Diazotrophic Community Research Articles

Broad‐scale distribution of diazotrophic communities is driven more by aridity index and temperature than by soil properties across various forests

AbstractAimMany studies have found that diazotrophic distribution is mainly determined by soil properties from field to regional scales. However, we lack strong evidence for the relative importance of different drivers controlling broad‐scale biogeography of forest diazotrophs, especially for soil multi‐nutrients.LocationChina’s forests.Time period2012−2013.Major taxa studiedDiazotrophic communities.MethodsWe investigated the distribution of soil diazotrophic communities from 146 sites along a 3,900‐km south–north transect by sequencing N‐fixing nifH gene amplicons. The relative contributions of environmental variables were assessed using a combination of stepwise multiple regression, variation partitioning analysis, multiple regression on distance matrices and partial least squares path modelling.ResultsOverall, aridity index and temperature were the predominant parameters governing diazotrophic community diversity and structure, mainly through their indirect effects on soil pH, nutrient contents and plant productivity. Although soil multi‐nutrients (Ca, Mg, Fe, Mn, Na, Cu and Zn) were included in the statistical analysis, they still exhibited lower impacts on diazotrophic communities than climate. Intriguingly, the microelement Mo could not explain the diazotrophic community patterns, despite its significance in nitrogenase enzymes. This unexpected phenomenon was attributed to the relatively high Mo supply in our work. Moreover, the distinct responses of diazotroph taxa to climatic factors and large heterogeneity of diazotrophic diversity among forests in different climatic zones further support the dominant role of climatic variation. These results indicate the presence of differentiated climatic niches for diazotrophs, such as warm‐adapted Bradyrhizobium and cool‐adapted Azospirillum.Main conclusionsOur findings suggest for the first time that unlike prior studies, the key roles of soil nutrient limitation (even for Mo) and pH‐dependent mechanisms in small‐scale diazotrophic communities can be surpassed by large‐scale climatic gradients. Future changes in drought severity and temperature might greatly shape diazotrophic distribution and its potential function in forest N2 fixation.

Diazotrophic Community Variation Underlies Differences in Nitrogen Fixation Potential in Paddy Soils Across a Climatic Gradient in China.

Biological nitrogen (N2) fixation as a source of new N input into the soil by free-living diazotrophs is important for achieving sustainable rice agriculture. However, the dominant environmental drivers or factors influencing N2 fixation and the functional significance of the diazotroph community structure in paddy soil across a climatic gradient are not yet well understood. Thus, we characterized the diazotroph community and identified the ecological predictors of N2 fixation potential in four different climate zones (mid-temperate, warm-temperate, subtropical, and tropical paddy soils) in eastern China. Comprehensive nifH gene sequencing, functional activity detection, and correlation analysis with environmental factors were estimated. The potential nitrogenase activity (PNA) was highest in warm-temperate regions, where it was 6.2-, 2.9-, and 2.2-fold greater than in the tropical, subtropical, and mid-temperate regions, respectively; nifH gene abundance was significantly higher in warm-temperate and subtropical zones than in the tropical or mid-temperate zones. Diazotroph diversity was significantly higher in the tropical climate zone and significantly lower in the mid-temperate zone. Non-metric multidimensional scaling and canonical correlation analysis indicated that paddy soil diazotroph populations differed significantly among the four climate zones, mainly owing to differences in climate and soil pH. Structural equation models and automatic linear models revealed that climate and nutrients indirectly affected PNA by affecting soil pH and diazotroph community, respectively, while diazotroph community, C/P, and nifH gene abundance directly affected PNA. And C/P ratio, pH, and the diazotroph community structure were the main predictors of PNA in paddy soils. Collectively, the differences in diazotroph community structure have ecological significance, with important implications for the prediction of soil N2-fixing functions under climate change scenarios.

Effect of soil chemical fertilization on the diversity and composition of the tomato endophytic diazotrophic community at different stages of growth.

