Soil organic matter (SOM) is crucial for ecosystem carbon cycling, soil fertility, and environmental quality. As the main component of SOM, humic substances (HS) are considered a unique category of nonuniformly assembled substances. It is widely accepted that HS are originated from small molecules produced during the decomposition of plant and animal residues or from residual macromolecules. These molecules can be recombined or condensed via enzymatic and mineral catalysis into quasi-macromolecular compounds or compound groups with high condensation (elevated C/H molar ratio) and relatively large molecular weight and are further stabilized by mineral association. Although HS can be regarded as an extension or a narrow definition of SOM, their properties are inherently more complex, and their chemical composition, structure, and formation processes remain controversial. Here, we examine the formation theories, compositional structure, stabilization mechanisms, and functional roles of SOM and HS. We propose that HS indeed differ from non-HS, with HS consisting of both residual and synthetic quasi-macromolecular substance components, which collectively form unique compounds or compound groups with independent characteristics. Consequently, HS remain fundamental to soil science and continue to be widely utilized through various HS-based technologies and products in agriculture and environmental fields.

Biobased biodegradable microplastics (Bio-MPs) can alter both the quantity and molecular composition of dissolved organic matter (DOM) in soil, which profoundly shapes the stability of soil organic matter (SOM). However, the microbial mechanisms underlying the Bio-MP-induced DOM turnover, particularly the role of fungi, remain largely unclear. Here, we tracked DOM molecular dynamics using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and characterized microbial communities using 16S rRNA gene and ITS amplicon sequencing during a 30-day soil incubation amended with polylactic acid (PLA-MPs, low biodegradability) and polyhydroxyalkanoate (PHA-MPs, high biodegradability). Our results showed that PLA-MPs exerted minimal impacts on DOM dynamics, whereas PHA-MPs rapidly increased DOM content and CO2 emission and shifted the DOM molecular composition from recalcitrant compounds (e.g., lignins and tannins) toward labile compounds (e.g., lipids and proteins/amino sugars). These alterations were primarily driven by fungal depolymerization of PHA-MPs and SOM to generate labile DOM, followed by bacterial assimilation, indicating a fungal-initiated metabolic cascade that governs soil DOM turnover under PHA-MP exposure. The increase in labile DOM was mainly associated with enrichment of fast-growing fungi (e.g., Neocosmospora). Overall, this study elucidates the pivotal role of fungi in mediating Bio-MP-induced DOM turnover and shaping SOM stability.

Fertilizer nitrogen (N) can accumulate in the soil-vadose zone-groundwater continuum as legacy N, exerting a consistent influence on key biogeochemical processes such as crop N uptake, N leaching, and gaseous N emissions. However, the regional stock of legacy N in soils and its contribution to soil N pools remain unknown due to the lack of effective estimation methods, hindering the understanding of its environmental and agricultural effects. Here, we proposed a method for quantifying legacy N stock at the regional scale by identifying an exponential decay pattern in multi-year N retention rates and establishing a validated power function fitting between a parameter related to the initial retention rate and the N application rate. We further proposed a novel N isotope model to constrain the contribution of legacy N to soil N pools. Overall, China's upland croplands have accumulated legacy N of 54.7 +/- 24.9 Tg N in top 30 cm soils over the past six decades (1961-2020). Croplands in southern China are hotspots for legacy N accumulation, likely due to their rich soil organic carbon. Legacy N may account for 43%-89% of soil organic N sequestration. The N isotope model estimates that legacy N constitutes 21.4% +/- 6.9% of total soil N, comparable with the compiled proportion of total N increases due to fertilization (16.0%). These findings suggest that the legacy N from historical fertilizers substantially enhances soil fertility, and may inform biogeochemical parameterization given that the components of soil N pools may differ in stability.

Bioavailable carbon (C) is a key determinant of free-living nitrogen (N) fixation, yet the regulatory role of dissolved organic matter (DOM) in paddy soils remains unclear. Using 15N2 isotope tracing, metatranscriptomics, and high-resolution mass spectrometry, we showed that phenology-dependent DOM transformations controlled rhizosphere N2 fixation throughout rice growth. N2 fixation varied significantly across phenological stages, peaking at the seedling stage (0.33 f 0.13 nmol N g- 1 h- 1) and decreasing to 0.05 f 0.02, 0.07 f 0.01, 0.13 f 0.08, and 0.14 f 0.03 nmol N g- 1 h- 1 at the tillering, heading, maturing, and harvesting stages, respectively. These changes were significantly associated with nifA transcription. Seasonal shifts in DOM toward lower combustion enthalpy and higher carbon oxidation state suggested the accumulation of more recalcitrant and energy-poor molecules. Notably, tannin-like compounds were identified as important factors influencing N2 fixation rates and nif gene expression. A potential association between methane (CH4) transformation and N2 fixation was also observed. The increase in N2 fixation rates after CH4 addition in straw-amended soils suggested that straw-derived oxidized DOM may promote diazotrophic activity under elevated CH4 conditions. Together, these results highlight the important roles of DOM dynamics and CH4 metabolism in sustaining free-living N2 fixation and provide mechanistic insights into balancing rice productivity with environmental sustainability.

