CH4 emissions from agricultural activities in response to anticipated climate changes, such as elevated temperature (warming) and elevated CO2 levels (eCO2), remain highly uncertain. In this study, warming, eCO2, and their combined effects were simulated using open-top chambers to elucidate the underlying mechanisms regulating CH4 emission potential from paddy soils. We found that single or combined warming and eCO2 uniformly reduced CH4 emission potential, decreased by 17.0 to 32.7%. A suite of complementary analyses, including carbon isotopic tracing, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), and microbial metagenomic and metabolomic profiling, were conducted to uncover the underlying mechanisms. We discovered only a marginal change of microbial community, metabolism and dead residuals (microbial necromass carbon). In comparison, CH4 production was primarily mediated by shift in dissolved organic matter (DOM) molecular composition. An increase of lignin-like compounds combined with a decrease of carbohydrate explained the changes in CH4 production. Variance partition analysis and structural equation model also evidenced the importance of DOM molecular composition rather than microbial traits on regulating CH4 production. This study highlights an important role of DOM chemical stability in regulating CH4 emissions in a changing world.

Coastal wetlands are critical blue carbon reservoirs, yet the depth-resolved impacts of warming on belowground carbon dynamics remain poorly understood. Over the course of an 8-year in situ experiment, we investigated plant-derived carbon inputs, soil carbon losses via respiration, and microbially mediated carbon fixation across a 60 cm soil profile under a projected 2°C atmospheric warming scenario. Plant carbon fixation (above- and belowground net primary productivity) and soil respiration exhibited synchronized responses to warming, with an initial increase, followed by a decline in the mid-term, and no significant response in the later stages. Soil and microbial respiration stabilized after prolonged exposure to elevated temperatures, as these processes were constrained by substrate availability. In contrast, phospholipid fatty acid profiling, amino sugar biomarkers, and metagenome-assembled genomes consistently indicated a greater than one-third reduction in microbial carbon fixation within subsoils (40-60 cm). Our fully factorial, depth-stratified warming design reveals the particular vulnerability of deep soil microbial carbon retention to long-term climate warming, independent of shifts in plant input or respiratory carbon loss. This work highlights underappreciated pathways influencing soil blue carbon dynamics in a changing world.

While dam-based regulation modifies the natural transport rhythm of riverine materials, the effects of artificial flooding on river mouth dissolved organic matter (DOM) dynamics remain poorly constrained. The Water-Sediment Regulation Scheme in the Yellow River provides a unique snapshot. Distinct organic matter signatures were observed between the water release and sediment flushing phases using optical and molecular techniques. Here we show that during water release phase, rapid discharge of upper-layer water from Xiaolangdi Reservoir increased the proportion of bio-labile DOM in the Yellow River mouth. Conversely, during sediment flushing phase, resuspension and then release of bottom sediments from the reservoir enhanced aromatic DOM components. Although the relative abundance and composition of particulate organic matter remained unchanged during water release phase, it increased during sediment flushing phase. These findings demonstrate that reservoir operations reconfigure river-ocean carbon flows, emphasizing the need to integrate dam management strategies into global carbon cycling models. During water release, rapid discharge of upper-layer water from Xiaolangdi Reservoir increased bio-labile dissolved organic matter at the Yellow River mouth, while sediment flushing resuspended bottom sediments and enhanced aromatic components, as shown by optical spectroscopy and isotope analysis.

Livestock feces contribute to approximately 32% of global methane emissions. Although ruminants are generally believed to have a higher methane production potential than non-ruminants, the dominant pathways and key regulatory processes underlying methane generation in ruminants remain poorly understood, impeding effective manure management and accurate livestock emission assessments. In this study, metagenomic and carbon isotope techniques were employed to investigate methane production potential and key pathways in sheep, pig, chicken, and duck feces. Methane production potential of ruminant sheep feces was significantly higher (approximately threefold) compared to that of non-ruminants. Isotopic analysis of methane sources revealed that sheep feces primarily produce methane through the acetoclastic pathway, whereas the other three likely rely on CO2 reduction. Metagenomic analysis of methanogenic pathways further indicated that the abundance of functional genes associated with acetoclastic methanogenesis is significantly higher in sheep feces compared to the other three. Moreover, the co-occurrence network analysis highlighted a tightly coordinated, cross-species partnership between fermentative acetogenic bacteria and methanogenic archaea in the sheep fecal microbiome. Together, our findings provide insights into some key methanogenic pathways, such as acetoclastic methanogenesis, contributing to high methane production from ruminant feces.