The aim of this work was to gain a more comprehensive and perspicacious view of the endophytic diazotrophic community (EDC) of tomato plant bacteria and assess the effects of chemical fertilization and the plant phenologic stage on the status of those microbes. When the EDC of stem and roots from tomato plants grown in a greenhouse with and without exogenous chemical fertilization was examined by pyrosequencing the nifH gene during the growth cycle, a high taxonomic and phylogenetic diversity was observed. The abundant taxa were related to ubiquitous endophytes such as Rhizobium or Burkholderia but also involved anaerobic members usually restricted to flooded plant tissues, such as Clostridium, Geobacter, and Desulfovibrio. The EDC composition appeared to be dynamic during the growth phase of the tomato, with the structure of the community at the early stages of growth displaying major differences from the late stages. Inorganic fertilization negatively affected the diversity and modified the profile of the predominant components of the EDC in the different growth stages. Populations such as Burkholderia and Geobacter plus the Cyanobacteria appeared particularly affected by fertilization.Our work demonstrates an extensive endophytic diazotrophic diversity, suggesting a high potential for nitrogen fixation. The effect of the phenologic stage and inorganic-chemical soil fertilization on the community structure indicated a dynamic community that responded to environmental changes. These findings contribute to a better understanding of endophytic associations that could be helpful in assisting to shape the endomicrobiome that provides essential benefits to crops.

Significant dose effects of fertilizers on soil diazotrophic diversity, community composition, and assembly processes in a long‐term paddy field fertilization experiment

AbstractAlthough diazotrophs are major participants in biological nitrogen fixation (BNF) processes, the variations in community structure and assembly processes in response to long‐term fertilization with different fertilizer application are still ambiguous, especially under various doses with balance or not. Using nifH amplicon sequencing, we investigated the soil diazotrophic communities in a paddy field which received 20‐year fertilization including control (CK), balanced fertilization (three doses of nitrogen (N), phosphorus (P), and potassium (K) at same ratio (NPK, 2(NPK), and 3(NPK)), unbalanced fertilization (three ratios, (2 N)PK, N(2P)K, and NP(2K)), and manure (M). Our results showed that the diazotrophic diversity increased with the long‐term fertilization, but it was tapering‐off with doses increment. However, the community dissimilarities increased with fertilizer doses. These variations were attributed to the trade‐off between the variation of soil pH and nutrients. The increasing of fertilizer doses induced the transformation of community assembly processes, which changed from mainly dominated by deterministic processes in CK, NPK, NP(2K) and M to stochastic processes in the other treatments. Our findings suggest that the long‐term fertilization, especially different fertilizer doses, strongly affect soil diazotrophic diversity, community structure, and assembly processes, which could help to improve the sustainable agricultural management in the future.

Drivers of diazotroph community structure and co-occurrence in a Northern Great Plains pulse crop rotation system

The Northern Great Plains of the United States and Canada have seen an increase in pulse crops like pea, lentil, and chickpea acreage in dryland and irrigated systems. Rotating pulse crops into cereal crop production systems could reduce the demand for nitrogen fertilizer due to the symbiotic relationship with nitrogen fixing bacteria called diazotrophs. Legume-diazotroph relationships are well studied, but little is known about the free-living bacteria of the bulk soil in the legume-cereal rotation systems. Here we determine the community structure of the overall (16S) and diazotroph (nifH) soil microbiome in pulse-cereal rotations across the state of Montana. Many variables influenced the microbial community structure, but and among them, irrigation caused a significant shift in the composition of the communities and co-occurrence network topology. Interestingly, the diazotrophic communities in dryland soil are more robust and contain more generalist compared to irrigated soils. Co-occurrence networks support the identification of different diazotrophic keystone taxa such as Firmicutes in irrigated and Proteobacteria in dryland soils. Linking farm management practices and crop productivity with microbial community structure dynamics is an initial step in establishing a knowledge base for farmers to exploit the soil microbiota to maintain healthy soil for crop production.

Depicting Temporal, Functional, and Phylogenetic Patterns in Estuarine Diazotrophic Communities from Environmental DNA and RNA.

