Methanogenesis was previously considered to be strictly confined to anoxic environments, but aerobic methane (CH4) production is increasingly being recognized, although it is typically ascribed to biological activity or photothermal reactions. Here we describe rhizosphere soil incubation experiments and biogeochemical analyses and our findings that photothermally inert rhizospheres of aquatic plants sustain widespread oxic CH4 formation in the absence of microbial CH4 oxidation. Circadian radial oxygen loss from plant roots induces redox oscillations in the rhizosphere that drive the CH4 production. At night, iron minerals in the soil are reduced, and during the day, the Fe(II) produced overnight is re-oxidized. This sequence generates reactive oxygen species and oxo-iron(IV) complexes, which mediate demethylation of organic substrates and release CH4 as a by-product. The extent of this redox-driven CH4 formation depends largely on the soil iron reactivity and the composition of the organic matter. Using a random forest model, we estimate that the global CH4 production potential in rice rhizospheres is on the order of 0.7-3.3 Tg yr(-)(1), corresponding to 1.9-13.2% of total paddy CH4 emissions. These findings uncover a redox-driven CH4 source, with implications for global CH4 budgets and organic carbon cycling.

Mitigating eutrophication in shallow lakes necessitates accurate measurement of internal phosphorus loading—a task complicated by intricate sediment–water interactions and insufficient long-term monitoring. This study integrates high-resolution porewater monitoring with machine learning (ML) to quantify internal soluble reactive phosphorus (SRP) release in the large eutrophic Lake Taihu, China. Monitoring across the whole of Taihu Lake throughout 2021–2022 indicated that SRP release fluxes varied from −0.32 to 1.84 mg/(m2·day), with peaks occurring in summer–autumn within algal-dominated zones, where internal SRP release was closely associated with the reductive dissolution of Fe-oxides. Building on these observations, an ML approach using an optimized Ridge model predicted SRP release fluxes over the past two decades. The results showed that internal SRP release contributed an average of 45 % of total phosphorus (TP) increases during critical bloom periods. Structural equation modeling indicated that chlorophyll-a variability was strongly linked to internal SRP release, temperature, and TP, which together accounted for 65 % of the variation, suggesting internal SRP release is a key driver for sustaining algal blooms. Furthermore, two-decade dynamics of internal SRP release revealed that its predominant control factors are shifting from past anthropogenic disturbance pressures to climate-induced processes. The findings highlight the important role of internal SRP release in sustaining eutrophication, and provide scientific guidance for formulating adaptive remediation measures to control its release.

Aquatic phosphorus (P) transformations are fundamental to water quality. While microscale experiments and macroscale models have advanced understanding of P cycling, their largely independent development has yielded a fragmented view of aquatic P dynamics, limiting the accuracy of P management across scales. Given the growing urgency of global water-quality governance, closer integration across scales is needed to provide a more predictive foundation for managing and mitigating P dynamics.

While microplastics (MPs) are known to profoundly disrupt carbon cycling in lake sediments, the distinct mechanistic pathways by which biodegradable MPs reconfigure organic carbon fractions (OCFs) remain largely unknown. This study elucidates how poly(butylene adipate-co-terephthalate) (PBAT) and polyethylene (PE) MPs dynamically reconfigure OCFs and modulate carbon dioxide (CO2) and methane (CH4) emissions in anaerobic lake sediment microcosms. Incubation experiments showed that PBAT serves a dual role: it acts as a potent short-term carbon source that accelerates the release of bioavailable oligomers and dissolved organic carbon (exceeding PE), selectively enriching fermentative microbiota (e.g., p_Firmicutes) and enhancing carbon cycle-related functions, thereby increasing CO2 and CH4 emissions by up to 68.59 % and 191.07 % (compared to the control), respectively. Concurrently, PBAT-amended sediment accelerated the decomposition of potential mineralized carbon and readily oxidizable organic carbon, resulting in reduced absolute stability (heavy/light fraction organic carbon= 1.03-2.63) compared to natural sediment (4.45-6.64). Nonetheless, PBAT degradation appeared to inhibit late-stage methanogenesis while facilitating a net redistribution of carbon towards stabilized fractions. Crucially, PBAT enhanced carbon stabilization by promoting humification (increased SUVA254 and humic-like fluorescence), mineral association, and humin accumulation. These findings reveal a significant environmental trade-off: biodegradable MPs, although less physically persistent, introduce complex biogeochemical disruptions by serving as substantial short-term carbon sources and modifying carbon sequestration processes. This challenges the assumptions of PBAT's environmental safety, emphasizing significant risks to carbon-sensitive lake ecosystems and necessitating the consideration of these cascading carbon-climate feedbacks in policy frameworks.

