Soil microbial subcommunity assembly mechanisms are highly variable and intimately linked to their ecological and functional traits.

Revealing the mechanisms underlying soil microbial community assembly is a fundamental objective in molecular ecology. However, despite increasing body of research on overall microbial community assembly mechanisms, our understanding of subcommunity assembly mechanisms for different prokaryotic and fungal taxa remains limited. Here, soils were collected from more than 100 sites across southwestern China. Based on amplicon high-throughput sequencing and iCAMP analysis, we determined the subcommunity assembly mechanisms for various microbial taxa. The results showed that dispersal limitation and homogenous selection were the primary drivers of soil microbial community assembly in this region. However, the subcommunity assembly mechanisms of different soil microbial taxa were highly variable. For instance, the contribution of homogenous selection to Crenarchaeota subcommunity assembly was 70%, but it was only around 10% for the subcommunity assembly of Actinomycetes, Gemmatimonadetes and Planctomycetes. The assembly of subcommunities including microbial taxa with higher occurrence frequencies, average relative abundance and network degrees, as well as wider niches tended to be more influenced by homogenizing dispersal and drift, but less affected by heterogeneous selection and dispersal limitation. The subcommunity assembly mechanisms also varied substantially among different functional guilds. Notably, the subcommunity assembly of diazotrophs, nitrifiers, saprotrophs and some pathogens were predominantly controlled by homogenous selection, while that of denitrifiers and fungal pathogens were mainly affected by stochastic processes such as drift. These findings provide novel insights into understanding soil microbial diversity maintenance mechanisms, and the analysis pipeline holds significant value for future research.

Mucilaginibacterpsychrotolerans sp. nov., isolated from peatlands.

A Gram-stain-negative, rod-shaped, non-flagellated, pink, cold-tolerant bacterial strain, NH7-4T, was isolated from the Riganqiao peatlands on the Tibetan Plateau. The 16S rRNA gene sequence of the novel isolate shared a pairwise similarity ranging from 96.84 to 93.02 % with type strains of species of the genus Mucilaginibacter. Growth of strain NH7-4T occurred between 0 and 30 °C and at pH 5.0-9.0, with an optimum growth temperature at 20 °C and an optimum pH for growth of approximately 7.0. The major isoprenoid quinone was MK-7. The major cellular fatty acids were summed feature 3 (C16 : 1ω7c and/or iso-C15 : 0 2-OH), iso-C15 : 0, iso-C17 : 0 3-OH, C16 : 0, iso-C15 : 0 3-OH and C16 : 1ω5c. The major polar lipid of strain NH7-4T was phosphatidylethanolamine. Strain NH7-4T did not assimilate any substrates in API 20NE strips without low concentrations of yeast extract being present and had a lower optimal growth temperature, which distinguished it from other type strains of species of the genus Mucilaginibacter. The DNA G+C content of strain NH7-4T was 48.6 mol%. Based on phylogenetic, phenotypic and chemotaxonomic data, strain NH7-4T (=JCM 30607T=CGMCC1.14937T) represents a novel species of the genus Mucilaginibacter for which the name Mucilaginibacter psychrotolerans sp. nov. is proposed.

Salinity Affects the Composition of the Aerobic Methanotroph Community in Alkaline Lake Sediments from the Tibetan Plateau.