Excessive ammonia (NH3) can result in visible foliar injury on vegetation and promote the formation of fine particulate matter (PM2.5), thus it is urgently necessary to reduce NH3 to protect ecosystems, environmental quality and human health. However, we still lack effective strategies to achieve the dual objectives of protecting vegetation and meeting the PM2.5 air quality standard. Here, we used the Community Multiscale Air Quality model to evaluate the NH3 and PM2.5 concentrations and provided synergistic emission reduction strategies to meet both of the above targets in China. Results showed that NH3 concentrations in 2019 in most provinces were much higher than the critical level of 3 μg m-3 for vegetation protection. Nearly 50% of NH3 emissions need to be reduced to meet the critical level, especially in central and northeast China. Meanwhile, provinces in eastern and central China still cannot reach the air quality standard target of PM2.5 (35 μg m-3) even with further reduction of NH3 emissions on this basis. Alternatively, reducing other species of pollutants, such as volatile organic compounds, nitrogen oxides, and sulfur dioxide, with a range of 0-60% will relieve the pressure of NH3 emission reduction in China to meet the air quality targets. The reduction of these pollutants in the industrial, transportation, and fugitive dust sectors should be prioritized for the synergistic management of NH3 and PM2.5 pollution.

Despite the recognized importance of dissolved organic matter (DOM) in soil carbon cycling, the molecular complexity and pronounced vertical heterogeneity of DOM have hindered a mechanistic understanding of how depth-structured DOM is associated with bacterial community organization in paddy soils. Here, we combined high-resolution mass spectrometry and 16S rRNA gene sequencing to investigate linkages between DOM molecular traits and bacterial composition across soil layers (0–20, 20–40, and 40–60 cm) in four representative paddy sites in South China. DOM molecular numbers decreased markedly with depth (on average from 2,848 formulas at 0–20 cm to 1,854 formulas at 40–60 cm), accompanied by a consistent decline in DOM chemodiversity (from 7.13 to 7.08). The middle soil layer functioned as a chemical transition zone, where intensified sulfur-associated transformations enhanced DOM condensation and aromaticity. These molecular changes coincided with decreased bacterial Shannon and Chao1 indies, reduced co-occurrence network connectivity (average degree decreased from 30.83 to 24.44), and a shift from oxidative metabolism toward fermentation-dominated functions. Random forest analysis identified lipid- and protein/amino sugar-like compounds as key predictors of bacterial diversity. However, their enrichment in deeper soil layers reflected the accumulation of refractory compounds with limited microbial accessibility rather than an increase in bioavailable substrates, thereby constraining microbial niche differentiation. These findings provide insight into how depth-structured DOM covaries with microbial ecology in paddy soils and offer a foundation for improving predictions of soil carbon cycling under long-term agricultural management.

Dissolved organic matter (DOM) plays a critical role in soil carbon cycling, yet its molecular dynamics and the associated microbial mechanisms in paddy fields remain poorly understood. Here, we combined FT-ICR MS, 16S rRNA sequencing, and metatranscriptomics across rice phenological stages (seedling, tillering, heading, maturity, and harvest) to trace DOM molecular succession and reveal directional couplings between microbial activity and DOM transformation. Our results revealed stage-dependent shifts in DOM composition, with labile and oxygen-rich compounds prevailing during early rice growth, followed by the accumulation in later stages. A total of 38,420 potential transformations were detected across rice phenological stages, over 55% of which were thermodynamically limited processes, indicating that DOM succession was predominantly influenced by microbial regulation. Changes in DOM molecular composition coincided with shifts in microbial community structure, with the relative abundance of core taxa such as Thermoleophilia and Actinobacteria co-varying across rice phenological stages. Granger causality analysis suggested a directional shift from labile compounds toward aromatic and stable forms. Metatranscriptomics further showed that early expression of energy- and amino acid metabolism facilitated turnover of labile DOM, while later enrichment of lipid metabolism and auxiliary activities supported the processing of complex substrates. Together, these findings highlight stage-dependent associations between microbial dynamics and DOM molecular succession in paddy soils, providing mechanistic insights into how microbial activity influences DOM transformation processes with implications for soil carbon persistence in agroecosystems.

