Changes in Methane Emission and Community Composition of Methane-Cycling Microorganisms Along an Elevation Gradient in the Dongting Lake Floodplain, China

Variations of methane fluxes and methane microbial community composition with soil depth in the riparian buffer zone of a sponge city park

Riparian buffers benefit both natural and man-made ecosystems by preventing soil erosion, retaining soil nutrients, and filtering pollutants. Nevertheless, the relationship between vertical methane fluxes, soil carbon, and methane microbial communities in riparian buffers remains unclear. This study examined vertical methane fluxes, soil carbon, and methane microbial communities in three different soil depths (0-5 cm, 5-10 cm, and 10-15 cm) within a riparian buffer of a Sponge City Park for one year. Structural equation model (SEM) results demonstrated that vertical methane fluxes varied with soil depths (λ = -0.37) and were primarily regulated by methanogenic community structure (λ = 0.78). Notably, mathematical regression results proposed that mcrA/pmoA ratio (R² = 0.8) and methanogenic alpha diversity/methanotrophic alpha diversity ratio (R² = 0.8) could serve as valid predictors of vertical variation in methane fluxes in the riparian buffer of urban river. These findings suggest that vertical variation of methane fluxes in riparian buffer soils is mainly influenced by carbon inputs and methane microbial abundance and community diversity. The study's results quantitatively the relationship between methane fluxes in riparian buffer soils and abiotic and biotic factors in the vertical direction, therefore contributing to the further development of mathematical models of soil methane emissions.

Emission dynamics of greenhouse gases regulated by fluctuation of water level in river-connected wetland

The application of reservoirs in the upper reaches of rivers will change the hydrological rhythm of river-connected wetlands in the lower reaches, causing changes in the distribution of wetland vegetation. The differences of carbon and nitrogen sequestration and emission potential in different vegetations may lead to the dynamics of greenhouse gas emissions from wetlands during hydrological periods. For a wetland connected to the Yangzi River, China, the dynamic changes of vegetation and water areas were identified by remote sensing, and the water level, the emission fluxes of greenhouse gases and the functional bacteria of carbon and nitrogen in soil were measured in-situ. Compared with drought period, the area of phragmites zone in flooding period increased by 28.2%, while the areas of carex and phalaris zones decreased by 42.9%. The carbon and nitrogen accumulation in the soil of phragmites zone is the highest, while the cumulative amount of phalaris is the lowest. The emission fluxes of CH₄ and N₂O in mud/water and various vegetations were positively correlated with water level and reached the maximum during flooding period. Although the global warming potential of mud/water was highest than that of vegetations, carex zone had the highest warming potential among vegetation zones. CH₄ contributes 8-37 times as much as N₂O to global warming potential in the wetland. The increase of flooding time promoted the emissions of CH₄ and N₂O in the wetland. The anaerobic condition caused by flooding stimulated the activities of denitrifying and methanogenic bacteria, thus increasing the emission of greenhouse gases. The sequestrations and emissions of carbon and nitrogen regulated by a reservoir in the upstream suggest that the operation of water conservancies should be considered to alleviate the greenhouse gas emission from river-connected wetland.

Characteristics of microbial community composition and its relationship with carbon, nitrogen and sulfur in sediments

Microorganisms play an important role in the biogeochemical cycles of lacustrine sediments. However, little is known about their vertical responses to sedimentary depths and their contributions to important element cycles such as carbon, nitrogen and sulfur. Here, we investigated the community features of sedimental prokaryotes along with vertical profiles (i.e., sedimental depths of 0-230 cm) in the Dongting watershed. In the entire profile, the bacterial diversity wavelike decreased with increasing sediment depths, whereas archaeal diversity increased monotonically in the deep layer (100-230 cm). Bacteria were more sensitive to sedimental depths than archaea (B: P < 0.001, R = 0.534; A: P < 0.001, R = 0.327). The relative abundance of the primary phylum of bacteria (e.g. Proteobacteria) and archaea (e.g. Thaumarchaeota) changed significantly with sediment layers (P < 0.05). The vertical distribution of prokaryotes in sediments was related to the changes in sediment properties (P < 0.05, e.g., the content of nutrients, pH, texture, etc.). The co-occurrence network analysis further indicated that the superficial (0-40 cm) and deep (100-230 cm) networks contained more tightly connected node groups (more modules number), while the tighter connections (shorter APL) and more complex networks (higher avgK) were seen in sub-deep (40-100 cm) sediments. Based on the FAPROTAX database, we found that the predicted prokaryotic microbial functional groups involved in the N cycle had the highest abundance (87.47%), followed by the C cycle (9.48%) and the S cycle (2.39%). In addition, these groups were enriched in the superficial and deep layers. Taken together, these results reveal a new perspective on the vertical spatial variation in microorganisms at the fine-scale of the lake sediments. Distinct microbiome response patterns may have important implications for carbon, nitrogen and sulfur cycling along with the sediment profile in Dongting lake ecosystems.