Lakes are widely distributed on the Tibetan Plateau, which plays an important role in natural methane emission. Aerobic methanotrophs in lake sediments reduce the amount of methane released into the atmosphere. However, no study to date has analyzed the methanotroph community composition and their driving factors in sediments of these high-altitude lakes (>4000 m). To provide new insights on this aspect, the abundance and composition in the sediments of six high-altitude alkaline lakes (including both freshwater and saline lakes) on the Tibetan Plateau were studied. The quantitative PCR, terminal restriction fragment length polymorphism, and 454-pyrosequencing methods were used to target the pmoA genes. The pmoA gene copies ranged 104-106 per gram fresh sediment. Type I methanotrophs predominated in Tibetan lake sediments, with Methylobacter and uncultivated type Ib methanotrophs being dominant in freshwater lakes and Methylomicrobium in saline lakes. Combining the pmoA-pyrosequencing data from Tibetan lakes with other published pmoA-sequencing data from lake sediments of other regions, a significant salinity and alkalinity effect (P = 0.001) was detected, especially salinity, which explained ∼25% of methanotroph community variability. The main effect was Methylomicrobium being dominant (up to 100%) in saline lakes only. In freshwater lakes, however, methanotroph composition was relatively diverse, including Methylobacter, Methylocystis, and uncultured type Ib clusters. This study provides the first methanotroph data for high-altitude lake sediments (>4000 m) and shows that salinity is a driving factor for the community composition of aerobic methanotrophs.

Anthropogenic protection alters the microbiome in intertidal mangrove wetlands in Hainan Island.

Intertidal mangrove wetlands are of great economic and ecological importance. The regular influence of tides has led to the microbial communities in these wetlands differing significantly from those in other habitats. In this study, we investigated the microbiomes of the two largest mangrove wetlands in Hainan Island, China, which have different levels of anthropogenic protection. Soil samples were collected from the root zone of 13 mangrove species. The microbial composition, including key functional groups, was assessed using Illumina sequencing. Bioinformatics analysis showed that there was a significant difference in the microbiomes between the protected Bamen Bay and the unprotected Dongzhai Bay. The overall microbiome was assigned into 78 phyla and Proteobacteria was the most abundant phylum at both sites. In the protected wetland, there were fewer marine-related microbial communities, such as sulfate-reducing bacteria, and more terrestrial-related communities, such as Verrucomicrobia methanotrophs. We also observed distinct microbial compositions among the different mangrove species at the protected site. Our data suggest that the different microbiomes of the two mangrove wetlands are the result of a complex interaction of the different environmental variables at the two sites.

Aerobic methanotroph diversity in Sanjiang wetland, Northeast China.

Aerobic methanotrophs present in wetlands can serve as a methane filter and thereby significantly reduce methane emissions. Sanjiang wetland is a major methane source and the second largest wetland in China, yet little is known about the characteristics of aerobic methanotrophs in this region. In the present study, we investigated the diversity and abundance of methanotrophs in marsh soils from Sanjiang wetland with three different types of vegetation by 16S ribosomal RNA (rRNA) and pmoA gene analysis. Quantitative polymerase chain reaction analysis revealed the highest number of pmoA gene copies in marsh soils vegetated with Carex lasiocarpa (10(9) g(-1) dry soil), followed by Carex meyeriana, and the least with Deyeuxia angustifolia (10(8) g(-1) dry soil). Consistent results were obtained using Sanger sequencing and pyrosequencing techniques, both indicating the codominance of Methylobacter and Methylocystis species in Sanjiang wetland. Other less abundant methanotrophy, including cultivated Methylomonas and Methylosinus genus, and uncultured clusters such as LP20 and JR-1, were also detected in the wetland. Methanotroph diversity was almost the same in three different vegetation covered soils, suggesting that vegetation types had very little influence on the methanotroph diversity. Our study gives an in-depth insight into the community composition of aerobic methanotrophs in the Sanjiang wetland.

Bacterial community structure in two permafrost wetlands on the Tibetan Plateau and Sanjiang Plain, China.

