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Diurnal freeze-thaw cycles (FTCs) occur in the spring and autumn in boreal wetlands as soil temperatures rise above freezing during the day and fall below freezing at night. A surge in methane emissions from these systems is frequently documented during spring FTCs, accounting for a large portion of annual emissions. In boreal wetlands, methane is produced as a result of syntrophic microbial processes, mediated by a consortium of fermenting bacteria and methanogenic archaea. Further research is needed to determine whether FTCs enhance microbial metabolism related to methane production through the cryogenic decomposition of soil organic matter. Previous studies observed large methane emissions during the spring thawed period in the Sanjiang seasonal frozen marsh of Northeast China. To investigate how FTCs impact the soil microbial community and methanogen abundance and activity, we collected soil cores from the Sanjiang marsh during the FTCs of autumn 2014 and spring 2015. Methanogens were investigated based on expression level of the methyl coenzyme reductase (mcrA) gene, and soil bacterial and archaeal community structures were assessed by 16S rRNA gene sequencing. The results show that a decrease in bacteria and methanogens followed autumns FTCs, whereas an increase in bacteria and methanogens was observed following spring FTCs. The bacterial community structure, including Firmicutes and certain Deltaproteobacteria, was changed following autumn FTCs. Temperature and substrate were the primary factors regulating the abundance and composition of the microbial communities during autumn FTCs, whereas no factors significantly contributing to spring FTCs were identified. Acetoclastic methanogens from order Methanosarcinales were the dominant group at the beginning and end of both the autumn and spring FTCs. Active methanogens were significantly more abundant during the diurnal thawed period, indicating that the increasing number of FTCs predicted to occur with global climate change could potentially promote CH4 emissions in seasonal frozen marshes.

The question of where and how to carry out reclamation work in coastal areas is still not well addressed in coastal research. To answer the question, it is essential to quantify the impact of reclamation and the associated ecological and/or environmental responses. In this study, ordinary least square (OLS) analysis and geographical weighted regression (GWR) analysis were performed to identify the reclamation variables that affect soil and vegetation characteristics. Reclamation related variables, including residential population (RP), years of reclamation (YR), income per capita (IP), and land use-based human impact index (HII), were used to explain nitrate, ammonium, total phosphorous, and heavy metals in soil, and the height, density, and above-ground biomass of native hydrophytic vegetation. It was found that variables IP, RP, and HII could be used to explain the height of Scirpus and Phragmites australis as well as above-ground biomass with a R 2 value of no > 0.55, and almost all the variables could explain the hydrophytic vegetation characteristics with a higher R 2 value. In comparison to OLS, GWR more reliably reflected the reclamation effects on soil and vegetation characteristics. By GWR analysis, total soil phosphorous, and nitrate and ammonium nitrogen could be explained by RP, YR, and HII, with the highest Ad-R 2 value of 0.496, 0.631 and 0.632, respectively. Both of the GWR and OLS analysis revealed that HII and RP were the better variables for explaining the soil and vegetation characteristics. This work demonstrated that coastal reclamation was highly spatial dependent, which sheds a light on the future development of spatial explicit and process-based models to guide coastal reclamation around the world.

Due to the enhancement of human activities on the global scale, the total amount of atmospheric nitrogen (N) deposition and the rate keep increasing, which seriously affect the structure and function of terrestrial ecosystems. In order to study the effects of N deposition on the soil structure and function of coastal saline wetlands, we established a long-term nitrogen deposition simulation platform in 2012 in the Yellow River delta (YRD). Herein, we analyzed the composition and diversity of the soil microbial community under different N deposition treatments (LNN, MNN and HNN, which stand for 50 kg N ha−1 yr−1, 100 kg N ha−1 yr−1, and 200 kg N ha−1 yr−1) and in a water-only control (CK). The results showed that with the increasing level of N deposition, α-diversity (Shannon and Simpson indices) decreased significantly, and the composition of the microbial community changed. At the phylum level, compared with CK, the relative abundance of Chloroflexi increased significantly under the treatment of HNN (P = 0.002), but the relative abundance of Chlorobi (P = 0.013) and Verrucomicrobia (P = 0.035) decreased significantly. At the genus level, compared with CK, the relative abundance of Bacillus (P = 0.01) and Halomonas (P = 0.042) increased significantly with HNN treatment. Bacillus and Nitrococcus showed a significant correlation with soil NH4+-N. The results suggest that the response of microorganisms to N deposition treatments varied by the concentration, and the deposition of a high concentration would increase the nutrients in the soil, but reduce the diversity of soil microorganisms, causing a negative impact on the coastal wetland ecosystem of the YRD.

In coastal wetlands, salinity significantly influences the vegetation successional series. Arbuscular mycorrhizal fungal (AMF) are ubiquitous soil microorganisms which play important roles in plant establishment, diversifying plant communities and accelerating successional progress by improving the soil health. However, the structure and function of AMF communities in coastal wetlands are poorly understood. Here we investigated the AMF communities along a vegetation successional series (from intertidal to supratidal flats) and the sediment physicochemical factors affecting the variations. AMF community composition and diversity were analyzed by 18S rRNA genes using high-throughput sequencing technology. The results revealed that the maximum number of AMF sequences belonged to Glomus 74,660, 62.75%. The relative abundance of the top three genera differed significantly along the vegetation successional series. Redundancy analysis (RDA) results demonstrated that salinity was a major environmental factor structuring the AMF community in the sediment along the vegetation successional series. No significant differences of AMF diversity were observed between different sediment layers. Additionally, total nitrogen (TN) and total carbon (TC), important soil nutrients in young emergent wetland ecosystems, also contributed to AMF community structure. The results of this study provide a better understanding of the AMF community and the major factors impacting it along a coastal wetland successional series composed of different vegetation types.

Plant species are characterized by high adaptability to water level fluctuation in freshwater wetlands. Alteration in defence allocation patterns is one of the plant defence strategies in response to increasing water level gradients. Two dominant species, flood-sensitive species Deyeuxia angustifolia and flood-tolerant species Carex pseudocuraica, from freshwater marshes in the Sanjiang Plain were collected and treated with four water level gradients (–5, 0, 5, and 15 cm, relative to the soil surface), and both plant chemical (total polyphenols and condensed tannins) and structural (cellulose and lignin) defence compounds in leaves, stems and roots were measured. Increasing water level significantly decreased concentration of condensed tannins in C. pseudocuraica roots. For structural defence compounds, lignin concentration in C. pseudocuraica roots increased, which performed the contrary results in D. angustifolia roots and leaves. On the whole plant level, concentrations of chemical defence compounds in plant leaves were higher than that in stems and roots. Moreover, both the total phenols allocation to D. angustifolia leaves and condensed tannins allocation to C. pseudocuraica roots decreased with the increasing water level. Negative correlations between concentrations of condensed tannins and cellulose could be found in both D. angustifolia and C. pseudocuraica leaves. Plants adopt different defence strategies in response to the increasing water level, which provide us new purviews for revealing the plant distribution patterns in freshwater wetlands.