Seasonally nitrogen-limited and phosphorus-replete temperate coastal waters generally host dense and diverse diazotrophic communities. Despite numerous studies in marine systems, little is known about diazotrophs and their functioning in oligohaline estuarine environments. Here we applied a combination of nifH transcript and metagenomic shotgun sequencing approaches to investigate temporal shifts in taxonomic composition and nifH activity of size-fractionated diazotrophic communities in a shallow and mostly freshwater coastal lagoon. Patterns in active nifH phylotypes exhibited a clear seasonal succession, which reflected their different tolerances to temperature change and nitrogen (N) availability. Thus, in spring, heterotrophic diazotrophs (Proteobacteria) dominated the nifH phylotypes, while increasing water temperature and depletion of inorganic N fostered heterocystous Cyanobacteria in summer. Metagenomic data demonstrated four main N-cycling pathways and three of them with a clear seasonal pattern: denitrification (spring) → N2 fixation (summer) → assimilative NO3- reduction (fall), with NH4+ uptake into cells occurring across all seasons. Although a substantial denitrification signal was observed in spring, it could have originated from the re-suspended benthic rather than planktonic community. Our results contribute to a better understanding of the realized genetic potential of pelagic N2 fixation and its seasonal dynamics in oligohaline estuarine ecosystems, which are natural coastal biogeochemical reactors.

Depression of soil nitrogen fixation by drying soil in a degraded alpine peatland

Soil biological nitrogen fixation (BNF) represents a major pathway through which nitrogen enters pristine peatlands. Many peatlands have been undergoing human-induced changes in environmental factors, while environmental changes dramatically affect the community composition and activity of nitrogen-fixing prokaryotes (i.e., diazotrophs). However, the impact of peatland degradation on soil BNF remains unclear. By carrying out a field campaign, we examined how soil BNF varies along a natural gradient from pristine marshes to moderately-degraded meadows and sandy meadows on the Zoige Plateau. Plant and topsoil samples from four pristine marshes, three moderately-degraded meadows, and three sandy meadows were collected to determine the potential rate of nitrogen fixation (RNfix), abundance of the nifH gene, diazotrophic community composition, and soil and plant characteristics. Our results showed that topsoil RNfix varied in the range 0.018–3.00 μmol N g d.w.−1 day−1 (i.e. 21.74–1632.37 mg N m−2 day−1) across the ten sites, being lowest in sandy meadows and highest in pristine marshes. Topsoil RNfix and diazotrophic abundance were positively correlated with soil water content, sedge cover, plant biomass, soil organic carbon content, and total nitrogen and phosphorus contents. Soil water content, which affected most plant and soil characteristics, had dominant influences on the abundance and community structure of diazotrophs. The RNfix was closely correlated with the abundance of dominant diazotroph groups. The community composition of diazotrophs differed markedly among sites of different degradation levels. Proteobacteria was the most abundant diazotrophic phylum across the ten sites. Heterotrophic diazotrophs acted as the major contributor to BNF, especially in pristine marshes and moderately-degraded meadows. We conclude that soil water content was the main factor driving the depressed soil BNF during peatland degradation in Zoige, due to soil water effects on plant cover and biomass, soil organic carbon and total phosphorus, and the abundance and community structure of diazotrophs.

Changes of diazotrophic communities in response to cropping systems in a Mollisol of Northeast China.

Nitrogen-fixing microorganisms play important roles in N cycling. However, knowledge related to the changes in the diazotrophic community in response to cropping systems is still rudimentary. In this study, the nifH gene was used to reveal the abundance and community compositions of diazotrophs in the cropping systems of continuous cropping of corn (CC) and soybean (SS) and soybean-corn rotation for growing corn (CSC) and soybean (SCS) in a black soil of Northeast China. The results showed that the abundance of the nifH gene was significantly higher in cropping soybean than in cropping corn under the same cropping system, while remarkably increased in the rotation system under the same crop. The Shannon index in the CC treatment was significantly higher than that in the other treatments, but the OTU number and Chao1 index had no significant change among the four treatments. Bradyrhizobium japonicum was the dominant diazotrophic species, and its relative abundance was at the lowest value in the CC treatment. In contrast, Skermanella sp. had the highest relative abundance in the CC treatment. A PCoA showed that the diazotrophic communities were separated between different cropping systems, and the variation caused by continuous corn cropping was the largest. Among the tested soil properties, the soil available phosphorus was a primary factor in determining diazotrophic community compositions. Overall, the findings of this study highlighted that the diazotrophic communities in black soils are very sensitive to cropping systems.

