Simultaneous methane mitigation and nitrogen removal by denitrifying anaerobic methane oxidation in lake sediments.

Methane (CH4) is a potent greenhouse gas, with lake ecosystems significantly contributing to its global emissions. Denitrifying anaerobic methane oxidation (DAMO) process, mediated by NC10 bacteria and ANME-2d archaea, links global carbon and nitrogen cycles. However, their potential roles in mitigating methane emissions and removing nitrogen from lake ecosystems remain unclear. This study explored the spatial variations in activities of nitrite- and nitrate-DAMO and their functional microbes in Changdanghu Lake sediments (Jiangsu Province, China). The results showed that although the average abundance of ANME-2d archaea (5.0 × 106 copies g-1) was significantly higher than that of NC10 bacteria (2.1 × 106 copies g-1), the average potential rates of nitrite-DAMO (4.59 nmol 13CO2 g-1 d-1) and nitrate-DAMO (5.01 nmol 13CO2 g-1 d-1) showed no significant difference across all sampling sites. It is estimated that nitrite- and nitrate-DAMO consumed approximately 6.46 and 7.05 mg CH4 m-2 d-1, respectively, which accordingly achieved 15.07-24.95 mg m-2 d-1 nitrogen removal from the studied lake sediments. Statistical analyses found that nitrite- and nitrate-DAMO activities were both significantly related to sediment nitrate contents and ANME-2d archaeal abundance. In addition, NC10 bacterial and ANME-2d archaeal community compositions showed significant correlations with sediment organic carbon content and water depth. Overall, this study underscores the dual roles of nitrite- and nitrate-DAMO processes in CH4 mitigation and nitrogen elimination and their key environmental impact factors (sediment organic carbon and inorganic nitrogen contents, and water depth) in shallow lake, enhancing the understanding of carbon and nitrogen cycles in freshwater aquatic ecosystems.

Comprehensive evaluation of factors influencing selenium fertilization biofortification.

BACKGROUND: Dietary selenium (Se) deficiency, stemming from low Se concentrations in agricultural products, threatens human health. While Se-containing fertilizers can enhance the Se content in crops, the key factors governing Se biofortification with Se fertilization remain unclear. RESULTS: This study constructed a global meta-analysis dataset based on field experiments comprising 364 entries on Se content in agricultural products and 271 entries on their yield. Random forest models and mixed effects meta-analyses revealed that plant types (i.e., cereals, vegetables, legumes, and forages) primarily influenced Se biofortification, with Se fertilization rates being the next significant factor. The random forest model, which included variables like plant types, Se fertilization rates, methods and types of Se application, initial soil conditions (including Se content, organic carbon content, and pH), soil types, mean annual precipitation, and temperature, explained 82.14% of the variation in Se content and 48.42% of the yield variation in agricultural products. For the same agricultural products, the increase in Se content decreased with higher rates of Se fertilization. The increase in Se content in their edible parts will be negligible for cereals, forages, legumes, and vegetable crops, when Se fertilization rates were 164, 103, 144, and 147 g Se ha-1 , respectively. Conversely, while low Se fertilization rates enhanced yields, high rates led to a yield reduction, particularly in cereals. CONCLUSION: Our findings highlight the need for balanced and precise Se fertilization strategies to optimize Se biofortification benefits and minimize the risk of yield reduction. © 2024 Society of Chemical Industry.

Vertical distribution of Candidatus Methylomirabilis and Methanoperedens in agricultural soils.