Methane (CH4) emission from livestock feces, led by ruminants, shows a profound impact on global warming. Despite this, we have almost no information on the syntrophy of the intact microbiome metabolisms, from carbohydrates to the one-carbon units, covering multiple stages of ruminant development. In this study, syntrophic effects of polysaccharide degradation and acetate-producing bacteria, and methanogenic archaea were revealed through metagenome-assembled genomes from water saturated dairy cattle feces. Although CH4 is thought to be produced by archaea, more edges, nodes, and balanced interaction types revealed by network analysis provided a closed bacteria-archaea network. The CH4 production potential and pathways were further evaluated through dynamic, thermodynamic and 13C stable isotope analysis. The powerful CH4 production potential benefited from the metabolic flux: classical polysaccharides, soluble sugar (glucose, galactose, lactose), acetate, and CH4 produced via typical acetoclastic methanogenesis. In comparison, a cooperative model dominated by hydrogenotrophic methanogenic archaea presented a weak ability to generate CH4. Our findings comprehensively link carbon and CH4 metabolism paradigm to specific microbial lineages which are shaped related to developmental stages of the dairy cattle, directing influencing global warming from livestock and waste treatment.

Understanding the source of methane (CH4) is of great significance for improving the anaerobic fermentation efficiency in bioengineering, and for mitigating the emission potential of natural ecosystems. Microbes involved in the process named direct interspecies electron transfer coupling with CO2 reduction, i.e., electrons released from electroactive bacteria to reduce CO2 into CH4, have attracted considerable attention for wastewater treatment in the past decade. However, how the synergistic effect of microbiota contributes to this anaerobic carbon metabolism accompanied by CH4 production still remains poorly understood, especial for wastewater with antibiotic exposure. Results show that enhancing lower-abundant acetoclastic methanogens and acetogenic bacteria, rather than electroactive bacteria, contributed to CH4 production, based on a metagenome-assembled genomes network analysis. Natural and artificial isotope tracing of CH4 further confirmed that CH4 mainly originated from acetoclastic methanogenesis. These findings reveal the contribution of direct acetate cleavage (acetoclastic methanogenesis) and provide insightsfor further regulation of methanogenic strategies.

. The nitrogen stable isotope composition (δ 15 N) of nitrogen oxides (NO x ) is a powerful 20 indicator for source apportionment of atmospheric NO x ; however, δ 15 N–NO x values emitted from ships have not been reported, affecting the accuracy of source partitioning of atmospheric NO x in coastal zones with a lot of ocean vessel activity. This study systemically analyzed the δ 15 N–NO x variability and main influencing factors of ship emissions. Results showed that δ 15 N–NO x values from ships ranged from −35.8‰ to 2.04‰ with a mean ± standard deviation of −18.5 ± 10.9‰. The δ 15 N–NO x values increased 25 monotonically with the ongoing tightening of emission regulations, presenting a significantly negative logarithmic relationship with NO x concentrations (p < 0.01). The selective catalytic reduction (SCR) system was the most important factor affecting changes in δ 15 N–NO x values, compared with fuel types and operation states of ships. Based on the relationship between δ 15 N–NO x values and emission regulations observed in this investigation, the temporal variation in δ 15 N–NO x values from ship emissions 30 in the international merchant fleet was evaluated by developing a mass-weighted model. These simulated δ 15 N–NO x values can be used to select suitable δ 15 N–NO x values for a more accurate assessment of the contribution of ship-emitted exhaust to atmospheric NO x .