Combined effects of elevated CO2, N fertilizer and water deficit stress on diazotrophic community in sub-humid tropical paddy soil

Elevated carbon dioxide (eCO2), higher dose of recommended nitrogen (HDN), and water deficit stress (WDS) are three important factors which influence individually on diazotrophic community in paddy soil, thereby affecting crop yield and its nitrogen use efficiency. However, the combined effects of eCO2, HDN, and WDS on soil diazotrophic community remains inadequate particularly in rice. Hence, the present study was conducted in CO2-maintained open-top chamber to assess the diazotrophic community under combined influence of eCO2 (550 ± 20 μmol mol−1 as compared to ambient CO2i.e. 400 ± 10 μmol mol−1) and HDN (120 kg N ha−1, 50% higher than recommended dose of N for rice i.e. 80 kg N ha−1, in sub-humid tropical region of eastern India) with and without WDS (−60 kPa) at two growth stages of rice (tillering, TI and panicle initiation, PI) and correlated with soil carbon (C) pools and enzyme activities. The results indicated that soil organic C, readily mineralizable C, microbial biomass C, acid and alkaline phosphatase activities were significantly (p < 0.05) increased under combined influence of eCO2 and HDN at both TI and PI stages of rice without WDS. The results also showed that abundance of nifH varied from ambient CO2 (aCO2) to eCO2 and found higher in aCO2 under recommended dose of nitrogen (RDN) without WDS, whereas under eCO2, nifH copy number was found higher under HDN at both TI and PI stages of rice without WDS. Biplot analysis revealed that abundance of nifH and nitrifiers were highly correlated under aCO2 coupled with RDN without WDS. Illumina MiSeq®-based targeted nifH metagenome showed that eCO2 encouraged the relative abundance of Proteobacteria and Cyanobacteria, whereas suppressed the other diazotrophic phyla like Actinobacteria, Firmicutes, and Chlorobi, but the reverse trend was observed in HDN treatments. Heatmap analysis revealed a unique differentiation of Rhizobiales and Pseudomonadales under eCO2 coupled with RDN without WDS. Moreover, three-way interactions of eCO2, HDN, and WDS also showed significant (p < 0.05) difference of soil C pools and enzymatic activities at both growth stages of rice. Overall, the present study showed that diazotrophic community in paddy soil was significantly (p < 0.05) altered by the additive effects of eCO2, HDN, and WDS.

Ridge Tillage Improves Soil Properties, Sustains Diazotrophic Communities, and Enhances Extensively Cooperative Interactions Among Diazotrophs in a Clay Loam Soil

Reduced tillage practices [such as ridge tillage (RT)] have been potential solutions to the weed pressures of long-term no tillage (NT) and the soil-intensive disturbances caused by conventional tillage [such as moldboard plow (MP) tillage]. Although soil diazotrophs are significantly important in global nitrogen (N) cycling and contribute to the pool of plant-available N in agroecosystems, little is currently known about the responses of diazotrophic communities to different long-term tillage practices. In the current study, we investigated the differences among the effects of NT, RT, and MP on soil properties, diazotrophic communities, and co-occurrence network patterns in bulk and rhizosphere soils under soybean grown in clay loam soil of Northeast China. The results showed that RT and MP led to higher contents of total C, N, and available K compared to NT in both bulk and rhizosphere soils, and RT resulted in higher soybean yield than NT and MP. Compared to NT and RT, MP decreased the relative abundances of free-living diazotrophs, while it promoted the growth of copiotrophic diazotrophs. Little differences of diazotrophic community diversity, composition, and community structure were detected between RT and NT, but MP obviously decreased diazotrophic diversity and changed the diazotrophic communities in contrast to NT and RT in bulk soils. Soil nitrogenous nutrients had negative correlations with diazotrophic diversity and significantly influenced the diazotrophic community structure. Across all diazotrophs’ networks, the major diazotrophic interactions transformed into a cooperatively dominated network under RT, with more intense and efficient interactions among species than NT and MP. Overall, our study suggested that RT, with minor soil disturbances, could stabilize diazotrophic diversity and communities as NT and possessed highly positive interactions among diazotrophic species relative to NT and MP.