The development of binding gels with a fast uptake rate, high capacity, and good selectivity could be beneficial for trace Hg(II) detection based on the DGT technology. In this study, a novel PAN@MoS2/rGO-DGT was assembled by using the nanocomposite embedded in polyacrylonitrile membrane (PAN@MoS2/rGO) the binding phase. The interior regular finger-like macropore of the gel provided a convenient channel for the rapid mass diffusion of Hg(II), and the abundant sulfur offered the paramount driving force for trapping Hg(II). These endowed the PAN@MoS2/rGO with an impressive reaction rate, capacity, and selectivity toward Hg(II) and featured the PAN@MoS2/rGO-DGT with excellent diffusion rate (D) and adaptability in the complex matrixes across a wide range of pH, ion strength, common cations. However, the uptake of Hg(II) was influenced by the high content of chloride, thus a calibrated model was established based on the chloride concentration correct the accumulated mass and D . After that, the high accuracy of this method was confirmed through the good consistency between Hg(II) concentration assessed by DGT and in bulk solution when the DGT was deployed to the river water, seawater, and domestic wastewater at the static and dynamic Hg(II) concentration. Field trials in the prawn farming seawater and lake water also showed a negligible deviation of Hg(II) content from the DGT and the conventional method, acquiring actual Hg(II) level as 1.07-3.69 ng/L. The findings highlighted the application potential of macropore PAN gel hybrid with nanocomposite as a promising binding phase for trace Hg(II) or other pollutant detection.

Remediation of arsenic (As)-contaminated sediments is challenging, due to surface sediment often being subjected to hypoxic/anoxic conditions where As(Ⅲ) is the dominant species. In this study, a novel capping material comprising zirconium-manganese binary oxides (ZMBO) was synthesized and its feasibility in controlling sedimentary As release investigated using high-resolution sampling, X-ray absorption near edge structure (XANES) spectroscopy, and scanning electron microscopy (SEM) techniques. Results showed ZMBO exhibited both high oxidation efficiency (94 %) and strong adsorption capacity (151.8 mg As/g) for As(Ⅲ). Capping As-contaminated sediments with ZMBO resulted in a negative diffusive flux of -0.08 ng/cm2/s, effectively maintaining low As levels in the overlying water over 150 days. XANES spectra showed As in surface sediments existed predominantly As(V), consistent with high-resolution data indicating ∼90 % of labile As(Ⅲ) was oxidized and adsorbed by ZMBO. Furthermore, ZMBO also promoted Fe(Ⅱ) oxidation to stable hematite in sediments, providing additional adsorption sites for As. By comparing with current capping materials, ZMBO exhibited a balanced performance in terms of its cost-effectiveness, adsorption capacity, remediation effects, and environmental adaptability. This study highlights the potential of ZMBO as a promising capping material for remediating As-contaminated sediments through combined chemical oxidation and adsorption mechanisms, offering sustainable solutions for improving water quality management worldwide.

Phosphorus (P) availability is vital for global primary productivity, yet it is often immobilized in soils by redox-inert crystalline iron (oxy)hydroxides. Here we show that diel radial oxygen loss (ROL) from plant roots induces redox fluctuations in the rhizosphere, activating these iron minerals and enhancing P mobilization. Nighttime reduction and daytime oxidation drive the formation of reactive metastable iron phases (RMPs) on root surfaces, forming a redox-active iron plaque. These RMPs undergo rapid dissolution-reformation cycles, facilitating P transfer from soil to porewater for plant uptake. Using multiple aquatic plants from agriculturally developed regions, we demonstrate that ROL broadly enhances soil P availability. In rice paddies, ROL-activated P release accounts for 8.7% of global P fertilizer input, contributing an estimated economic value of USD 0.52 billion annually. Our findings uncover a previously overlooked redox mechanism by which plants enhance P acquisition, with broad implications for nutrient cycling and agricultural sustainability.