Permafrost wetlands are important methane emission sources and fragile ecosystems sensitive to climate change. Presently, there remains a lack of knowledge regarding bacterial communities, especially methanotrophs in vast areas of permafrost on the Tibetan Plateau in Northwest China and the Sanjiang Plain (SJ) in Northeast China. In this study, 16S rRNA-based quantitative PCR (qPCR) and 454 pyrosequencing were used to identify bacterial communities in soils sampled from a littoral wetland of Lake Namco on the Tibetan Plateau (NMC) and an alluvial wetland on the SJ. Additionally, methanotroph-specific primers targeting particulate methane monooxygenase subunit A gene (pmoA) were used for qPCR and pyrosequencing analysis of methanotrophic community structure in NMC soils. qPCR analysis revealed the presence of 10(10) 16S rRNA gene copies per gram of wet soil in both wetlands, with 10(8) pmoA copies per gram of wet soil in NMC. The two permafrost wetlands showed similar bacterial community compositions, which differed from those reported in other cold environments. Proteobacteria, Actinobacteria , and Chloroflexi were the most abundant phyla in both wetlands, whereas Acidobacteria was prevalent in the acidic wetland SJ only. These four phyla constituted more than 80 % of total bacterial community diversity in permafrost wetland soils, and Methylobacter of type I methanotrophs was overwhelmingly dominant in NMC soils. This study is the first major bacterial sequencing effort of permafrost in the NMC and SJ wetlands, which provides fundamental data for further studies of microbial function in extreme ecosystems under climate change scenarios.

Methylocystis dominates methane oxidation in glacier foreland soil at elevated temperature.

Methane-oxidizing bacteria (methanotrophs) play an important role in mitigating methane emissions in various ecological environments, including cold regions. However, the response of methanotrophs in these cold environments to extreme temperatures above the in-situ temperature has not been thoroughly explored. Therefore, this study collected soil samples from Longxiazailongba (LXZ) and Qiangyong (QY) glacier forelands and incubated them with 13CH4 at 35°C under different soil water conditions. The active methanotroph populations were identified using DNA stable isotope probing (DNA-SIP) and high throughput sequencing techniques. The results showed that the methane oxidation potential in LXZ and QY glacier foreland soils was significantly enhanced at an unusually high temperature of 35°C during microcosm incubations, where abundant substrate (methane and oxygen) was provided. Moreover, the influence of soil water conditions on this potential was observed. Interestingly, Methylocystis, a type II and mesophilic methanotroph, was detected in the unincubated in-situ soil samples and became the active and dominant methanotroph in methane oxidation at 35°C. This suggests that Methylocystis can survive at low temperatures for a prolonged period and thrive under suitable growth conditions. Furthermore, the presence of mesophilic methanotrophs in cold habitats could have potential implications for reducing greenhouse gas emissions in warming glacial environments.

Methylomonadaceae was the active and dominant methanotroph in Tibet lake sediments.

Methane (CH4), an important greenhouse gas, significantly impacts the local and global climate. Our study focused on the composition and activity of methanotrophs residing in the lakes on the Tibetan Plateau, a hotspot for climate change research. Based on the field survey, the family Methylomonadaceae had a much higher relative abundance in freshwater lakes than in brackish and saline lakes, accounting for ~92% of total aerobic methanotrophs. Using the microcosm sediment incubation with 13CH4 followed by high throughput sequencing and metagenomic analysis, we further demonstrated that the family Methylomonadaceae was actively oxidizing CH4. Moreover, various methylotrophs, such as the genera Methylotenera and Methylophilus, were detected in the 13C-labeled DNAs, which suggested their participation in CH4-carbon sequential assimilation. The presence of CH4 metabolism, such as the tetrahydromethanopterin and the ribulose monophosphate pathways, was identified in the metagenome-assembled genomes of the family Methylomonadaceae. Furthermore, they had the potential to adapt to oxygen-deficient conditions and utilize multiple electron acceptors, such as metal oxides (Fe3+), nitrate, and nitrite, for survival in the Tibet lakes. Our findings highlighted the predominance of Methylomonadaceae and the associated microbes as active CH4 consumers, potentially regulating the CH4 emissions in the Tibet freshwater lakes. These insights contributed to understanding the plateau carbon cycle and emphasized the significance of methanotrophs in mitigating climate change.

Distinct responses of soil methanotrophy in hummocks and hollows to simulated glacier meltwater and temperature rise in Tibetan glacier foreland.