Candidatus Methylomirabilis-related bacteria conduct anaerobic oxidation of methane (AOM) coupling with NO2- reduction, and Candidatus Methanoperedens-related archaea perform AOM coupling with reduction of diverse electron acceptors, including NO3-, Fe (III), Mn (IV) and SO42-. Application of nitrogen fertilization favors the growth of these methanotrophs in agricultural fields. Here, we explored the vertical variations in community structure and abundance of the two groups of methanotrophs in a nitrogen-rich vegetable field via using illumina MiSeq sequencing and quantitative PCR. The retrieved Methylomirabilis-related sequences had 91.12%-97.32% identity to the genomes of known Methylomirabilis species, and Methanoperedens-related sequences showed 85.49%-97.48% identity to the genomes of known Methanoperedens species which are capable of conducting AOM coupling with reduction of NO3- or Fe (III). The Methanoperedens-related archaeal diversity was significantly higher than Methylomirabilis-related bacteria, with totally 74 and 16 operational taxonomic units, respectively. In contrast, no significant difference in abundance between the bacteria (9.19 × 103-3.83 × 105 copies g-1 dry soil) and the archaea (1.55 × 104-3.24 × 105 copies g-1 dry soil) was observed. Furthermore, the abundance of both groups of methanotrophs exhibited a strong vertical variation, which peaked at 30-40 and 20-30 cm layers, respectively. Soil water content and pH were the key factors influencing Methylomirabilis-related bacterial diversity and abundance, respectively. For the Methanoperedens-related archaea, both soil pH and ammonium content contributed significantly to the changes of these archaeal diversity and abundance. Overall, we provide the first insights into the vertical distribution and regulation of Methylomirabilis-related bacteria and Methanoperedens-related archaea in vegetable soils. KEY POINTS: • The archaeal diversity was significantly higher than bacterial. • There was no significant difference in the abundance between bacteria and archaea. • The abundance of bacteria and archaea peaked at 30-40 and 20-30 cm, respectively.

Equal importance of humic acids and nitrate in driving anaerobic oxidation of methane in paddy soils.

Methane (CH4) is both generated and consumed in paddy soils, where anaerobic oxidation of methane (AOM) serves as a crucial process for mitigating CH4 emissions. Although the participation of humic acids (HA) and nitrate in AOM has been recognized, their relative roles and significance in paddy soils remain insufficiently investigated. In this study, we explored the potential activity of AOM driven by HA and nitrate, as well as the composition of archaeal communities in paddy soils across different rice growth periods and fertilization treatments. AOM activity ranged from 0.81 to 1.33 and 1.26 to 2.38 nmol of 13CO2 g-1 (dry soil) day-1 with HA and nitrate, respectively. No significant differences (p < 0.05) were observed between the AOM activity driven by HA and nitrate across the three fertilization treatments. According to AOM activity, the annual consumption of CH4 was estimated at approximately 0.49 ± 0.06 and 0.83 ± 0.19 Tg for AOM processes driven by HA and nitrate in Chinese paddy soils. Nitrate-driven AOM activity exhibited a positive (p < 0.05) correlation with the abundance of the ANME-2d mcrA gene but a negative (p < 0.05) correlation with the content of dissolved organic carbon. Intriguingly, HA-driven AOM activity was only correlated positively with the nitrate-driven AOM activity. Soil water content, soil organic carbon, nitrate and nitrite contents were significantly correlated with the relative abundance of methanogenic and methanotrophic archaea. These results identified the potential importance of HA and nitrate in driving AOM processes within paddy soils, providing a comprehensive understanding of the complex microbial processes regulating greenhouse gas emissions from paddy soils.

Nitrogen fertilization rate affects communities of ammonia-oxidizing archaea and bacteria in paddy soils across different climatic zones of China.

Nitrogen fertilization has important effects on nitrification. However, how the rate of nitrogen fertilization affects nitrification potential, as well as the communities of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB), remains unclear. We performed a large-scale investigation of nitrification potential and ammonia-oxidizer communities in Chinese paddy fields at different nitrogen fertilization rates across different climatic zones. It was found that the nitrification potential at the high nitrogen fertilization rate (≥150 kg-1 N ha-1) was 23.35 % higher than that at the intermediate rate (100-150 kg-1 N ha-1) and 20.77 % higher than that at the low rate (< 100 kg-1 N ha-1). The nitrification potential showed no significant variation among different nitrogen fertilization rates across climatic zones. Furthermore, the AOA and AOB amoA gene abundance at the high nitrogen fertilization rate was 481.67 % and 292.74 % higher (p < 0.05) than that at the intermediate rate, respectively. Correlation analysis demonstrated a significant positive correlation between AOB abundance and nitrification potential. AOA and AOB community composition differed significantly among nitrogen fertilization rates. Moreover, soil NH4+ content, pH, water content, bulk density, and annual average temperature were regarded as key environmental factors influencing the community structure of ammonia-oxidizers. Taken together, the nitrogen fertilization rate had a significant impact on the communities of AOA and AOB but did not significantly alter the nitrification potential. Our findings provide new insights into the impact of nitrogen fertilization management on nitrification in rice paddy fields.