New insights into the distributions of nitrogen fixation and diazotrophs revealed by high-resolution sensing and sampling methods.

Nitrogen availability limits marine productivity across large ocean regions. Diazotrophs can supply new nitrogen to the marine environment via nitrogen (N2) fixation, relieving nitrogen limitation. The distributions of diazotrophs and N2 fixation have been hypothesized to be generally controlled by temperature, phosphorus, and iron availability in the global ocean. However, even in the North Atlantic where most research on diazotrophs and N2 fixation has taken place, environmental controls remain contentious. Here we measure diazotroph composition, abundance, and activity at high resolution using newly developed underway sampling and sensing techniques. We capture a diazotrophic community shift from Trichodesmium to UCYN-A between the oligotrophic, warm (25-29 °C) Sargasso Sea and relatively nutrient-enriched, cold (13-24 °C) subpolar and eastern American coastal waters. Meanwhile, N2 fixation rates measured in this study are among the highest ever recorded globally and show significant increase with phosphorus availability across the transition from the Gulf Stream into subpolar and coastal waters despite colder temperatures and higher nitrate concentrations. Transcriptional patterns in both Trichodesmium and UCYN-A indicate phosphorus stress in the subtropical gyre. Over this iron-replete transect spanning the western North Atlantic, our results suggest that temperature is the major factor controlling the diazotrophic community structure while phosphorous drives N2 fixation rates. Overall, the occurrence of record-high UCYN-A abundance and peak N2 fixation rates in the cold coastal region where nitrate concentrations are highest (~200 nM) challenges current paradigms on what drives the distribution of diazotrophs and N2 fixation.

Phosphorus but not nitrogen addition significantly changes diazotroph diversity and community composition in typical karst grassland soil

Diazotrophs have a vital role in nitrogen (N) supply to ecosystems. Yet, response of diazotroph community profiles and N2-fixation to N deposition and phosphorus (P) addition remains poorly understood. Two-year nutrient addition experiments (control, N, P, and a combination of N plus P) were conducted in a karst grassland. The relative abundances of diazotroph orders Rhizobiales and Rhodospirillales were higher in July, while those of Nostocales and Burkholderiales were higher in December. Diazotroph abundances and N2-fixation activity in July were significantly higher compared with December under control, while an opposite pattern was observed for diazotroph richness and Shannon diversity index. N, P, and N plus P plots showed no significant changes between July and December in terms of diazotroph abundance and diversity. Diazotroph abundances and N2-fixation activity in N and NP plots were significantly reduced compared with control plots in July. P addition plots in July showed a significant increase in diazotroph diversity and stronger interaction patterns among diazotroph taxa compared with control plots. Diazotroph diversity and community compositions were primarily affected by P availability. Overall, our findings suggested diazotroph diversity and community compositions were more sensitive to P addition than to N addition, and P availability played an important role in regulating N2 fixation by increasing diazotroph diversity in karst ecosystems. N and P addition may alter the dry-cold and wet-hot seasonal dynamics of diazotroph communities, while sampling months also affect diazotroph community changes among treatments (no difference between treatments in December).