Glacier foreland soils are known to be essential methane (CH4) consumers. However, global warming and increased glacier meltwater have turned some foreland meadows into swamp meadows. The potential impact of this change on the function of foreland soils in methane consumption remains unclear. Therefore, we collected Tibetan glacier foreland soils in the non-melting season from typical microtopography in swamp meadows (hummock and hollow). Three soil moisture conditions (moist, saturated, and submerged) were set by adding glacier runoff water. Soil samples were then incubated in the laboratory for two weeks at 10 °C and 20 °C. About 5 % of 13CH4/12CH4 was added to the incubation bottles, and daily methane concentrations were measured. DNA stable isotope probing (DNA-SIP) and high-throughput sequencing were combined to target the active methanotroph populations. The results showed that type Ia methanotrophs, including Crenothrix, Methylobacter, and an unclassified Methylomonadaceae cluster, actively oxidized methane at 10 °C and 20 °C. There were distinct responses of methanotrophs to soil moisture rises in hummock and hollow soils, resulting in different methane oxidation potentials. In both hummock and hollow soils, the methane oxidation potential was positively correlated with temperature. Furthermore, saturated hummock soils exhibited the highest methane oxidation potential and methanotroph populations, while submerged hollow soils had the lowest. This suggests that the in-situ hummock soils, generally saturated with water, are more essential than in-situ hollows, typically submerged in water, for alleviating the global warming potential of swamp meadows in the Tibetan glacier foreland during the growing season.

Vertical variations in microbial diversity, composition, and interactions in freshwater lake sediments on the Tibetan plateau.

Microbial communities in freshwater lake sediments exhibit a distinct depth-dependent variability. Further exploration is required to understand their biodiversity pattern and microbial interactions in vertical sediments. In this study, sediment cores from two freshwater lakes, Mugecuo (MGC) and Cuopu (CP), on the Tibetan plateau were sampled and subsequently sliced into layers at a depth of every centimeter or half a centimeter. Amplicon sequencing was used to analyze the composition, diversity, and interaction of microbial communities. Results showed that sediment samples of both lakes could be clustered into two groups at a sediment depth of about 20 cm, with obvious shifts in microbial community compositions. In lake MGC, the richness component dominated ß-diversity and increased with depth, indicating that the microbial communities in the deep layer of MGC was selected from the surface layer. Conversely, the replacement component dominated ß-diversity in CP, implying a high turnover rate in the surface layer and inactive seed banks with a high variety in the deep layer. A co-occurrence network analysis showed that negative microbial interactions were prevalent in the surface layers with high nutrient concentrations, while positive microbial interactions were more common in the deep layers with low nutrient concentrations, suggesting that microbial interactions are influenced by nutrient conditions in the vertical sediments. Additionally, the results highlight the significant contributions of abundant and rare taxa to microbial interactions and vertical fluctuations of ß-diversity, respectively. Overall, this work deepens our understanding of patterns of microbial interactions and vertical fluctuation in ß-diversity in lake sediment columns, particularly in freshwater lake sediments from the Tibetan plateau.

Sink or Source: Alternative Roles of Glacier Foreland Meadow Soils in Methane Emission Is Regulated by Glacier Melting on the Tibetan Plateau.