Differences in Gut Microbiota between Healthy Individuals and Patients with Perianal Abscess before and after Surgery.

Surgery is the most important treatment for perianal abscesses. However, the gut microbiota of patients with perianal abscess and the effects of perianal abscess on the gut microbiota after surgery are unknown. In this study, significant changes in interleukin 6 and tumor necrosis factor-α in the blood of healthy subjects, patients with perianal abscesses, and patients after perianal abscess surgery were identified. 16S rRNA gene sequencing technology was used to detect the changes in the gut microbiota among 30 healthy individuals and 30 patients with perianal abscess before and after surgery. Venn diagrams and alpha diversity analyses indicated differences in the abundance and uniformity of gut microbiota between the healthy individuals and patients with perianal abscesses before and after surgery. Beta diversity analysis indicated that the grouping effects among the control, abscess, and surgery groups were good. The classification and compositional analysis showed significant differences in the gut microbiota between healthy individuals and patients with perianal abscesses before and after surgery. LEfSe analysis, random forest analysis, and ROC curve analysis showed that Klebsiella (AUC = 0.7467) and Bilophila (AUC = 0.72) could be potential biomarkers for the diagnosis of perianal abscess. The functional prediction results showed that the differential microbiota is significantly enriched in the pathways related to nutrition and drug metabolism. This study may have important implications for the clinical management and prognostic assessment of patients with perianal abscesses.

Spatio-temporal variations of activity of nitrate-driven anaerobic oxidation of methane and community structure of Candidatus Methanoperedens-like archaea in sediment of Wuxijiang river.

Nitrate-driven anaerobic oxidation of methane (AOM), catalyzing by Candidatus Methanoperedens-like archaea, is a new addition in the global CH4 cycle. This AOM process acts as a novel pathway for CH4 emission reduction in freshwater aquatic ecosystems; however, its quantitative importance and regulatory factors in riverine ecosystems are nearly unknown. Here, we examined the spatio-temporal changes of the communities of Methanoperedens-like archaea and nitrate-driven AOM activity in sediment of Wuxijiang River, a mountainous river in China. These archaeal community composition varied significantly among reaches (upper, middle, and lower reaches) and between seasons (winter and summer), but their mcrA gene diversity showed no significant spatial or temporal variations. The copy numbers of Methanoperedens-like archaeal mcrA genes were 1.32 × 105-2.47 × 107 copies g-1 (dry weight), and the activity of nitrate-driven AOM was 0.25-1.73 nmol CH4 g-1 (dry weight) d-1, which could potentially reduce 10.3% of CH4 emissions from rivers. Significant spatio-temporal variations of mcrA gene abundance and nitrate-driven AOM activity were found. Both the gene abundance and activity increased significantly from upper to lower reaches in both seasons, and were significantly higher in sediment collected in summer than in winter. In addition, the variations of Methanoperedens-like archaeal communities and nitrate-driven AOM activity were largely impacted by the sediment temperature, NH4+ and organic carbon contents. Taken together, both time and space scales need to be considered for better evaluating the quantitative importance of nitrate-driven AOM in reducing CH4 emissions from riverine ecosystems.

Effects of abrupt and gradual increase of atmospheric CO2 concentration on methanotrophs in paddy fields.