Diazotrophic communities are more responsive to maize cultivation than phosphorus fertilization in an acidic soil

Although phosphorus (P) availability and plant cultivation affect diazotrophic populations, studies often consider individual factors rather than their combined effects. Their relative importance to diazotrophs remains poorly understood, especially in acidic soils with P deficiency. It is hypothesized that the influences of P fertilization and plant cultivation on diazotrophic communities differ in acidic soils. The objectives were to investigate these influences and identify the key determining factors. Maize was grown in an acidic soil that was supplemented with 0, 20, and 50 mg P kg− 1 for 42 days. Maize biomasses, plant nutrient contents, and soil physicochemical properties were determined. Based on the nifH gene, the abundances and community compositions of diazotrophs in the different plant/soil compartments (bulk soils, rhizosphere soils, and roots) were respectively investigated using quantitative PCR and high-throughput sequencing. P fertilization significantly improved the diazotrophic abundances in the bulk and rhizosphere soils, but not in the roots. The plant/soil compartments had stronger effects on the abundance and diversity of diazotrophs than did P fertilization. Furthermore, the plant/soil compartments influenced diazotrophic community composition, but P fertilization did not. However, in the same sampling site, P fertilization caused community variations in the bulk and rhizosphere soils, rather than in the roots. Although P fertilization affected the abundances and compositions of diazotrophic communities in the bulk and rhizosphere soils, the plant/soil compartments resulting from maize cultivation had more noticeable effects than did P fertilization. These findings improved the understanding of biological nitrogen fixation in acidic soils.

Soil properties and heavy metal concentrations affect the composition and diversity of the diazotrophs communities associated with different land use types in a mining area

Biological nitrogen fixed by diazotrophs is important because it is the largest natural source of nitrogen to support primary productivity and maintain ecosystem sustainability when anthropogenic nitrogen inputs are lacking, especially in mining areas. The present study selected six different land use types (a corn field, a vegetable field, an area of shrubs, a mine tailing, a forest and a citrus orchard) as variables related to the mining activities around the Siding mining area, Liuzhou, Guangxi Province, China. A 0–10 cm soil layer was sampled, and the heavy metal concentrations and soil physicochemical properties were measured. Illumina MiSeq sequencing technology was used to analyze the abundance and structural diversity of the diazotroph community. The distribution of diazotroph assemblages among the different land use types and the main soil environmental properties that drive diazotroph community compositions were determined. The results indicated that the phylum Cyanobacteria (85.89%) was the most dominant taxon in the diazotrophic community in the citrus orchard. However, in the soils of the five other land use types, Proteobacteria was the most dominant taxon in the diazotrophic community, with a high abundance that ranged from 60.08% to 99.96%. Spearman's correlation analysis and redundancy analysis results indicated that the soil organic matter (OM) content in the citrus orchard was significantly higher than that in other areas, and the difference in soil OM content might be the main reason for the difference in the diazotrophic communities. The diazotrophic communities and their associated diversity under different land use types were driven by soil heavy metal and soil N (including NH4+-N, NO3−-N and total N (TN)) contents. Specifically, TN was highly correlated with the NMDS1 score of the soil diazotrophic community, which indicated that the soil TN content was the main reason for the high similarity among the diazotrophic communities in the corn field, vegetable field and shrub soils. Overall, our study offers a detailed description of the characteristics of the diazotroph community structure under different land use types around a mining area.

Microbial community overlap between the phyllosphere and rhizosphere of three plants from Yongxing Island, South China Sea.

Phyllosphere and rhizosphere are unique and wide‐ranging habitats that harbor various microbial communities, which influence plant growth and health, and the productivity of the ecosystems. In this study, we characterized the shared microbiome of the phyllosphere and rhizosphere among three plants (Ipomoea pes‐caprae, Wedelia chinensis, and Cocos nucifera), to obtain an insight into the relationships between bacteria (including diazotrophic bacteria) and fungi, present on these host plants. Quantitative PCR showed that the abundances of the microbiome in the soil samples were significantly higher than those in the phyllosphere samples, though there was an extremely low abundance of fungi in bulk soil. High‐throughput sequencing showed that the alpha‐diversity of bacteria and fungi was higher in the rhizosphere than the phyllosphere samples associated with the same plant, while there was no obvious shift in the alpha‐diversity of diazotrophic communities between all the tested phyllosphere and soil samples. Results of the microbial composition showed that sample‐specific bacteria and fungi were found among the phyllosphere and rhizosphere of the different host plants. About 10%–27% of bacteria, including diazotrophs, and fungi overlapped between the phyllosphere and the rhizosphere of these host plants. No significant difference in microbial community structure was found among the tested rhizosphere samples, and soil properties had a higher influence on the soil microbial community structures than the host plant species.