Developing green technologies to mitigate soil carbon dioxide (CO2) and nitrous oxide (N2O) emissions is critical for sustainable agriculture. Here, we investigated the effects of two water-soluble metalloporphyrins (FeTPPS and MnTPPS), with and without glucose addition, on CO2 and N2O emissions and the associated mechanisms in an agricultural soil under natural solar irradiation. Metalloporphyrin effects depended on metal identity and labile carbon (C) availability. FeTPPS alone significantly reduced cumulative CO2 emissions by 9.9%, and FeTPPS with glucose reduced emissions by 7.2% relative to the glucose treatment. These effects were accompanied by redistribution of organic C into < 250 μm aggregates, suppressed cellulase activity, and decreased humic-like DOM components, suggesting altered organic C accessibility and aggregate-associated C distribution. However, FeTPPS did not significantly alter cumulative N2O emissions. MnTPPS exhibited a biphasic effect on CO2 flux, but still significantly reduced cumulative CO2 emissions by 13.3%, consistent with increased abundances of cbbL/cbbM. MnTPPS also significantly reduced cumulative N2O emissions by 63.3%, despite increased urease activity and abundances of nitrification-related genes, and this reduction coincided with increased nosZII abundance. However, glucose addition eliminated the suppressive effects of MnTPPS on CO2 and N2O emissions, recovered cellulase activity, and did not alter cbbL/cbbM abundances, indicating that labile C can override MnTPPS-induced changes in CO2 and N2O emissions. Overall, these results demonstrate that solar-driven metalloporphyrin catalysis can regulate soil CO2 and N2O emissions, and its effectiveness depends on labile C availability. These findings provide useful information for developing innovative strategies to mitigate soil CO2 and N2O emissions.

Global heatwave intensification under climate change will impact the nitrogen cycle; yet, its effect on active nitrifier groups or their interactions with viruses remains unclear. Using 13CO2-DNA-based stable-isotope probing coupled with metagenomics, we show that elevated temperatures under heatwave conditions fundamentally restructure active nitrifying communities and their associated viruses in Yangtze River estuary upper tidal flats and adjacent agricultural soils. In tidal flats, sustained high temperature constrained nitrification by reducing the abundance of active ammonia-oxidizing archaea and bacteria (AOA, AOB) and canonical nitrite-oxidizing bacteria (NOB). This was accompanied by a shift in the active community from marine to more thermotolerant but less salt-tolerant terrestrial ecotypes. Conversely, heatwave conditions in agricultural soils suppressed AOB but enhanced nitrification activity in thermotolerant terrestrial AOA ecotypes. Across both ecosystems, inferred virus-nitrifier interactions were temperature dependent. 13C-labeled nitrifier-infecting viruses exhibited coordinated shifts in virus-to-host abundance ratios and predicted lifestyles with their hosts, with sustained high temperatures reducing virus-to-host abundance ratios and favoring temperate infections, relative to higher abundance ratios and a greater proportion of predicted lytic cycles at lower temperatures. We identified AOA-infecting viruses that carry plastocyanin (pcy), encoding a key copper-dependent electron carrier in the AOA respiratory chain, with conserved active sites and a predicted protein fold that supports its capacity for electron transfer, potentially augmenting host energy metabolism. Together, our findings demonstrate that prolonged heatwaves drive coupled shifts in nitrifier community composition and virus-host interaction strategies in a land-use-dependent manner, with implications for nitrogen transformations and ecosystem feedbacks under climate extremes.