The simulation of abrupt atmospheric CO2 increase is a common way to examine the response of soil methanotrophs to future climate change. However, atmosphere is undergoing a gradual CO2 increase, and it is unknown whether the previously reported response of methanotrophs to abrupt CO2 increase can well represent their response to the gradual increase. To improve the understanding of the effect of elevated CO2 (eCO2) on methanotrophs in paddy ecosystems, the methane oxidation potential and communities of methanotrophs were examined via open top chambers under the three following CO2 treatments: an ambient CO2 concentration (AC); an abrupt CO2 increase by 200 ppm above AC (AI); a gradual CO2 increase by 40 ppm each year until 200 ppm above AC (GI). Relative to AC treatment, AI and GI treatments significantly (p < 0.05) increased the methane oxidation rate by 43.8% and 36.7%, respectively, during rice growth period. Furthermore, the abundance of pmoA genes was significantly (p < 0.05) increased by 62.4% and 32.5%, respectively, under AI and GI treatments. However, there were no significant variations in oxidation rate or gene abundance between the two eCO2 treatments. In addition, no obvious change of overall community composition of methanotrophs was observed among treatments, while the proportions of Methylosarcina and Methylocystis significantly (p < 0.05) changed. Taken together, our results indicate similar response of methanotrophs to abrupt and gradual CO2 increase, although the magnitude of response under gradual increase was smaller and the abrupt increase may somewhat overestimate the response.

Characteristics of soil N2O emission and N2O-producing microbial communities in paddy fields under elevated CO2 concentrations.

The effects of elevated carbon dioxide (CO2) concentration (e[CO2]) on nitrous oxide (N2O) emissions from paddy fields and the microbial processes involved in N2O emissions have recently received much attention. Ammonia-oxidizing microorganisms and denitrifying bacteria dominate the production of N2O in paddy soils. To better understand the dynamics of N2O production under e[CO2], a field experiment was conducted after five years of CO2 fumigation based on three treatments: CK (ambient atmospheric CO2), T1 (CK + increase of 40 ppm per year until 200 ppm), and T2 (CK + 200 ppm). N2O fluxes, soil physicochemical properties, and N2O production potential were quantified during the rice-growth period. The functional gene abundance and community composition of ammonia-oxidizing archaea (AOA) and bacteria (AOB) were analyzed using quantitative polymerase chain reaction (qPCR) and those of ammonia-denitrifying bacteria (nirS- and nirK-type) were analyzed using Illumina MiSeq sequencing. N2O emissions decreased by 173% and 41% under the two e[CO2] treatments during grain filling and milk ripening, respectively (P < 0.05). N2O emissions increased by 279% and 172% in the T2 treatment compared with T1 during the tillering and milk-ripening stages, respectively (P < 0.05). Furthermore, the N2O production potential was significantly higher in the CK treatment than in T1 and T2 during the elongation stage. The N2O production potential and abundance of AOA amoA genes in T1 treatment were significantly lower than those in CK treatment during the high N2O emission phase caused by mid-season drainage (P < 0.05). Although nirK- and nirS-type denitrifying bacteria community structure and diversity did not respond significantly (P > 0.05) to e[CO2], the abundance of nirK-type denitrifying bacteria significantly affected the N2O flux (P < 0.05). Linear regression analysis showed that the N2O production potential, AOA amoA gene abundance, and nirK gene abundance explained 47.2% of the variation in N2O emissions. In addition, soil nitrogen (N) significantly affected the nirK- and nirS-type denitrifier communities. Overall, our results revealed that e[CO2] suppressed N2O emissions, which was closely associated with the abundance of AOA amoA and nirK genes (P < 0.05).

Role and regulation of anaerobic methane oxidation catalyzed by NC10 bacteria and ANME-2d archaea in various ecosystems.

Freshwater wetlands, paddy fields, inland aquatic ecosystems and coastal wetlands are recognized as important sources of atmospheric methane (CH4). Currently, increasing evidence shows the potential importance of the anaerobic oxidation of methane (AOM) mediated by NC10 bacteria and a novel cluster of anaerobic methanotrophic archaea (ANME)-ANME-2d in mitigating CH4 emissions from different ecosystems. To better understand the role of NC10 bacteria and ANME-2d archaea in CH4 emission reduction, the current review systematically summarizes different AOM processes and the functional microorganisms involved in freshwater wetlands, paddy fields, inland aquatic ecosystems and coastal wetlands. NC10 bacteria are widely present in these ecosystems, and the nitrite-dependent AOM is identified as an important CH4 sink and induces nitrogen loss. Nitrite- and nitrate-dependent AOM co-occur in the environment, and they are mainly affected by soil/sediment inorganic nitrogen and organic carbon contents. Furthermore, salinity is another key factor regulating the two AOM processes in coastal wetlands. In addition, ANME-2d archaea have the great potential to couple AOM to the reduction of iron (III), manganese (IV), sulfate, and even humics in different ecosystems. However, the study on the environmental distribution of ANME-2d archaea and their role in CH4 mitigation in environments is insufficient. In this study, we propose several directions for future research on the different AOM processes and respective functional microorganisms.