Dams worldwide commonly accelerate the eutrophication of reservoirs. While the seasonal hypoxia in deep reservoirs is widely acknowledged, there is limited research on its impact on benthic phosphorus (P) cycling and P fraction release from the reservoir sediments. Here we show that seasonal hypoxia enhances sediment P release and P fluxes at the sediment -water interface (SWI) which might alter P dynamics in deep reservoirs. We conducted a detailed measurement of sediment P fractions through the SEDEX approach, combined with a labile P gradient analysis using the diffusive gradients in thin films (DGT) technique to understand P cycling patterns in sediments during the transition period from spring (oxic) to late summer (hypoxic) conditions. The sediment P pool was predominantly composed of iron-bound phosphorus (Fe -P, 76 -80 %), primarily due to the widespread occurrence of lateritic red soil (rich in Fe 2 O 3 /MnO 2 ) in subtropical areas. More organic-P was observed in summer compared to spring. A significant increase in labile P occurred at the depth of 0 -4 cm and 0 -1 cm in spring and summer, respectively, where sediment P release was primarily governed by the reduction of Fe -P and the generation of S 2- . A higher apparent fluxes of phosphate across the SWI were observed in summer characterized by higher temperature and lower oxygen levels. The current results suggest that seasonal hypoxia was a crucial factor affecting P cycling and diffusion in deep reservoirs. These findings present important implications for the ecology and management of the watershed -coast ecosystem.

AbstractBiochar is widely used for sediment remediation owing to its excellent adsorption properties and low carbon footprint. However, the impacts of biochar capping on phosphorus (P) bioavailability and mobility in the sediment are little known. In this study, the P mobilization processes in sediments capped with biochar were investigated by combining advanced high-resolution sampling techniques and microbiome analysis. The results showed that biochar is a double-edged sword for the sediment P release, depending on the application dosage and the capping time. In the short term (30 days), 2-cm biochar capping decreased the release flux of soluble reactive phosphorus (SRP) by 73.1%, whereas the 1-cm biochar capping significantly increased the release flux of SRP by 51.0%. After aging of biochar (80 days), the resupply capacity of sediment P was improved, resulting in increases of more than 33.7% and 121.5% in the release fluxes of SRP in the 1-cm and 2-cm capping groups, respectively, compared to the control group. Chemisorption played a pivotal role in regulating the levels of SRP, particularly during the short-term capping period. And more biochar can provide more adsorption sites on P. The P mobilization increase could be attributed to P desorption from biochar after biochar aging. Furthermore, biochar capping intensified the microbial-mediated iron reduction and organic matter decomposition, which enhanced P mobility. Our study highlights the importance of biochar application dosage and the capping time in sediment remediation, providing a scientific basis for the optimization of biochar capping techniques. Graphical Abstract

Mercury (Hg), especially in the form of methylmercury (MeHg), poses a significant threat to both organisms and the environment due to its extreme toxicity. While methylation process of Hg in sediments has been extensively studied, recognition of its associated risks and mechanisms during cyanobacterial blooms remains limited. This study investigated the distribution characteristics of Hg and MeHg in sediments of Taihu Lake, China. The concentration of Hg and MeHg varied within the range of 96.0-212.0 ng g-1 and 0.1-0.5 ng g-1, respectively. Higher ecological risks of Hg were found in algal-dominated regions compared to macrophyte areas. The significant correlations observed between Hg, MeHg, and algal-derived dissolved organic matter (ADOM) components C1 and C2 in algal-dominated regions indicate a close association between ADOM components and the Hg methylation process. These components are involved in the absorption or complexation of Hg, participate in redox reactions, and modulate microbial activity. The dsrB gene in sulfate-reducing bacteria (SRB) was found to accelerate the metabolic pathways of Hg methylation. These findings indicate that ADOM could enhance the methylation process of Hg during cyanobacterial blooms, which warrants attention.

This study investigated seasonal variations in spatial distribution, mobilization kinetic and toxicity risk of arsenic (As) in sediments of three representative ecological lakes in Lake Taihu. Results suggested that the bioavailability and mobility of As in sediments depended on the lake ecological types and seasonal changes. At the algal-type zones and macrophyte-type zones, elevated As concentrations were observed in April and July, while these occurred at the transition areas in July and October. The diffusion flux of soluble As ranged from 0.03 to 3.03 ng/ cm2/d, indicating sediments acted as a source of As. Reductive dissolution of As-bearing iron/manganese-oxides was the key driver of sediment As remobilization. However, labile S(-II) caused by the degradations of algae and macrophytes buffered sediment As release at the algal-type and macrophyte-type zones. Furthermore, the resupply ratio was less than 1 at three ecological lakes, indicating the resupply As capacity of sediment solid phase was partially sustained case. The risk quotient values were higher than 1 at the algal-type zones and transition areas in July, thereby, the adverse effects of As should not be ignored. This suggested that it is urgently need to be specifically monitored and managed for As contamination in sediments across multi-ecological lakes.