Abstract. The nitrogen stable isotope composition (δ15N) of nitrogen oxides (NOx) is a powerful indicator for source apportionment of atmospheric NOx; however, δ15N–NOx values emitted from ships have not been reported, affecting the accuracy of source partitioning of atmospheric NOx in coastal zones with a lot of ocean vessel activity. This study systemically analyzed the δ15N–NOx variability and main influencing factors of ship emissions. Results showed that δ15N–NOx values from ships ranged from −35.8 ‰ to 2.04 ‰ with a mean ± standard deviation of −18.5 ± 10.9 ‰. The δ15N–NOx values increased monotonically with the ongoing tightening of emission regulations, presenting a significantly negative logarithmic relationship with NOx concentrations (p < 0.01). The selective catalytic reduction (SCR) system was the most important factor affecting changes in δ15N–NOx values, compared with fuel types and operation states of ships. Based on the relationship between δ15N–NOx values and emission regulations observed in this investigation, the temporal variation in δ15N–NOx values from ship emissions in the international merchant fleet was evaluated by developing a mass-weighted model. These simulated δ15N–NOx values can be used to select suitable δ15N–NOx values for a more accurate assessment of the contribution of ship-emitted exhaust to atmospheric NOx.

The nitrogen stable isotope composition (δ15N) of nitrogen oxides (NOx) is a powerful indicator of source apportionment of atmospheric NOx; however, δ15N–NOx values emitted from ships have not been reported, affecting the accuracy of source partitioning of atmospheric NOx in coastal zones with a lot of vessel activity. In addition, δ15N–NOx values from ship emissions could also be important for source apportionment of atmospheric nitrogen deposition in remote ocean regions. This study systemically analysed the δ15N–NOx variability and main influencing factors of ship emissions. The results showed that δ15N–NOx values from ships, which were calculated by weighting the emission values from the main engine and auxiliary engine of the vessel, ranged from −35.8 ‰ to 2.04 ‰ with a mean ± standard deviation of −18.5 ± 10.9 ‰. The δ15N–NOx values increased monotonically with the ongoing tightening of emission regulations, presenting a significantly negative logarithmic relationship with NOx concentrations (p<0.01). The selective catalytic reduction (SCR) system was the most important factor affecting changes in δ15N–NOx values, followed by the ship category, fuel types, and operation states of ships. Based on the relationship between δ15N–NOx values and emission regulations observed in this investigation, a mass-weighted model to compute accurate assessments over time was developed, and the temporal variation in δ15N–NOx values from ship emissions in the international merchant fleet was evaluated. These simulated δ15N–NOx values can be used to select suitable δ15N–NOx values for a more accurate assessment, including the contribution of ship-emitted exhaust to atmospheric NOx and its influence on atmospheric nitrate (NO3-) air quality and nitrogen deposition studies.

Porous CuO/CeO2 nanospheres are synthesized by a facile route, which involves a template-free solvothermal method to obtain porous CeO2 nanospheres, followed by a deposition–precipitation (DP) method of as-prepared porous CeO2 nanospheres and Cu(NO3)2 solution. XRD, TEM, SEM, ICP-MS, and EDS elemental mapping confirm that the obtained porous CuO/CeO2 nanospheres are composed of CeO2 nanospheres decorated with small particle size and well-dispersed CuO nanoparticles (NPs). Meanwhile, the activity of CO catalytic oxidation study is tested; porous CuO/CeO2 nanospheres exhibit signally higher activity than undecorated CeO2 nanospheres and solid CuO/CeO2 nanospheres at low temperature. The improved performance of porous CuO/CeO2 nanospheres is possibly ascribed to the synergetic effect and strong interaction between porous CeO2 nanospheres and decorated CuO NPs.