Effect of gradual increase of atmospheric CO2 concentration on nitrification potential and communities of ammonia oxidizers in paddy fields.

Currently, the influence of elevated atmospheric CO2 concentration (eCO2) on ammonia oxidation to nitrite, the rate-limiting step of nitrification in paddy soil, is poorly known. Previous studies that simulate the effect of eCO2 on nitrification are primarily based on an abrupt increase of atmospheric CO2 concentration. However, paddy ecosystems are experiencing a gradual increase of CO2 concentration. To better understand how the nitrification potential, abundance and communities of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) respond to eCO2 in paddy ecosystems, a field experiment was conducted using the following two treatments: a gradual increase of CO2 (EC, increase of 40 ppm per year until 200 ppm above ambient) and ambient CO2 (CK). The results demonstrated that the EC treatment significantly (P < 0.05) stimulated the soil potential nitrification rate (PNR) at the jointing and milky stages, which increased by 127.83% and 27.35%, respectively, compared with CK. Furthermore, the EC treatment significantly (P < 0.05) stimulated the AOA and AOB abundance by 56.60% and 133.84%, respectively, at the jointing stage. Correlation analysis showed that the PNR correlated well with the abundance of AOB (R2 = 0.7389, P < 0.001). In addition, the EC treatment significantly (P < 0.05) altered the community structure of AOB, while it had little effect on that of AOA. A significant difference in the proportion of Nitrosospira was observed between CO2 treatments. In conclusion, the gradual increase of CO2 positively influenced the PNR and abundance of ammonia oxidizers, and AOB could be more important than AOA in nitrification under eCO2.

Detection and Quantification of Candidatus Methanoperedens-Like Archaea in Freshwater Wetland Soils.

Candidatus Methanoperedens-like archaea, which can use multiple electron acceptors (nitrate, iron, manganese, and sulfate) for anaerobic methane oxidation, could play an important role in reducing methane emissions from freshwater wetlands. Currently, very little is known about the distribution and community composition of Methanoperedens-like archaea in freshwater wetlands, particularly based on their alpha subunit of methyl-coenzyme M reductase (mcrA) genes. Here, the community composition, diversity, and abundance of Methanoperedens-like archaea were investigated in a freshwater wetland through high-throughput sequencing and quantitative PCR on their mcrA genes. A large number of Methanoperedens-like mcrA gene sequences (119,250) were recovered, and a total of 31 operational taxonomic units (OTUs) were generated based on 95% sequence similarity cut-off. The majority of Methanoperedens-like sequences can be grouped into three distinct clusters that were closely associated with the known Methanoperedens species which can couple anaerobic methane oxidation to nitrate or iron reduction. The community composition of Methanoperedens-like archaea differed significantly among different sampling sites, and their mcrA gene abundance was 1.49 × 106 ~ 4.62 × 106 copies g-1 dry soil in the examined wetland. In addition, the community composition of Methanoperedens-like archaea was significantly affected by the soil water content, and the archaeal abundance was significantly positively correlated with the water content. Our results suggest that the mcrA gene is a good biomarker for detection and quantification of Methanoperedens-like archaea, and provide new insights into the distribution and environmental regulation of these archaea in freshwater wetlands.