Glacier foreland soils have long been considered as methane (CH4) sinks. However, they are flooded by glacial meltwater annually during the glacier melting season, altering their redox potential. The impacts of this annual flooding on CH4 emission dynamics and methane-cycling microorganisms are not well understood. Herein, we measured in situ methane flux in glacier foreland soils during the pre-melting and melting seasons on the Tibetan Plateau. In addition, high-throughput sequencing and qPCR were used to investigate the diversity, taxonomic composition, and the abundance of methanogenic archaea and methanotrophic bacteria. Our results showed that the methane flux ranged from -10.11 to 4.81 µg·m-2·h-1 in the pre-melting season, and increased to 7.48-22.57 µg·m-2·h-1 in the melting season. This indicates that glacier foreland soils change from a methane sink to a methane source under the impact of glacial meltwater. The extent of methane flux depends on methane production and oxidation conducted by methanogens and methanotrophs. Among all the environmental factors, pH (but not moisture) is dominant for methanogens, while both pH and moisture are not that strong for methanotrophs. The dominant methanotrophs were Methylobacter and Methylocystis, whereas the methanogens were dominated by methylotrophic Methanomassiliicoccales and hydrogenotrophic Methanomicrobiales. Their distributions were also affected by microtopography and environmental factor differences. This study reveals an alternative role of glacier foreland meadow soils as both methane sink and source, which is regulated by the annual glacial melt. This suggests enhanced glacial retreat may positively feedback global warming by increasing methane emission in glacier foreland soils in the context of climate change.

Properties of sediment dissolved organic matter respond to eutrophication and interact with bacterial communities in a plateau lake.

Sediment dissolved organic matter (DOM) in inland waters is commonly affected by environmental changes. However, knowledge about how sediment DOM responds to eutrophication and the associations between sediment DOM and bacterial communities requires further investigation. We selected a sediment core from Dianchi Lake (China) that was dated from 1864 to 2019 by the activity of radionuclides (210Pb and 137Cs). δ13CDOC changes fit well with the historical record that heavy eutrophic status in Dianchi Lake were observed since 1980s. Large amounts of dissolved organic carbon (DOC), chromophoric (CDOM) and fluorescent (FDOM) DOM accumulated at the top of the sediments during the eutrophication period (1982-present). The additional algae sources with a higher degradation rate altered the composition, aromaticity and humification of DOM. After long-term mineralization, the remaining DOM became more and more recalcitrant and kept a relatively stable level at older sediments. A co-occurrence network analysis revealed that Proteobacteria, Chloroflexi, Acidobacteriota, Bacteroidota and Desulfobacterota were the most abundant species at the phylum level and clustered into three primary modules. Different microbes shared unique preferences for niches, causing a heterogeneous bacterial distribution at different depths. We conducted Spearman's correlation and redundancy analysis (RDA) to explore potential interactions between bacterial community and sediment DOM. The richness and diversity of bacterial communities were positively related to DOM content, suggesting abundant DOM can produce more available resources for bacteria. RDA results showed some specific species might modify DOM composition and structure. This study suggests that sediment DOM properties were regulated by source transformation during eutrophication, and emphasizes the importance of microbial role on sediment biogeochemical process.

Downward aeration promotes static composting by affecting mineralization and humification.

A composting experiment with sewage sludge and green waste was conducted to explore the effects of aeration directions (i.e., upward and downward) on static composting systems. The compost properties, including humification indexes and organic matter loss rate, and microbial diversity during the composting, were determined. Results showed that the downward aeration promoted the homogenization of temperature and moisture of the static composting system, thereby stimulating microbial metabolism and accelerating mineralization and humification. Microbial community profiles significantly changed among the composting phases. The humification dynamics were significantly correlated with the relative abundance of multiple microbial functional groups. However, no significant effects of aeration direction on the microbial community profiles were observed. The findings indicate that downward aeration is promising to improve the quality of static compost production, by stimulating microbial metabolism rather than altering microbial community profiles.

Bacterial community composition in soils covered by different vegetation types in the Yancheng tidal marsh.

