A groundbreaking new investigation into the microbial mechanisms that regulate nitrogen removal in China’s vast riverine wetlands reveals profound insights into the biogeochemical processes that maintain freshwater ecosystem health on a continental scale. This study, encompassing 30 major wetlands along a staggering 3,500-kilometer north-south transect, leverages cutting-edge isotope tracing combined with state-of-the-art genetic analyses to unpack the intricate balance between two globally significant microbial pathways: denitrification and anaerobic ammonium oxidation, better known as anammox.
Nitrogen, a fundamental building block of life, paradoxically becomes an ecological threat when present in excessive amounts. Anthropogenic activities such as intensified agriculture, fossil fuel combustion, and urban development continuously augment reactive nitrogen loads in aquatic systems. These surpluses instigate eutrophication, characterized by explosive algal blooms, hypoxic dead zones, and consequential fish mortality events. The critical role microbes play in mitigating this nitrogen excess through conversion into inert nitrogen gas (N₂) has been recognized for decades, yet the spatial variability and relative contributions of microbial processes remain underexplored across large heterogeneous landscapes.
In this ambitious study published in Nitrogen Cycling, researchers set out to delineate how the denitrification and anammox pathways contribute to nitrogen gas production across diverse riverine wetland habitats in China. Denitrification, a facultative anaerobic process, enzymatically reduces nitrate (NO₃⁻) to dinitrogen gas via intermediate nitrogen oxides, a process that can generate greenhouse gases if incomplete. Anammox, in contrast, is an obligate anaerobic reaction closing the nitrogen cycle by combining ammonium (NH₄⁺) and nitrite (NO₂⁻) directly into dinitrogen gas without releasing nitrous oxide (N₂O), a potent greenhouse gas, positioning it as an environmentally advantageous pathway.
Spatial analyses yielded striking latitudinal gradients in denitrification activity, with northern river wetlands exhibiting significantly elevated rates compared to their southern counterparts. This gradient likely reflects variations in temperature regimes, nutrient availability, and organic carbon sources that differentially modulate microbial community composition and enzymatic efficiency. Conversely, the anammox process showed weaker correlation with latitude but was distinctly prevalent in deeper riparian soil layers characterized by sandy textures and unique redox conditions, emphasizing niche partitioning within the wetland matrix.
Senior author Wenzhi Liu of the Wuhan Botanical Garden emphasizes the paradigm shift these findings represent: “Historically, denitrification has been deemed the primary nitrogen sink in aquatic sediments, yet our data compellingly demonstrate that anammox rivals and sometimes surpasses denitrification in certain habitats, particularly in riparian, sandy soils adjacent to rivers.” This recognition has profound implications for biogeochemical modeling and ecosystem management, as current nitrogen cycling models frequently omit anammox, potentially underestimating total nitrogen removal capacity and mischaracterizing greenhouse gas emissions.
Breaking down the overall contributions, denitrification accounted for approximately 56 to 64 percent of nitrogen gas production predominantly within sediments and root-associated microbial niches. In contrast, anammox dominated nitrogen removal in bulk soils of riparian zones, contributing up to 58 percent, a figure that underscores its ecological significance in previously underappreciated microhabitats. The research further identifies soil carbon content, iron levels, and nitrate availability as principal environmental drivers shaping these microbial processes, revealing complex interdependencies between geochemistry and microbial function.
Analytical techniques utilized such as stable isotope probing (SIP) enabled precise quantification of these processes at fine spatial scales, while genetic sequencing unveiled the community structure of key denitrifier and anammox bacteria. These insights align with emerging global perspectives that the nitrogen cycle’s microbial backbone is more intricate than traditionally considered, with diverse microbial guilds responsive to microenvironmental gradients in resource availability and physicochemical parameters.
Consequently, this comprehensive study advocates for the integration of both denitrification and anammox pathways into predictive nitrogen cycling models to enhance their accuracy and applicability across landscapes. Enhanced models will better inform water quality forecasting and conservation planning, allowing policymakers and environmental managers to design targeted interventions that leverage natural microbial processes for pollution mitigation.
Moreover, the research casts natural riverine wetlands as unsung heroes in the battle against human-induced nitrogen pollution. As agricultural intensification and urban expansion continue to escalate nitrogen inputs into freshwater systems globally, safeguarding and restoring these microbial hotspots becomes paramount for sustaining ecosystem resilience and biodiversity conservation.
These findings hold promise beyond China, offering conceptual frameworks applicable to riverine wetlands worldwide. They underscore the necessity of preserving not only extensive wetland area but also the heterogeneity of soil habitats that support diverse microbial communities capable of robust nitrogen removal under varying environmental conditions.
As climate change alters hydrological cycles, temperature profiles, and nutrient fluxes, this nuanced understanding of microbial nitrogen processing assumes even greater urgency. Adaptive management strategies founded on microbial ecology will be critical in buffering aquatic ecosystems against escalating anthropogenic pressures, thereby preserving their ecological functions and services foundational to human well-being.
Ultimately, this work charts a new course for nitrogen biogeochemistry research—one that embraces the complexity and spatial dynamics of microbial processes in large, interconnected freshwater landscapes. It calls for interdisciplinary approaches combining advanced molecular tools, geochemical assays, and ecological modeling to holistically capture the drivers and outcomes of nitrogen transformations shaping Earth’s critical water resources.
Subject of Research: Not applicable
Article Title: Relative contributions of denitrification and anammox to nitrogen removal in riverine wetlands across China
News Publication Date: 17-Sep-2025
Web References: http://dx.doi.org/10.48130/nc-0025-0004
References: Deng D, Xu D, He G, Ding B, Liu W. 2025. Relative contributions of denitrification and anammox to nitrogen removal in riverine wetlands across China. Nitrogen Cycling 1: e003
Image Credits: Danli Deng, Di Xu, Gang He, Bangjing Ding & Wenzhi Liu
Keywords: Nitrogen, Nitrogen cycle, Atmospheric chemistry, Rhizosphere, Wetlands