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Nitrate-dependent anaerobic oxidation of methane (AOM) is a new pathway to reduce methane emissions from paddy ecosystems. The elevated atmospheric CO2 concentration can affect methane emissions from paddy ecosystems, but its impact on the process of nitrate-dependent AOM is poorly known. Based on the automatic CO2 control platform with open top chambers and the 13CH4 stable isotope experiments, the responses of the activity of nitrate-dependent AOM, abundance and community composition of Candidatus Methanoperedens nitroreducens (M. nitroreducens)-like archaea to the gradual increase of CO2 concentration were investigated in paddy fields. We set up two CO2 concentration treatments, including an ambient CO2 and a gradual increase of CO2(increase of 40 µL·L-1 per year above ambient CO2 concentration until 160 µL·L-1). The results showed the nitrate-dependent AOM rate of 0.7-11.3 nmol CO2·g-1·d-1 in the studied paddy fields, and quantitative PCR showed the abundance of M. nitroreducens-like archaeal mcrA genes of 2.2×106-8.5×106 copies·g-1. Compared to the ambient CO2 treatment, the slow elevated CO2 treatment enhanced the nitrate-dependent AOM rate and stimulated the abundance of M. nitroreducens-like archaea, particularly in 5-10 cm soil layer. The gradual increased CO2 concentration treatment did not change the community composition of M. nitroreducens-like archaea, but significantly decreased their diversity. The soil organic carbon content was an important factor influencing the nitrate-dependent AOM process. Overall, our results showed that the gradual increase of CO2 concentration could promote the nitrate-dependent AOM, suggesting its positive role in mitigating methane emissions from paddy ecosystems under future climate change.

Spatial variations of activity and community structure of nitrite-dependent anaerobic methanotrophs in river sediment.

Rivers are an important site for methane emissions and reactive nitrogen removal. The process of nitrite-dependent anaerobic methane oxidation (n-damo) links the global carbon cycle and the nitrogen cycle, but its role in methane mitigation and nitrogen removal in rivers is poorly known. In the present study, we investigated the activity, abundance, and community composition of n-damo bacteria in sediment of the upper, middle, and lower reaches of Wuxijiang River (Zhejiang Province, China). The 13CH4 stable isotope experiments showed that the methane oxidation activity of n-damo was 0.11-1.88 nmol CO2 g-1 (dry sediment) d-1, and the activity measured from the middle reaches was significantly higher than that from the remaining regions. It was estimated that 3.27 g CH4 m-2 year-1 and 8.72 g N m-2 year-1 could be consumed via n-damo. Quantitative PCR confirmed the presence of n-damo bacteria, and their 16S rRNA gene abundance varied between 5.45 × 105 and 5.86 × 106 copies g-1 dry sediment. Similarly, the abundance of n-damo bacteria was significantly higher in the middle reaches. High-throughput sequencing showed a high n-damo bacterial diversity, with totally 152 operational taxonomic units being detected at 97 % sequence similarity cut-off. In addition, the n-damo bacterial community composition also varied spatially. The inorganic nitrogen (NH4+, NO2-, NO3-) level was found to be the key environmental factor controlling the n-damo activity and bacterial community composition. Overall, our results showed the spatial variations and environmental regulation of the activity and community structure of n-damo bacteria in river sediment, which expanded our understanding of the quantitative importance of n-damo in both methane oxidation and reactive nitrogen removal in riverine systems.

Efficient removal of Cr(VI) at alkaline pHs by sulfite/iodide/UV: Mechanism and modeling.

Efficient removal of toxic hexavalent chromium (Cr(VI)) under alkaline conditions is still a challenge due to the relatively low reactivity of CrO42-. This study proposed a new sulfite/iodide/UV process to remove Cr(VI). The removal of Cr(VI) followed pseudo-zero-order kinetics at alkaline pHs, and was enhanced by sulfite and iodide with synergy. Compared with sulfite/UV, iodide in sulfite/iodide/UV showed about 40 times higher concentration-normalized enhancement for Cr(VI) removal, and reduced the requirement of sulfite ([S(IV)]0/[Cr(VI)]0 of about 2.1:1) by more than 90%. The Cr(VI) removal was accelerated by decreasing pH and by increasing temperature, and was slightly influenced by dissolved oxygen, carbonate, and humic acid. The process was still effective in real surface water and industrial wastewater. Mechanism and pathways of Cr(VI) removal were revealed by quenching experiments, competition kinetic analysis, product identification and quantification, and mass and electron balance. Both eaq- and SO3•- were responsible for Cr(VI) removal, making contributions of about 75% and 25%, respectively. When eaq- mainly reacted with Cr(VI), SO3•- participated in reduction of Cr(V) and Cr(IV) intermediates, with Cr(III), S2O62-, and SO42- as the final products. A model was developed to predict removal kinetics of Cr(VI), and well interpreted the roles of S(IV) and iodide in the process. This study sheds light on mechanism of Cr(VI) removal at alkaline pHs by kinetic modeling, and thus advances the applicability of this promising process for water decontamination.