Coastal wetland vegetation plays an important role in maintaining ecological function and is a key factor affecting the soil bacterial community. Spartina alterniflora was introduced to the Yancheng tidal marsh to stabilize the sediments and gradually replaced the native plants. However, the changes in the soil bacterial community profile caused by S. alterniflora invasion are poorly characterized. Here, we used MiSeq sequencing to compare the composition of the bacterial community in soil at different depths under exotic S. alterniflora (SA), native Phragmites australis (PA), and native Suaeda salsa (SS). The results showed that the pH value was lower, but the salinity, soil organic carbon, total nitrogen, and number of 16S rRNA genes were higher in SA soils than in PA and SS soils. Overall, Proteobacteria was the dominant bacterial phylum, followed by Chloroflexi, Acidobacteria, Planctomycetes, Gemmatimonadetes, and Nitrospirae. Anaerolineae in the Chloroflexi phylum showed the greatest difference based on vegetation, accounting for 14.4% of the overall bacterial community in SA soils but only about 3.8% of those in PA and SS soils. The composition, interaction, and predicted functional profiles of the bacterial community in SA soils were significantly different from those in PA and SS soils, especially for functions related to the sulfur and nitrogen cycles. Salinity was negatively correlated with the Shannon index and accounted for 37.7% of the total variation in the bacterial community, making it the most important environmental factor. Our results showed the differences in bacterial community composition among different vegetation types and soil depths in the Yancheng tidal marsh, which provides a microbial basis for a better understanding of the ecological functions in this ecosystem.

Methylococcaceae are the dominant active aerobic methanotrophs in a Chinese tidal marsh.

Although coastal marshes are net carbon sinks, they are net methane sources. Aerobic methanotrophs in coastal marsh soils are important methane consumers, but their activity and populations are poorly characterized. DNA stable-isotope probing followed by sequencing was used to determine how active methanotrophic populations differed in the main habitats of a Chinese coastal marsh. These habitats included mudflat, native plant-dominated, and alien plant-dominated habitats. Methylococcaceae was the most active methanotroph family across four habitats. Abundant methylotroph sequences, including methanotrophs and non-methane-oxidizing methylotrophs (Methylotenera and Methylophaga), constituted 50-70% of the 16S rRNA genes detected in the labeled native plant-dominated and mudflat soils. Methylotrophs were less abundant (~ 20%) in labeled alien plant-dominated soil, suggesting less methane assimilation into the target community or a different extent of carbon cross-feeding. Canonical correspondence analysis indicated a significant correlation between the active bacterial communities and soil properties (salinity, organic carbon, total nitrogen, pH, and available phosphorus). Importantly, these results highlight how changing vegetation or soil features in coastal marshes may change their resident active methanotrophic populations, which will further influence methane cycling.

The voltage signals of microbial fuel cell-based sensors positively correlated with methane emission flux in paddy fields of China.

Previous studies showed that exoelectrogenic bacteria in paddy soil could suppress methanogens and methanogenesis after they were enriched by application of Fe3+ or running microbial fuel cells (MFCs). However, the relationship between exoelectrogenic bacteria and methanogens without the enrichment process is unknown. Our study was conducted in three paddy fields in China and over three seasons. We explored novel MFC-based sensors to in situ detect voltage signals that were generated from paddy soil within 10 min. The voltage and methane emission flux were determined as an indicator of the exoelectrogenic activity and methanogenic activity, respectively. The abundance of exoelectrogenic bacteria was assessed by quantifying five exoelectrogenic bacterial-associated genera including Geobacter, Shewanella, Anaeromyxobacter, Desulfovibrio and Clostridium, while the methanogens were studied by quantifying and sequencing the mcrA gene. The results showed that the abundance of exoelectrogenic bacteria and the voltage signals were positively correlated to the abundance of mcrA gene and methane emission flux, respectively. Moreover, non-metric dimensional scaling reveals that the abundance of Geobacter, Desulfovibrio and Clostridium significantly correlated with that of Methanomassiliicoccus, Methanoregula and Methanolinea. The present study suggests that the voltage signals might act as a novel indicator of methane emission flux in paddy fields.

Upland Soil Cluster Gamma dominates methanotrophic communities in upland grassland soils.