Rhizoremediation of wetland plants is an environmentally friendly strategy for sediment phosphorous (P) removal, the basic underlying principle of which is the complex interactions between roots and microorganisms. This study investigated the immobilization and mobilization mechanisms of P in the rhizosphere of wetland plants using high-resolution spatial visualization techniques and metagenomic sequencing. Two-dimensional visualization of the spatial distribution of P, iron (Fe) and manganese (Mn) indicated that the sequestration of Fe-oxides rather than Mn-oxides caused the depletion of labile P, resulting in an increase in the Fe-adsorbed P fraction. Plants altered the rhizospheric environments and P-cycling microbial community to mobilize low-availability P from sediments. Mineral P solubilization and organic P mineralization were enhanced by local acidification and increased phosphatase activity, respectively. Microbial P mobilization also increased with increasing relative abundances of P solubilization and mineralization genes (gcd and phnW) and decreasing P transportation genes (ugpA, ugpB, and pit) genes in the rhizosphere. These processes led to the remobilization of 10.04% of inorganic P, and 15.23% of organic P, in the rhizosphere during the incubation period. However, the resupply of P via the above processes did not compensate for the depletion of rhizospheric P via root uptake and mineral sequestration. Our results provide novel insights into the mechanisms of rhizospheric P cycling, which will help to inform future phytoremediation strategies.

Excess nickel (Ni) entering lakes can pose adverse effects on aquatic ecosystems and human health. This study aimed to reveal the spatiotemporal distribution, mobilization kinetics, and potential risk of Ni in sediments of a typical multi-ecological lake, Lake Taihu, China. We conducted seasonal monitoring of the spatial distribution of soluble and labile Ni in sediments using high-resolution dialysis samplers (HR-Peeper) and the diffusive gradient in thin-films technique (DGT), respectively. We found that the total Ni concentrations in sediments (mean: 37.56 mg kg−1) all exceeded the background value (19.5 mg kg−1). The spatial distributions of soluble and labile Ni showed no notable fluctuations along the vertical profiles of sediments in all seasons. The DGT-induced fluxes model implied that there is a partial Ni resupply capacity in the sediment of all three ecological zones, but it is higher in the algal-type zones than in the macrophyte-type and transition zones. Furthermore, an assessment of the ecotoxicological risk found that the risk quotient values for Ni were less than 1 in all sampling seasons, indicating a low ecotoxicological risk of Ni in the sediments of Lake Taihu. Our results indicate that the ecological risk in the algal-type lake zones requires special attention. Our findings help towards improving the level of understanding regarding the mobilization process and potential risk of Ni in sediments, which in turn can provide guidance for the prevention and control of sediment Ni pollution in lakes with multiple types of ecological zones.

Changes in the oxygen and acidic environments of water caused by algal blooms in lakes aggravate the uncertainty of the eutrophication process, profoundly influencing water ecosystems and lake biogeochemical cycles. This study aimed to detail the mechanisms of benthic oxygen and acid processes by determining high-resolution and high-quality chemical gradients and acquiring heterogeneous data for diurnal changes in dissolved oxygen (DO)/pH at the water-sediment interface (SWI) during different stages of algal degradation. Planar optode (PO) systems were used to obtain dynamic two-dimensional (2D) images of DO/pH at a laboratory microcosm interface. The benthic gradient and diurnal variation of DO/pH at the SWI were jointly regulated by the photosynthesis/respiration and migration behaviors of algae, through their influence on the water-carbon chemistry. The DO/pH at the SWI showed unique diurnal variation characteristics at different stages of algal degradation; however, characteristics consistent with the diurnal variation in photosynthesis were limited to the algal growth period. The increase in DO/pH during the daytime resulted from O2-rich and carbonate hydrolysis, to compensate for the reduction in CO2 saturation caused by algal photosynthesis. Nocturnal DO/pH decreased owing to O2 depletion and H+ release, because of increased water CO2 content from algal respiration. When photosynthesis and respiration were inhibited, or not smooth, DO/pH showed a limited increase with uncertainty. The sediment was not completely anaerobic, and DO/pH penetrated to - 20 mm and even to the bottom of the interface (- 40 mm) as a result of algal migration, whereas DO was exhausted and high pH areas transferred to sediments following algal decay. Sediments acted as storage pools in alkaline environments, which posed a potential risk for water eutrophication. The trajectory of algal activity at the SWI was highly consistent with the dynamic variation in DO/pH. Our findings provide new insights for in situ tracking and early prediction of algal blooms, and can be used to formulate a theoretical basis for the mechanism of eutrophication processes and for exploring new algal control technologies.