Response of potential activity, abundance and community composition of nitrite-dependent anaerobic methanotrophs to long-term fertilization in paddy soils.

The process of nitrite-dependent anaerobic methane oxidation (n-damo) catalysed by Candidatus Methylomirabilis oxyfera (M. oxyfera)-like bacteria is a novel pathway in regulating methane (CH4 ) emissions from paddy fields. Nitrogen fertilization is essential to improve rice yields and soil fertility; however, its effect on the n-damo process is largely unknown. Here, the potential n-damo activity, abundance and community composition of M. oxyfera-like bacteria were investigated in paddy fields under three long-term (32 years) fertilization treatments, i.e. unfertilized control (CK), chemical fertilization (NPK) and straw incorporation with chemical fertilization (SNPK). Relative to the CK, both NPK and SNPK treatments significantly (p < 0.05) increased the potential n-damo activity (88%-110%) and the abundance (52%-105%) of M. oxyfera-like bacteria. The variation of soil organic carbon (OrgC) content and inorganic nitrogen content caused by the input of chemical fertilizers and straw returning were identified as the key factors affecting the potential n-damo activity and the abundance of M. oxyfera-like bacteria. However, the community composition and diversity of M. oxyfera-like bacteria did not change significantly by the input of fertilizers. Overall, our results provide the first evidence that long-term fertilization greatly stimulates the n-damo process, indicating its active role in controlling CH4 emissions from paddy fields.

Fabrication of 3D self-supported MoS2-Co-P/nickel foam electrode for adsorption-electrochemical removal of Cr(â ¥).

Electrochemistry is considered to be one of the most efficient and environment-friendly methods for removing highly toxic Cr (Ⅵ). In this study, a 3D self-supported MoS2-Co-P/nickel foam (NF) electrode was prepared via a calcination-hydrothermal process to remove the Cr (Ⅵ) in aqueous medium. Scanning electron microscope (SEM) analysis indicated that the pine-needle-like Co2P nanoneedle and flower-like MoS2 nanosheets were successfully loaded on the three-dimensional (3D) framework of NF, which provided abundant active sites. The electrode modified by Co, P and MoS2 exhibited high removal efficiency of Cr (Ⅵ) (96.9%) at pH 3.0, current of 0.128 mA and voltage of 2.5 V. Co, P and MoS2 have the synergistic promotion on the catalytic performance of electrodes, and the reduction efficiency of Cr (Ⅵ) was greatly improved by 127.5 times relative to pure NF. The enhanced removal of Cr (Ⅵ) was related to the coupling effect of adsorption and electrocatalytic reduction. The mechanism study indicated that electron (e-) is the active species of Cr (Ⅵ) reduction. The Cr (Ⅵ) removal rate was maintained at 90 ± 1% after five successive cycle experiments, demonstrating good stability and potential industrial applications of MoS2-Co-P/NF.

Potential role of nitrite-dependent anaerobic methane oxidation in methane consumption and nitrogen removal in Chinese paddy fields.

