well as the soil salinity and soil organic carbon. The results revealed the following. (1) The leaf stoichiometric characteristics and soil properties of S. alterniflora differed significantly between tidal levels; for example, total carbon, nitrogen, soil organic carbon were detected at their highest levels at 3 km and lowest levels at 5 km. (2) Significant correlations were detected between the leaf stoichiometric characteristics and soil characteristics. Additionally, nitrogen limitation was evident in the study area, as indicated by the nitrogen-phosphorus ratio being less than 14 and the soil nitrogen-phosphorus ratio being less than 1. (3) Soil salinity and the soil carbon-nitrogen ratio were shown to be the key factors that affect the eco-stoichiometric characteristics of S. alterniflora. These findings furthered our understanding of the nutrient distribution mechanisms and invasion strategy of S. alterniflora and can thus be used to guide S. alterniflora control policies formulated by government management departments in China.

Changes of the denitrifying communities in a multi-stage free water surface constructed wetland.

Microorganisms play crucial roles in the nitrogen removal processes of wetlands. However, the key functional genes and microbes related to the nitrogen removal remain largely unknown in the free water surface constructed wetland (FWS CW). Here we studied the abundances of denitrifiers by targeting the key functional genes (nirS, nirK and nosZ) and investigated the community compositions of denitrifiers and their correlations with the abiotic variables in a FWS CW. The increase of nosZ/(nirS + nirK) and nirS/nirK ratios in the outlet indicated a shift of denitrifiers' communities which tended to release less nitrous oxide at the genetic potential level. The denitrifiers dominated the bacterial community which also remarkably changed from the inlet to the outlet. PICRUSt analysis revealed that the denitrifiers contributed to 39.1% of the nitrogen metabolism, 38.9% of the amino acid metabolism and 25.6% of the amino acid related enzymes. Four bacterial genera including Hydrogenophaga, Hylemonella, Aquabacterium and Cellvibrio were detected as the putative keystone denitrifiers. The abundance (nirS, nirK and nosZ) and the relative abundance of putative keystone denitrifiers were significantly correlated with total organic carbon, oxidation-reduction potential and C/N ratio, which could be regarded as the determinants for the denitrification process in the free water.

[Characteristics of Bacterial Communities in Surface-flow Constructed Wetlands].

Employing high-throughput sequencing as the method, this study analyzed the relationship between the water environment and the microbial community structure in the surface-flow constructed wetland. The results showed that: the mean Shannon-Wiener index was 6.57 and there were mainly 18 categories in the microbial community, including Proteobacterice (38.97%), Bacteroidetes (15.63%) etc. Of these, the total content of 22 genera was over 1%. The microbial biodiversity showed an increasing trend at the beginning and then turned to a decreasing trend in the flowing direction. The results also revealed that pH, ORP, NH4+-N, NO2--N and TN acted as important restricting factors for the microbial community.