Chemolithoautotropic Diazotrophy Dominates the Nitrogen Fixation Process in Mine Tailings.

Nutrient deficiency, especially bio-available nitrogen deficiency, often impedes the bioremediation efforts of mining generated tailings. Biological nitrogen fixation is a critical process necessary for the initial nitrogen buildup in tailings. Current knowledge regarding the diazotrophs that inhabit tailings is still in its infancy. Therefore, in this study, a comprehensive investigation combining geochemical characterization, sequence analyses, molecular techniques, and activity measurements was conducted to characterize the diazotrophic community residing in tailing environments. Significant differences between tailings and their adjacent soils in prokaryotic and diazotrophic communities were detected. Meanwhile, strong and significant correlations between the absolute abundance of the nitrogen fixation (nifH), carbon fixation (cbbL), sulfur oxidation (soxB), and arsenite oxidation (aioA) genes were observed in the tailings but not in the soils. The reconstructed nif-containing metagenome-assembled genomes (MAGs) suggest that the carbon fixation and sulfur oxidation pathways were important for potential diazotrophs inhabiting the tailings. Activity measurements further confirmed that diazotrophs inhabiting tailings preferentially use inorganic electron donors (e.g., elemental sulfur) compared to organic electron donors (e.g., sucrose), while diazotrophs inhabiting soils preferred organic carbon sources. Collectively, these findings suggest that chemolithoautotrophic diazotrophs may play essential roles in acquiring nutrients and facilitating ecological succession in tailings.

Revealing structure and assembly for rhizophyte-endophyte diazotrophic community in mangrove ecosystem after introduced Sonneratia apetala and Laguncularia racemosa

Biological nitrogen fixation (BNF) mediated by diazotrophic communities is a major source of bioavailable nitrogen in mangrove wetlands, which plays important roles in maintaining the health and stability of mangrove ecosystems. Recent large-scale mangrove afforestation activities have drawn great attention due to introduced mangrove species and their potential impacts on bio-functionalities of local ecosystems. However, the effects of introduced mangrove species on diazotrophic communities remain unclear. Here, we analyzed rhizosphere and endosphere diazotrophic communities between native mangrove species (Avicennia marina) and introduced mangrove species (Sonneratia apetala and Laguncularia racemose) by sequencing nifH gene amplicons. Our results showed that S. apetala and L. racemose introduction significantly (P < 0.05) increased nutrition components (e.g., total carbon and total nitrogen) in rhizosphere, as well as the diazotrophs richness in rhizosphere and endosphere. The relative abundance of clusters III diazotrophs in the rhizosphere and Rhizobium in the endosphere were significantly increased with L. racemosa or S. apetala introduction. Fe and pH were the main environmental factors driving the divergence of endophyte-rhizophyte diazotrophs between native and introduced mangroves. The correlation-based network analyses indicated that the interaction among rhizophyte-endophyte diazotrophs is more harmonious in native mangrove, while there exist more competition in introduced mangroves. These findings expand our current understanding of BNF in mangrove afforestation, and providing new perspectives to sustainable management of mangrove ecosystem.