Aerobic methanotrophs in upland soils consume atmospheric methane, serving as a critical counterbalance to global warming; however, the biogeographic distribution patterns of their abundance and community composition are poorly understood, especial at a large scale. In this study, soils were sampled from 30 grasslands across >2000 km on the Qinghai-Tibetan Plateau to determine the distribution patterns of methanotrophs and their driving factors at a regional scale. Methanotroph abundance and community composition were analyzed using quantitative PCR and Illumina Miseq sequencing of pmoA genes, respectively. The pmoA gene copies ranged from 8.2 × 105 to 1.1 × 108 per gram dry soil. Among the 30 grassland soil samples, Upland Soil Cluster Gamma (USCγ) dominated the methanotroph communities in 26 samples. Jasper Ridge Cluster (JR3) was the most dominant methanotrophic cluster in two samples; while Methylocystis, cluster FWs, and Methylobacter were abundant in other two wet soil samples. Interestingly, reanalyzing the pmoA genes sequencing data from existing publications suggested that USCγ was also the main methanotrophic cluster in grassland soils in other regions, especially when their mean annual precipitation was <500 mm. Canonical Analysis of Principal Coordinates including all soil samples indicated that the methanotrophic community composition was significantly correlated with local environmental factors, among which mean annual precipitation and pH showed the strongest correlations. Variance partitioning analysis showed that environmental factors and spatial distance were significant factors affecting the community structure of methanotrophs, and environmental properties were more important factors. Collectively, these findings indicate that atmospheric methane may be mainly oxidized by USCγ in upland soils. They also highlight the key role of water availability and pH in determining the abundance and community profiles of grassland soil methanotrophs.

Autotrophic and symbiotic diazotrophs dominate nitrogen-fixing communities in Tibetan grassland soils.

Biological nitrogen fixation, conducted by soil diazotrophs, is the primary nitrogen source for natural grasslands. However, the diazotrophs in grassland soils are still far from fully investigated. Particularly, their regional-scale distribution patterns have never been systematically examined. Here, soils (0-5 cm) were sampled from 54 grasslands on the Tibetan Plateau to examine the diazotroph abundance, diversity, and community composition, as well as their distribution patterns and driving factors. The diazotroph abundance was expressed as nifH gene copies, measured using real-time PCR. The diversity and community composition of diazotrophs were analyzed through MiSeq sequencing of nifH genes. The results showed that Cyanobacteria (47.94%) and Proteobacteria (45.20%) dominated the soil diazotroph communities. Most Cyanobacteria were classified as Nostocales which are main components of biological crusts. Rhizobiales, most of which were identified as potential symbiotic diazotrophs, were also abundant in approximately half of the soil samples. The soil diazotroph abundance, diversity, and community composition followed the distribution patterns in line with mean annual precipitation. Moreover, they also showed significant correlations with prokaryotic abundance, plant biomass, vegetation cover, soil pH values, and soil nutrient contents. Among these environmental factors, the soil moisture, organic carbon, available phosphorus, and inorganic nitrogen contents could be the main drivers of diazotroph distribution due to their strong correlations with diazotroph indices. These findings suggest that autotrophic and symbiotic diazotrophs are the predominant nitrogen fixers in Tibetan grassland soils, and highlight the key roles of water and nutrient availability in determining the soil diazotroph distribution on the Tibetan Plateau.

Identification of active aerobic methanotrophs in plateau wetlands using DNA stable isotope probing.

Sedge-dominated wetlands on the Qinghai-Tibetan Plateau are methane emission centers. Methanotrophs at these sites play a role in reducing methane emissions, but relatively little is known about the composition of active methanotrophs in these wetlands. Here, we used DNA stable isotope probing to identify the key active aerobic methanotrophs in three sedge-dominated wetlands on the plateau. We found that Methylocystis species were active in two peatlands, Hongyuan and Dangxiong. Methylobacter species were found to be active only in Dangxiong peat. Hongyuan peat had the highest methane oxidation rate, and cross-feeding of carbon from methanotrophs to methylotrophic Hyphomicrobium species was observed. Owing to a low methane oxidation rate during the incubation, the labeling of methanotrophs in Maduo wetland samples was not detected. Our results indicate that there are large differences in the activity of methanotrophs in the wetlands of this region.