While biochar (BC) is proposed to mitigate estrogen-associated ecological risks in soil, its effects on estrogen fate remain obscure. We studied mineralization, transformation, and non-extractable residue (NER) formation of 17β-estradiol (E2) in sterilized soil and active soil with 0 %, 0.1 %, and 1 % BC amendment under oxic conditions for 42 days. E2 dissipated rapidly in the active soil (half-life = 1.5 days), mainly forming NERs (65 ± 1 % at the end of experiments) or mineralized (11.1 ± 0.4 %), with generation of minor extractable residues containing estrone (2.0 ± 0.2 %) and one unknown biotransformation product TP (2.5 ± 0.2 %). E2-derived NERs were formed mainly via physico-chemical entrapment (59 ± 3 % of the total NERs) and were associated with humin fraction (66 ± 3 % of the total NERs). BC application had no significant effect on E2 mineralization, transformation into estrone and TP, and NERs quantity, but significantly promoted the formation of physico-chemical entrapped NERs and fulvic acids-bound NERs at 0.1 % level, while increasing BC level might increase the formation of humic acids-bound NERs. Our findings highlight a critical role of NERs in determining environmental risk of E2 and suggest that BC application may not be effective strategy for mitigating estrogen contamination in soil.

The persistence of soil organic matter (SOM) is shaped by its molecular features and stability, but the temporal dynamics of these features remain unclear. Here we investigate the molecular diversity (the number of molecules) and molecular thermodynamic stability (the theoretical Gibbs free energy for the half reaction of carbon oxidation) of SOM in soils from long-term (>30 years) paddy and upland experimental fields. Thermogravimetric analysis shows that enhanced SOM thermostability aligns with the temporal variation of molecular thermodynamic stability in these soils. Increased SOM molecular thermodynamic stability occurs alongside decreased molecular diversity over decades, and this temporal trade-off (negative relationship) is modulated by increased bacterial richness. These findings highlight the role of microbial diversity in enhancing SOM thermostability and support strategies that promote bacterial richness for improved SOM persistence in agriculture. Soil organic matter stability is critical for long-term soil health and carbon sequestration. This study reveals that increased bacterial richness enhances soil organic matter thermostability by driving a trade-off between molecular diversity and thermodynamic stability.

Atmospheric nitrous oxide (N2O) is a potent greenhouse gas and ozone-depleting substance. In this Review, we outline global N2O sources, with a focus on hotspots and hot moments, and discuss strategies to mitigate N2O emissions. N2O can be released by natural sources such as bedrock weathering, but anthropogenic sources such as agriculture account for 40% of total emissions. Hotspots are localized regions of high emissions and include cropland soils (2.1 Tg N yr−1), tropical forests (1.55 Tg N yr−1), pasture soils with animal waste return (1.7 Tg N yr−1), and streams and small lakes (0.4 Tg N yr−1). Brief periods of intense emissions, known as hot moments, include post-deforestation, upland soils after fertilizer application, and desert and grasslands after precipitation. N2O production from terrestrial and aquatic environments is mainly driven by two microbial processes: nitrification and denitrification. Bioaugmentation and biogeoengineering technologies hold potential for reducing N2O emissions; for example, nature-based anammox hotspot geoengineering in Jiaxing, China, reduces N2O emissions by 27.1%. However, the spatiotemporal heterogeneities and different production pathways of N2O emissions are poorly represented in existing models, hindering the quantification and mitigation of emissions. A global N2O database is needed to address this limitation. Additionally, artificial intelligence technology could enable real-time agricultural management to align nitrogen supply with crop demand. Nitrous oxide (N2O) emissions are continuously increasing owing to human activities. This Review discusses the temporal and spatial variability of N2O sources, N2O production pathways, and the potential of biogeoengineering strategies in agricultural soils and terrestrial water bodies to mitigate emissions.

Global heatwave intensification associated with climate change will impact the nitrogen cycle, yet its effect on specific nitrifier groups or their interactions with viruses remains unclear. Using 13CO2-DNA-based stable isotope probing (SIP) coupled with metagenomics, we show that elevated temperatures associated with heatwave conditions restructure active nitrifying communities and their viruses in Yangtze River estuary tidal flats and adjacent agricultural soils. In tidal flats, high temperatures shifted active ammonia-oxidizing archaea and bacteria (AOA and AOB), and nitrite-oxidizing bacteria (NOB) from marine to terrestrial ecotypes. In contrast, heatwave conditions stimulated terrestrial ecotypes of AOA but suppressed AOB in agricultural soils. 13C-labeled nitrifier-infecting viruses also showed temperature-driven shifts in activity, lifestyle, and auxiliary metabolic genes in concert with hosts. Notably, AOA viruses carried the plastocyanin gene, potentially augmenting host metabolism. These findings demonstrate heatwaves drive complex shifts in nitrifier communities and their interactions with viruses, impacting global nutrient cycling under climate extremes. ### Competing Interest Statement The authors have declared no competing interest. National Natural Science Foundation of China, 42477318, 42277304, U22A20590 US Department of Energy Early Career Research Program, DE-SC0025455