Pollutions of trace metals (TMs) in reservoirs are blooming due to TMs were trapped efficiently in reservoir sediments by dams. Despite the mobilization of TMs in sediments have been well-documented, the patterns of biogeochemical processes occurred in sediments remain poorly understanding. Herein, a deep reservoir was selected to investigate the patterns of TMs biogeochemical processes in sediments by using high-resolution ZrOChelex-AgI diffusive gradient in thin films technique (HR-ZCA DGT) and the laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). 2-dimension high-resolution (2D-HR) images showed significant differential spatial enrichment of TMs (V, Mn, Fe, Co, Zn and Sb) in sediments, indicating strong heterogeneity in sediments. Correlations of TMs within microniches (diameter < 1 mm) in horizontal were usually different even contrast with that in vertical profile, suggesting distinct biogeochemical process patterns occurred in vertical vs. in horizontal. Further analyses from 2D-HR images showed the distributions of TMs in microniches reflected their mobilization that was driven by microenvironmental conditions. In contrast, distributions in sediment vertical profile recorded the diagenesis in different deposition depth. The diagenesis in sediment vertical is continuously accumulated by the discrete, microniches mobilization of TMs in horizontal. Collectively, our findings evidenced that 2D-HR data is an update complement to 1-dimension data for better interpret the biogeochemical process patterns of TMs in sediments, that have implication for water management to metals pollution in reservoir ecosystems.

Arsenic (As)-contaminated water restoration is extremely challenging because As remobilization from sediments can result in episodic or long-term release of As to the overlying water. In this study, by combining high -resolution imaging techniques with microbial community profiling, we examined the feasibility of utilizing the rhizoremediation of submerged macrophytes (Potamogeton crispus) to decrease As bioavailability and regulate its biotransformation in sediments. Results showed that P. crispus considerably decreased the rhizospheric labile As flux to lower than 4 pg cm-2 s- 1 from larger than 7 pg cm-2 s- 1, suggesting its effectiveness in promoting As retention in sediments. Iron plaques induced by radial oxygen loss from roots decreased the mobility of As by sequestering it. Additionally, Mn-oxides may act as an oxidizer for the oxidation of As(III) to As(V) in the rhizosphere, which can further increase the As adsorption owing to the strong binding affinity between As(V) and Fe-oxides. Furthermore, microbially mediated As oxidation and methylation were intensified in the microoxic rhizosphere, which decreased the mobility and toxicity of As by changing its speciation. Our study demonstrated that root-driven abiotic and biotic transformation contribute to As retention in sediments, which lays a foun-dation for applying macrophytes to the remediation of As-contaminated sediments.

For soils and sediments, which are complex and heterogeneous, root-triggered processes, bioturbation of benthic communities, and local decomposition of reactive organic materials can greatly increase their biogeochemical heterogeneity. In practice, common monitoring methods can only be used to determine the centimeter- to millimeter-scale heterogeneity from vertical or horizontal one-dimensional profiles. However, significant heterogeneity exists in both soils and sediments on the millimeter to sub-millimeter scales. Therefore, in-situ and high-resolution sampling and imaging methods are essential to capture the spatial and temporal heterogeneity of soils and sediments. This article reviews micro-scale sampling methods in soil and sediments, including diffusive equilibration in thin films (DET), diffusive gradient in thin films (DGT), planar optodes (PO), nanoparticle- and microparticle-based luminescence imaging, and soil zymography. Besides, we introduce their imaging principles and approaches in detail, and discuss some problems and solutions during imaging. Micro-scale sampling and imaging methods have given us many insights into the biogeochemical processes in soils and sediments, but the full potential of these methods has not been exhausted yet. Finally, the potential directions for the development of micro-scale sampling methods in the study of complex biogeochemical processes in heterogeneous soils and sediments are prospected.

The cycling of iron (Fe) and carbon in the rhizosphere of submerged plants and the associated impact on organic carbon (OC) sequestration are poorly understood. We detected the spatiotemporal distribution of CO2 using a planar optode in the rhizosphere of a common submerged plant as an indicator of OC mineralization. We found that the rhizosphere was a hot spot of CO2 and that the CO2 concentration decreased rapidly from the root tip zone (maximum of 19.68 +/- 0.60 matm) to the root base zone (maximum of 12.10 +/- 0.29 matm), with the trend of change being the opposite of that of O2. The Fe plaques in the root tip zone had the highest concentration of amorphous Fe. However, the concentration of Fe-bound OC was not significantly different among the different root parts. Because the relative abundances of ferrobacteria decreased in the order tip (8.45%) > base (4.05%) approximate to bulk sediment (3.05%), the enrichment of CO2 in the rhizosphere was attributed to dissimilatory Fe(III) reduction, O2 induced microbial respiration, and root respiration. Our conclusion was that the root tip of submerged plants is an efficient engine for Fe oxidation-carbon sequestration and Fe reduction-carbon mineralization processes, which may affect the stability of sediment carbon pools.