Latitudinal constraints on the abundance and activity of the cyanobacterium UCYN‐A and other marine diazotrophs in the North Pacific

AbstractThe number of marine environments known to harbor dinitrogen (N2)‐fixing (diazotrophic) microorganisms is increasing, prompting a reassessment of the biogeography of marine diazotrophs and N2fixation rates (NFRs). Here, we investigate the diversity, abundance, and activity of diazotrophic microorganisms in the North Pacific Subtropical Gyre (NPSG), a diazotrophic habitat, and the North Pacific Transition Zone (NPTZ), a region characterized by strong physical, chemical, and biological gradients. Samples were collected on two springtime meridional cruises during 2016 and 2017, spanning from 23.5°N to 41.4°N along 158°W. We observed an abrupt decrease in diazotrophic abundances near the southern edge of the NPTZ, which coincided with a salinity front and with a ∼10‐fold increase inSynechococcusabundance, but without a concomitant change in phosphate or nitrate concentrations. In NPSG waters south of this diazotrophic boundary,nifHgenes and NFRs were consistently detected and diazotrophic communities were dominated by UCYN‐A, an uncultivated, symbiotic cyanobacterium (2.8 × 103to 1.0 × 106nifHgene copies L−1). There was a significant positive relationship between quantitative polymerase chain reaction‐derived UCYN‐AnifHgene abundances and community NFRs in the NPSG, suggesting a large contribution of UCYN‐A to community NFRs. In the NPTZ waters to the north, NFRs were low or undetected andnifHgenes were rare, with the few detected sequences represented by UCYN‐A and noncyanobacterial diazotrophs. The patterns we observed in UCYN‐A abundance in the context of local biogeochemistry suggest that the environmental controls of this organism may differ from those of cultivated marine cyanobacterial diazotrophs.

Insight to key diazotrophic community during composting of dairy manure with biochar and its role in nitrogen transformation

Analyzing diazotrophic community may help to understand nitrogen transformation in composting and improves the final compost quality. In this study, diazotrophic community dynamics were investigated in terms of nifH gene during dairy manure and corn straw composting with biochar addition using high-throughput sequencing. Biochar decreased the diversity of diazotrophic community and altered diazotroph community structure during composting. At phylum level, Proteobacteria, Actinobacteria and Firmicutes were dominant diazotrophic communities throughout composting process. Biochar addition registered higher correlation coefficient (R) between physicochemical factors (temperature, ammonium (NH4+-N) and nitrate (NO3−-N)) and diazotroph community composition. Rhodopseudomonas and Pseudoxanthomonas was the key diazotrophic communities influencing NH4+-N transformation in control (CK) and biochar compost (BC), respectively, while for NO3−-N transformation Clostridium and Bradyrhizobium in CK, Azospira and Methylocystis in BC served as predominant factors. These results indicated that addition of biochar altered the key diazotroph communities influencing nitrogen transformation. Furthermore, some diazotrophs (e.g. Rhodopseudomonas, Bradyrhizobium and Azospira) affecting NH4+-N and NO3−-N transformation were also observed to be mediating total nitrogen (TN). Interestingly, interactions between diazotrophic communities were observed and these interactions could also influence nitrogen transformation.

Seasonal variation of epiphytic bacteria in the phyllosphere of Gingko biloba, Pinus bungeana and Sabina chinensis.

Phyllosphere harbors diverse microorganisms, which influence plant growth and health. In order to understand the extent to which environmental factors affect epiphytic microbial communities, we characterized microbial communities on leaves of three separate tree species present on the college campus, and also present within a forest park over two seasons. Quantitative PCR analysis showed the quantity of 16S rRNA genes was lower in May compared with October, while the abundances of functional genes (nifH and bacterial amoA genes) were extremely high in May. High-throughput sequencing revealed a large variation in the diversity and composition of bacterial and diazotrophic communities over the two seasons, and showed the abundance of functional genera, such as Nocardioides, Bacillus and Zoogloea were significantly elevated in May. In addition, xenobiotic biodegradation pathways of bacterial communities were clearly elevated in May. Network analysis showed the correlations between phyllospheric bacteria in May were more complex than that in October and showed greater negative correlations. These results were consistent in all tree species in this study. This study showed that phyllospheric bacteria varied greatly in different seasons, which implies that different growing seasons should be considered in the exploitation of the interactions between phyllospheric microorganisms and host plants.