Research Background
Natural wetlands serve as significant carbon sinks. The long-term inundation conditions slow down the decomposition of organic matter, allowing large amounts of carbon to accumulate in the soil. As a result, wetlands play a crucial role in the global carbon cycle and in regulating greenhouse gases. However, with the expansion of land development and food production, an increasing number of natural wetlands are being converted into paddy fields. This not only alters land use types but also reshapes soil moisture levels, redox conditions, and microbial communities, thereby affecting emissions of methane (CH₄) and carbon dioxide (CO₂).
So, how will the ecosystem’s carbon balance change after wetlands are converted into paddy fields? Is this change directly driven by environmental factors, or is it regulated by microbial activity? Recently, the research team led by Professor Zhu Weihong from Yanbian University took the natural wetlands and their converted paddy fields in the Tumen River basin as their study object. They systematically assessed the impacts of wetland-to-paddy conversion on CH₄ and CO₂ fluxes as well as soil microbial communities, aiming to uncover the risks of carbon emissions from wetlands caused by agricultural reclamation. The research findings have been published in the journal Ecological Processes.
Why focus on “wetlands converted to paddy fields”?
Wetlands are highly coupled ecosystems involving the "hydrological–soil–microbial–carbon cycle." The anaerobic environment created by prolonged waterlogging not only facilitates the sequestration of organic carbon but also promotes the methanogenesis process, making wetlands an important source of CH₄ emissions. When natural wetlands are converted into paddy fields, agricultural practices such as tillage, drainage, fertilization, and monoculture alter the soil environment and microbial communities, thereby influencing CH₄ and CO₂ fluxes. Existing studies have shown that converting wetlands into paddy fields typically reduces CH₄ emissions but may increase CO₂ release; however, the underlying microbial regulatory mechanisms remain to be elucidated.
Figure 1. Overview of the study area: Geographical location of the Jingxin Wetland (a) and land use (b).
Research Methodology
(1) The study area is located in the Jingshin Wetland and its surrounding reclaimed paddy fields in the Tumen River basin, southeast of Hunchun City, Jilin Province, China. Natural wetlands serve as the control group, while paddy fields developed from wetlands constitute the experimental group. The paddy fields in the study area are managed using practices such as intermittent irrigation, mid-season drying of the fields, and winter drainage after harvest.
(2) Sample Collection: The research team simultaneously collected soil, microbial, and greenhouse gas samples in July and November 2023, and supplemented these with additional greenhouse gas observations at the same time in 2024, thereby creating a flux dataset covering two years and four time points.
Greenhouse gas fluxes: The PS-3010 Automatic Soil CO₂/CH₄ Flux System (Beijing Lijia United Technology Ltd.) is used to measure CH₄ and CO₂ fluxes in situ and in real time.
Soil physicochemical indicators: ST, SWC, pH, COND, DOC, EOC, TOC, TN, TP, microbial biomass carbon/nitrogen, and others.
Microbiome analysis: High-throughput sequencing was used to analyze the community composition and diversity of bacteria, methanogens, and methane-oxidizing microbes.
(3) Data Analysis: The study employed methods including two-factor ANOVA, Mantel test, RF analysis, variance partitioning analysis, and SEM.
Figure 2. Greenhouse gas fluxes from wetlands and paddy fields. The bars represent the average flux; the error bars represent the standard error (SE).
Figure 3. Heatmap depicting the correlation between greenhouse gas fluxes and soil physicochemical properties: wetlands (a) and paddy fields (b).
Figure 4. Microbial abundance and diversity: relative taxonomic abundance at the phylum level (a) and α-diversity index (d) for soil bacterial communities; relative genus-level abundance (B) and α-diversity index (e) for methanogen communities; and relative genus-level abundance (c) and α-diversity index (f) for methanotrophic bacterial communities in wetlands and paddy soils.
Figure 5. Variance decomposition analysis (VPA) of CH₄ and CO₂ fluxes. Wetland CH₄ flux (a), paddy field CH₄ flux (b), wetland CO₂ flux (c), and paddy field CO₂ flux (d). Microbial variables include bacteria, methanogens, and methanotrophs.
Figure 6. SEM images of soil, microbial, and greenhouse gas fluxes in wetlands (a) and paddy fields (b).
Research results
After wetlands were converted into paddy fields, CH₄ continued to be emitted net, but its flux significantly decreased; CO₂, on the other hand, shifted from net absorption to net emission.
In natural wetlands, the net flux of CH₄ is significantly correlated with COND, EOC, and inorganic nitrogen forms, while the net flux of CO₂ shows a moderate correlation with EOC.
In paddy fields, the net flux of CH₄ is strongly correlated with ST, SWC, NO₃⁻-N, and microbial biomass carbon, whereas the net flux of CO₂ is primarily influenced by pH and TOC.
After wetlands are converted into paddy fields, the bacterial community remains relatively stable, while the methanogenic microbial community undergoes significant restructuring; the composition of the methane-oxidizing microbial community also experiences noticeable changes.
In natural wetlands, the physicochemical properties of the soil indirectly influence CH₄ and CO₂ fluxes by altering the structure of the microbial community.
In paddy fields, the direct influence of soil physicochemical properties on carbon flux is enhanced, while the explanatory power of microbe-mediated pathways relatively decreases.
Conclusion
The impacts of converting wetlands into paddy fields are not merely a matter of reduced methane emissions or altered carbon dioxide emissions; rather, they represent a fundamental restructuring of the entire carbon-cycle regulation system. Studies have shown that the conversion to paddy fields weakens the CO₂-absorbing capacity of natural wetlands and disrupts the previously tight coupling between soil conditions and microbial communities, making carbon fluxes more susceptible to human management practices such as drainage, fertilization, and tillage. This underscores the need for wetland conservation and agricultural emission reductions to move beyond focusing solely on the rise or fall of individual greenhouse gases and instead adopt a holistic approach that considers the overall carbon balance of ecosystems, comprehensively assessing the long-term impacts of land-use changes. In the future, by integrating long-term flux observations, microbial functional analyses, and refined water- and fertilizer-management practices, we can gain a more accurate understanding of the carbon effects associated with wetland conversion, providing a scientific basis for regional wetland conservation and low-carbon agricultural management.
Journal of Publication: Ecological Processes [Impact Factor: 3.9]
Research Institution: Yanbian University
Study site: Jingxin Wetland in the Tumen River basin, located in the southeastern part of Hunchun City, Jilin Province, and its surrounding reclaimed paddy fields.
Equipment used: PS-3010 Automatic Soil CO₂/CH₄ Flux System
DOI: https://doi.org/10.1186/s13717-026-00698-1