Expanding worldwide aquaculture has greatly increased greenhouse gas emissions; however, the underlying microbial mechanisms are poorly understood. In particular, the role of ferric iron [Fe(III)] (hydro)oxides in carbon mineralization in aquaculture pond sediments remains unclear. Here, we studied the rates of microbial Fe(III) reduction, sulfate reduction, methanogenesis, and carbon mineralization in aquaculture shrimp (Litopenaeus vannamei) ponds of various salinities before, during, and after shrimp farming in subtropical estuaries in southeast China. Sediment samples (0–10 cm) were collected to investigate the content of iron species, characteristics of organic matter, and abundance of Geobacter, a proxy of iron reducers. Overall, Fe(III) reduction (46.1% ± 19.1%) dominated carbon mineralization, followed by sulfate reduction (39.6% ± 16.8%) and methanogenesis (1.5% ± 1.1%). Microbial Fe(III) reduction contributed more to carbon mineralization during farming than before and after farming. This enhancement in Fe(III) reduction is attributed to a significant increase in Fe(III) content during farming. Additionally, the contributions of microbial Fe(III) reduction to carbon mineralization were lower in the high‐salinity ponds than in the low‐salinity ponds due to the suppression of sulfate reduction, abiotic Fe(III) reduction by sulfides, and lower oxidation‐reduction potential. Our findings demonstrate that microbial Fe(III) reduction plays a significant role in carbon mineralization in aquaculture pond sediments. Future carbon flux prediction models of aquaculture pond systems should fully integrate microbial Fe(III) reduction.

Although salinization is widely known to affect cycling of soil carbon (C) in tidal freshwater wetlands, the role of the presence or absence of plants in mediating the responses of soil organic carbon (SOC) mineralization to salinization is poorly understood. In this study, we translocated soils collected from a tidal freshwater wetland to sites with varying salinities along a subtropical estuarine gradient and established unplanted and planted (with the salt-tolerant plant Cyperus malaccensis Lam.) mesocosms at each site. We simultaneously investigated cumulative soil CO2 emissions, C-acquiring enzyme activities, availability of labile organic C (LOC), and structures of bacterial and fungal communities. Overall, in the planted mesocosm, the soil LOC content and the activities of β-1,4-glucosidase, cellobiohydrolase, phenol oxidase, and peroxidase increased with salinization. However, in the unplanted mesocosm, soil LOC content decreased with increasing salinity, whereas all the C-acquiring enzyme activities did not change. In addition, salinization favored the dominance of bacterial and fungal copiotrophs (e.g., γ-Proteobacteria, Bacteroidetes, Firmicutes, and Ascomycota) in the planted mesocosms. Contrarily, in the unplanted mesocosms salinization favored bacterial and fungal oligotrophs (e.g., α-Proteobacteria, Chloroflexi, Acidobacteria, and Basidiomycota). In both planted and unplanted mesocosms, cumulative soil CO2 emissions were affected by soil LOC content, activities of C-acquiring enzymes, and microbial C-use trophic strategies. Overall, cumulative soil CO2 emissions increased by 35% with increasing salinity in the planted mesocosm but decreased by 37% as salinity increased in the unplanted mesocosm. Our results demonstrate that the presence or absence of salt-tolerant plants can moderate the effect of salinity on SOC mineralization in tidal wetland soils. Future C prediction models should embed both planted and unplanted modules to accurately simulate cycling of soil C in tidal wetlands under sea level rise.

Atmospheric nitrate (NO3−) pollution has become an obstacle to efforts to further reduce fine particulate (PM2.5) concentration in North China. However, there have been limited long‐term measurements of NO3− and isotopic knowledge (δ15N, δ18O) on the driving factors during NO3− changes. Here, we report observations of 10 voyages from 2014 to 2019 conducted in the Bohai Sea, a typical background area in North China. The results show that the average proportion of NO3− in PM2.5 increased from 0.08 to 0.16 over the study period. The δ15N–NO3− ranged from −4.1‰ to +20.5‰, with a significant annual decline (p < 0.01), especially in winter. The average δ18O–NO3− was +72.6 ± 13.5‰, and a Monte Carlo calculation revealed that the contribution of the •OH pathway in the NO3− formation declined by 27.4% in winter, implying an increase in O3 pollution. Coal combustion remained the most important contributor to NO3− (46.6 ± 15.9%), but its contribution showed a significant downward trend (p < 0.01), consistent with the control of disperse coal use in North China. Enhancement of atmospheric oxidation and the unexpected large increase in contribution of microbial processes were found to be the main causes of the increasingly serious NO3− pollution in North China. In addition, a spike in the contribution of coal combustion in 2018 indicates that the coal‐control policy needs to be reinforced.