Organic fertilizer can enhance soil health and multifunctionality in agroecosystems, but its impact on soil-borne greenhouse gas emissions needs mitigation. Fungal denitrification significantly contributes to N2O emissions in carbon-rich soils; yet, the interactions between bacterial and fungal denitrifers under organic fertilizer amendment, remain unclear. Here, we investigated the rates and proportions of N2O and N2 emissions, along with the interactions between fungal and bacterial denitrifiers in a high nitrogen (N) loading arable soil subjected to four treatments: ⅰ) Control, ⅱ) organic fertilizer (Manure), ⅲ) synthetic fertilizer (Urea), and ⅳ) synthetic plus organic fertilizer (Urea + Manure). Results showed that N2O and N2 fluxes increased by 35.4 and 7.7 folds, respectively, in the Manure treatment compared to Control treatment. And these fluxes increased by 62.9 and 37.0 folds, respectively, in the Manure + Urea treatment compared to Urea treatment. Meanwhile, the contribution of fungal denitrification to N2O emissions significantly increased in both Manure and Urea + Manure treatments, due to the significant enrichment of keystone fungal denitrifiers like Chaetomium among bacterial and fungal denitrifiers' co-occurrence networks. Additionally, N2O/(N2O + N2) ratio significantly decreased in the Manure and Urea + Manure treatments, which was primarily driven by significant enrichment of keystone bacterial denitrifiers carrying nosZ gene such as Achromobacter, Chelatococcus, and Shinella. These bacteria possess complete denitrification capability and can synergize with fungal denitrifiers, enhancing N2O reduction. Overall, our findings suggest that organic fertilizer amendment in high N loading arable soils decreases N2O/(N2O + N2) ratio mainly by enhancing fungal-bacterial denitrifier mutualism.

The Ningxia Yellow River irrigation area, characterized by an arid climate and high leaching of NO3--N, exhibits complex and unique groundwater nitrate (NO3--N) pollution, with denitrification serving as the principal mechanism for NO3--N removal. The characteristics of N leaching from paddy fields and NO3--N removal by groundwater denitrification were investigated through a two-year field observation. The leaching losses of total nitrogen (TN) and NO3--N accounted for 10.81-27.34% and 7.59-12.74%, respectively, of the N input. The linear relationship between NO3--N leaching and N input indicated that the fertilizer-induced emission factor (EF) of NO3--N leaching in direct dry seeding and seedling-raising and transplanting paddy fields was 8.2% (2021, R2 = 0.992) and 6.7% (2022, R2 = 0.994), respectively. The study highlighted that the quadratic relationship between the NO3--N leaching loss and N input (R2 = 0.999) significantly outperformed the linear relationship. Groundwater denitrification capacity was characterized by monitoring the concentrations of dinitrogen (N2) and nitrous oxide (N2O). The results revealed substantial seasonal fluctuations in excess N2 and N2O concentrations in groundwater, particularly following fertilization and irrigation events. The removal efficiency of NO3--N via groundwater denitrification ranged from 42.70% to 74.38%, varying with depth. Groundwater denitrification capacity appeared to be linked to dissolved organic carbon (DOC) concentration, redox conditions, fertilization, irrigation, and soil texture. The anthropogenic-alluvial soil with limited water retention accelerated the leaching of NO3--N into groundwater during irrigation. This process enhances the groundwater recharge capacity and alters the redox conditions of groundwater, consequently impacting groundwater denitrification activity. The DOC concentration emerged as the primary constraint on the groundwater denitrification capacity in this region. Hence, increasing carbon source concentration and enhancing soil water retention capacity are vital for improving the groundwater denitrification capacity and NO3--N removal efficiency. This study provides practical insights for managing groundwater NO3--N pollution in agricultural areas, optimizing fertilization strategies and improving groundwater quality.