Nitrite-dependent anaerobic methane oxidation (n-damo), catalyzed by bacteria closely related to Candidatus Methylomirabilis oxyfera, links the global carbon and nitrogen cycles. Currently, the contribution of n-damo in controlling methane emissions and nitrogen removal, and the key regulatory factors of this process in Chinese paddy fields are poorly known. Here, soil samples from 20 paddy fields located in different climate zones across China were collected to examine the n-damo activity and bacterial communities. The n-damo activity and bacterial abundance varied from 1.05 to 5.97 nmol CH4 g-1 (dry soil) d-1 and 2.59 × 105 to 2.50 × 107 copies g-1 dry soil, respectively. Based on the n-damo activity, it was estimated that approximately 0.91 Tg CH4 and 2.17 Tg N could be consumed annually via n-damo in Chinese paddy soils. The spatial variations in n-damo activity and community structure of n-damo bacteria were significantly (p < 0.05) affected by the soil ammonium content, labile organic carbon content and pH. Furthermore, significant differences in n-damo activity, bacterial abundance and community composition were observed among different climate zones. The n-damo activity was found to be positively correlated with the mean annual air temperature. Taken together, our results demonstrated the potential importance of n-damo in both methane consumption and nitrogen removal in Chinese paddy soils, and this process was regulated by local soil and climatic factors.

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Paddy fields are one of the most important methane sources, which have great impacts on climate change. The nitrite-dependent anaerobic methane oxidation, by NC10 phylum bacteria-Candidatus Methylomirabilis oxyfera (M. oxyfera)-like bacteria, is a new process regulating methane emission from paddy fields. However, little is known about the spatial and temporal variations of M. oxyfera-like bacterial communities and the regulating factors in paddy soils. We investigated the community composition, diversity, and abundance of M. oxyfera-like bacteria in 0-40 cm depth of paddy soils at key growth stages of rice, including tillering, jointing, flowering, and milky stages. Results of high-throughput sequencing showed that community composition of M. oxyfera-like bacteria differed significantly among different soil layers, while no significant variation was observed among different rice growth stages. The diversity of M. oxyfera-like bacteria increased with soil depth. Real-time quantitative PCR showed that the 16S rRNA gene abundance of M. oxyfera-like bacteria ranged from 5.73×106 to 2.56×107 copies·g-1 (dry weight), with the highest gene abundance in the 10-20 cm layer. Further, the abundance of these bacteria showed a decreasing trend with rice growth. Soil organic carbon content and soil pH were correlated with the M. oxyfera-like bacterial community structures and abundance. In all, our results suggested a certain degree of heterogeneity of spatial and temporal distribution of M. oxyfera-like bacterial communities in paddy soils, which was largely influenced by soil organic carbon and soil pH.

Response of nitrite-dependent anaerobic methanotrophs to elevated atmospheric CO2 concentration in paddy fields.

Nitrite-dependent anaerobic methane oxidation (n-damo) catalyzed by Candidatus Methylomirabilis oxyfera (M. oxyfera)-like bacteria is a new pathway for the regulation of methane emissions from paddy fields. Elevated atmospheric CO2 concentrations (e[CO2]) can indirectly affect the structure and function of microbial communities. However, the response of M. oxyfera-like bacteria to e[CO2] is currently unknown. Here, we investigated the effect of e[CO2] (ambient CO2 + 200 ppm) on community composition, abundance, and activity of M. oxyfera-like bacteria at different depths (0-5, 5-10, and 10-20 cm) in paddy fields across multiple rice growth stages (tillering, jointing, and flowering). High-throughput sequencing showed that e[CO2] had no significant effect on the community composition of M. oxyfera-like bacteria. However, quantitative PCR suggested that the 16S rRNA gene abundance of M. oxyfera-like bacteria increased significantly in soil under e[CO2], particularly at the tillering stage. Furthermore, 13CH4 tracer experiments showed potential n-damo activity of 0.31-8.91 nmol CO2 g-1 (dry soil) d-1. E[CO2] significantly stimulated n-damo activity, especially at the jointing and flowering stages. The n-damo activity and abundance of M. oxyfera-like bacteria increased by an average of 90.9% and 50.0%, respectively, under e[CO2]. Correlation analysis showed that the increase in soil dissolved organic carbon content caused by e[CO2] had significant effects on the activity and abundance of M. oxyfera-like bacteria. Overall, this study provides the first evidence for a positive response of M. oxyfera-like bacteria to e[CO2], which may help reduce methane emissions from paddy fields under future climate change conditions.