Microbial fuel cells (MFCs) are sensitive to acidity variations in both bioelectricity generation and biochemical digestion aspects, therefore online pH monitoring is of necessity to guarantee optimal function of MFCs. Present pH meters hardly fulfill this special need. In this work, we designed a novel voltammetric pH sensor based on electrochemically reduced graphene oxide (rGO) modified screen printed electrode. By surface doping of alizarin, good linearity of pH sensing over the range of 4.0-9.0 can be realized. Fast readout can be acquired within 15 s for each test. pH monitoring for artificial wastewater with inoculum of granular activated sludge in a MFC was successfully illustrated. Specially, it was verified that the performance was improved with alizarin doping due to the enhanced rGO surface proton diffusion. This approach provides an online, calibration-free and long stable pH monitoring method for the future MFC development.

Excessive nitrate (NO3-) in rivers can lead to water quality deterioration, and can also be directly input into estuaries and oceans, thus posing a serious threat to the stability of their ecosystems. In this study, the concentration, isotopes and sources of NO3- in 30 rivers discharging into the Bohai Sea were comprehensively investigated. The mean concentration of NO3--N was 2.24 ± 2.11 mg L-1, with obvious seasonal and spatial variations. In total, 104.24 kt of NO3--N was discharged into the Bohai Sea annually, to which the Yellow River Basin and Liao River Basin made the largest contributions. The range of δ15N-NO3- was -1.1‰ to +33.2‰ (mean value, +11.4 ± 5.0‰), with no significant seasonal or spatial differences; the mean value of δ18O-NO3- was +9.4 ± 7.2‰, with much higher values seen in June. Based on the MixSIAR model, manure (24.3 ± 7.5%) and sewage (19.1 ± 14.5%) were the primary sources of NO3- in the 30 rivers, followed by NO3- fertilizers (16.3 ± 12.5%), soil N (15.5 ± 11.9%), atmospheric deposition of NO3- (13.5 ± 5.7%) and NH4+ fertilizers (11.4 ± 8.9%). This finding highlights the vital roles of sewage and manure management in riverine NO3-. Using a mathematical method, the contributions of various sources to each river were simulated. The results indicated that management of the Yellow River, Daliao River, Liao River, and Xiaoqing River is more urgently needed than that of other rivers to control Bohai NO3- pollution. We believe that this finding will provide guidance for scientific management of NO3- pollution in these 30 rivers and the Bohai Sea.

Direct interspecies electron transfer (DIET) between electricigens and methanogens has been shown to favour CO2 reduction to produce biomethane. Furthermore, DIET is accelerated by conductive materials. However, whether conductive materials can promote other methanogenic pathways is unclear due to a lack of detailed experimental data and the poor mechanistic studies. Here, we hypothesized that conductive carbon nanotubes (CNTs) stimulate acetoclastic methanogenesis independently of electricigens in pure cultures of Methanosarcina spp. and anaerobic wetland soil. We found a significant increase in the methane production rate during the growth phase, e.g. from 0.169 mM to 0.241 mM after addition of CNTs on the 3rd day. CNTs did not increase the abundance of electromicroorganisms or the electron transfer rate in anaerobic soils, using via microbial diversity and electrochemical analysis. C-13-CH3COOH labelling, stable carbon isotope fractionation and the CH3F inhibitor of acetoclastic methanogenesis were used to distinguish methanogenic pathways. CNTs mainly accelerated acetoclastic methanogenesis rather than CO2 reduction in both pure cultures and anaerobic soils. Furthermore, the presence of CNTs slightly alleviate the inhibition of CH3F on acetoclastic methanogenesis during the pure culture of Methanosarcina barkeri and Methanosarcina mazei with the production of more than 0.3 mM methane. CNTs closely attached to the cell surface were observed by transmission electron microscopy. Proteome analysis revealed a stimulation of protein synthesis with about twice the improvement involved in -COOH oxidation and electron transfer. Overall, our findings demonstrate that conducting CNTs favor methane production and that the mechanism involved is acetoclastic methanogenesis via acetate dismutation, at least partly, rather than classical CO2 reduction.