Dissolved organic matter (DOM) is pivotal for soil biogeochemical processes, soil fertility, and ecosystem stability. While numerous studies have investigated the impact of fertilization practices on DOM content along soil profiles, variations in DOM chemodiversity and the underlying factors across soil profiles under long-term fertilization regimes remain unclear. Using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and high-throughput sequencing, this study investigated DOM composition characteristics and microbial community compositions across different soil layers (0-20, 20-40, 40-60, and 60-100 cm) in paddy soil under different long-term fertilization treatments, including Control (no fertilizer), NPK (mineral NPK fertilizer), NPKHS (NPK fertilizer with half straw return), and NPKS (NPK fertilizer with full straw return). The results revealed that fertilization regimes significantly increased soil TC, TN, and NO3- contents, as well as DOM chemodiversity in the top soil layer, particularly under NPKHS and NPKS treatments. Both the DOM chemodiversity and bacterial diversity decreased with soil depth. However, below 0-20 cm, DOM chemodiversity was not significantly affected by fertilization treatments. Co-occurrence network analysis further showed that microbial decomposition primarily drove the changes in DOM composition across soil profile. Overall, our study suggests that long-term NPK fertilization and straw return significantly increased DOM chemodiversity only in the top layer of paddy soil by regulating soil TC, TN, and NO3- contents. Our study provides useful information regarding the vertical molecular composition of DOM and enhances the understanding of DOM chemodiversity along soil profile in rice paddy ecosystems.

Soil salinization, impaired by climate change and poor management practices, poses a global threat, particularly in arid and semi-arid regions, leading to significant land degradation. This study aims to investigate the effects of different nitrogen (N) fertilizer sources (urea, ammonium-sulfate, and biogas waste) on CO2, 2 , N2O, 2 O, and NH3 3 emissions and soil enzyme activities in two soil types varying in salinity level (non-saline: EC = 1.15 dS m-1,-1 , and saline: EC = 35.80 dS m- 1 ) in a robotized continuous-flow soil incubation system. Our results showed a sharp increase in N2O 2 O and CO2 2 emissions (up to 0.51 +/- 0.02 g N2O-N 2 O-N ha- 1 day- 1 , 28.1 +/- 3.9 kg CO2-C 2-C ha- 1 day- 1 ) in non-saline soils following soil rewetting, attributed to bacterial denitrification. However, this pattern was not observed in saline soils, suggesting that salinity causes partial inhibition to the regeneration of soil organic matter mineralization and denitrification processes after rewetting. Although salinity did not alter the overall cumulative N2O 2 O losses in any fertilizer treatment, it significantly delayed the evolution of N2O 2 O peak during the incubation period. On the other hand, NH3 3 volatilization was significantly higher in N-fertilized saline soils compared to non-saline soils (241% and 157% in ammonium-sulfate and biogas waste treatments, respectively), except for urea treatment, likely due to the decrease in nitrification rates. Furthermore, the study clearly showed lower soil enzyme activity levels for both nitrate reductase and urease activity. Interestingly, the lowest NH3 3 emissions were measured in urea treatment in both soils. Overall, our findings highlight the complex interplay between soil salinity, nitrogen fertilizer sources, and microbial processes, significantly influencing gaseous nitrogen emissions and N cycling in agricultural soils. Identifying the specific fertilizer treatments that minimize or maximize gaseous nitrogen losses in varying soil salinity, may guide the selection of appropriate fertilization strategies for farmers and policymakers to mitigate environmental impacts of fertilizer use during agricultural production.

Redox condition is an important controlling factor for contaminant removal in constructed wetlands; however, the redox-sensitivity of antibiotic removal in wetland sediments under controlled conditions with specific electron acceptors remains unclear. Here, using a 14C radioactive tracer, we explored fate of sulfamethoxazole (SMX) in a wetland sediment slurry under oxic, nitrate-reducing, iron-reducing, and methanogenic conditions. In the sterile treatment, unlike the comparable SMX dissipation from the water phase under four redox conditions, non-extractable residues (NERs) of SMX was highest formed in the sediment under oxic condition, mainly in sequestered and ester/amide-linked forms. Microorganisms markedly promoted SMX transformation in the slurry. The dissipation rate of SMX and its transformation products (TPs) followed the order: oxic ≈ iron-reducing > methanogenic >> nitrate-reducing conditions, being consistent with the dynamics of microbial community in the sediment, where microbial diversity was greater and networks connectivity linking dominant bacteria to SMX transformation were more complex under oxic and iron-reducing conditions. Kinetic modeling indicated that the transformation trend of SMX and its TPs into the endpoint pool NERs depended on the redox conditions. Addition of wetland plant exudates and sediment dissolved organic matter at environmental concentrations affected neither the abiotic nor the biotic transformation of SMX. Overall, the iron-reducing condition was proven the most favorable and eco-friendly for SMX transformation, as it resulted in a high rate of SMX dissipation from water without an increase in toxicity and subsequent formation of significant stable NERs in sediment. Our study comprehensively revealed the abiotic and biotic transformation processes of SMX under controlled redox conditions and demonstrated iron-reducing condition allowing optimal removal of SMX in constructed wetlands.