In recent years, emergent pollutant’s accidents have occurred frequently in China, causing serious harm to the ecological environment. In this study, the impact of an accidental fire and explosion at Tianjin Port in 2015 on the atmosphere over the Bohai Sea was explored. Results showed sharp increases in the concentrations of several important components of fine particulate matter (e.g. NO3−, SO42−, NH4+, organic carbon, elemental carbon) over Beihuangcheng Island after the explosion. Among them, NO3− was most affected (about 10 days), with a maximum concentration of 16.45 μg m−3. The δ15N-NO3− ranged from −1.58‰ to +8.74‰, with an average of +2.79‰ ± 3.32‰. Influenced by the explosion, δ15N-NO3− decreased significantly, which was in accordance with the industrial processes of explosives. The δ18O-NO3− varied between +49.40‰ and +69.52‰, and showed a marked increase (+66.62‰ ± 3.92‰) in the explosion-affected period. Using Monte Carlo simulation, the •OH pathway for NO3− formation was 51.79% ± 10.94% at that time — much lower than in the regular period. The elevated dry deposition of NO3− caused by the explosion was 266.08 μmol N m−2 d−1 over the Bohai Sea — again, much higher than in the regular period. With the dry nitrogen deposition of NH4+ (42.41 μmol N m−2 d−1), the total nitrogen deposition increased by 308.49 μmol N m−2 d−1, leading to severe ecological risk. Through the inverse computation of the dry deposition flux of NO3−, the affected area over the Bohai Sea was less than 1.42 × 104 km2, which is about 20% of the total area.

In China, nitrate (NO3-) becomes the main contributor to fine particles (PM2.5) because the emissions of its precursor, nitrogen oxides (NOx), were not recognized and controlled well in recent years. In this work, sources, conversion, and geographical origin of NOx were interpreted combining the isotopic information (delta N-15 and delta O-18) of NO3- and dual modelling at five Chinese megacities (Beijing, Shanghai, Guangzhou, Wuhan and Chengdu) during 2013-2014. Results showed that the delta N-15-NO3- values (n = 512) ranged from -12.3 parts per thousand to +22.9 parts per thousand, and the average delta O-18-NO3- value was +83.4 parts per thousand +/- 17.2 parts per thousand. The isotopic compositions both had a rising tendency as ambient temperature dropped, attributing largely to the source changes. Bayesian model indicated the percentage for the center dot OH pathway of NOx conversion had a clear seasonal variation with a higher value during summer (58.0% +/- 9.82%) and a lower value during winter (11.1% +/- 3.99%); it was also significantly correlated with latitude (p < 0.01). Coal combustion was the most important source of NOx (31.1%-41.0%), which was geographically derived from North China and other south-central developed regions implied by Potential Source Contribution Function (PSCF). Apart from Chengdu, mobile sources was the second largest contributor to NOx. This source was extensive but uniformly distributed all around the typical urban agglomerations of China. Biomass burning and microbial processes shared similar source areas, mostly originating from the North China Plain and Sichuan Basin. Based on the NOx features, we infer that residential coal combustion was the primary source of heavy PM2.5 pollution in Chinese megacities. Controlling the source categories of these regional priorities would help mitigate atmospheric pollution in these areas.