Ecology of heterotrophic dinitrogen fixation in the rhizosphere of mangrove plant community at the Ganges river estuary in India

Summary Heterotrophic dinitrogen fixation in root associations of successional stages of the tropical mangrove plant community at the Ganges river estuary in India was investigated by excised-root acetylene reduction assay, and enumeration and identification of diazotrophic bacteria from sediment, root and tidal water samples. High to very high rates of nitrogenase activity (64–130 nmol C₂H₄/g dry root/h) were associated with washed excised roots of seven common early-successional mangrove species at the inundated swamps. Declining, late-successional mangroves at the occasionally inundated ridges had considerably lower values and the “declined” mangroves and other non-littoral species at embankment protected highlands had very low to insignificant values of root nitrogenase activity. Total and inorganic nitrogen contents of the mangrove sediments were low and were positively related to the stages of physiographic succession. Plant-associated sediments of particularly the old formation swamps had very high C/N ratios. Nine isolates of nitrogen-fixing bacteria belonging to all known O₂ response groups were distinguished from a large population of diazotrophs associated with roots of mangroves and other associate plant species of the community. The isolates differed with respect to their N₂-fixation efficiency and halotolerance in pure culture. There was no specificity of any of the bacterial isolates to any of the plant species of the community but a higher number of efficient isolates were seen to be associated with mangroves at the swampy succession. Sediment-free tidal water also contained a large population of microaerophilic and anaerobic N₂-fixing bacteria.     

Arbuscular mycorrhizal community composition associated with two plant species in a grassland ecosystem

Arbuscular mycorrhizal (AM) fungi are biotrophic symbionts colonizing about two-thirds of land plant species and found in all ecosystems. They are of major importance in plant nutrient supply and their diversity is suggested to be an important determinant of plant community composition. The diversity of the AM fungal community composition in the roots of two plant species (Agrostis capillaris and Trifolium repens) that co-occurred in the same grassland ecosystem was characterized using molecular techniques. We analysed the small subunit (SSU) ribosomal RNA gene amplified from a total root DNA extract using AM fungal-specific primers. A total of 2001 cloned fragments from 47 root samples obtained on four dates were analysed by restriction fragment length polymorphism, and 121 of them were sequenced. The diversity found was high: a total of 24 different phylotypes (groups of phylogenetically related sequences) colonized the roots of the two host species. Phylogenetic analyses demonstrate that 19 of these phylotypes belonged to the Glomaceae, three to the Acaulosporaceae and two to the Gigasporaceae. Our study reveals clearly that the AM fungal community colonizing T. repens differed from that colonizing A. capillaris, providing evidence for AM fungal host preference. In addition, our results reveal dynamic changes in the AM fungal community through time.     

Arbuscular mycorrhizal fungi affect plant community structure under various nutrient conditions and stabilize the community productivity

Soil biota could have a significant impact on plant productivity and diversity through benefiting plants and mediating plant–plant interaction. However, it is poorly understood how soil biotic factors interaction with abiotic environments affect plant community diversity and composition. Here, we investigate the community‐level consequences of arbuscular mycorrhizal fungi (AMF) interactions with multiple nutrients and their ecological stoichiometry. We conducted a greenhouse experiment manipulating nitrogen (N) and phosphorus (P) to create soil nutrient availability and N:P gradients for microcosm communities with and without AMF. We found that AMF suppressed plant diversity at low P levels, whereas it did not alter the diversity at high P levels because of trade‐offs in the abundance of the dominant and subordinate species. AMF reduced plant diversity at the intermediate N:P ratios, while AMF did not affect the diversity at low and high N:P ratios. P addition decreased the mycorrhizal contribution to community productivity, whereas N addition reduced the negative effects of AMF on productivity at high P levels. AMF decreased community productivity at low N:P ratios but increased it at high N:P ratios. AMF increased the stoichiometric homoeostasis of plant communities, which was positively correlated with the stability of productivity under variations in soil N:P ratios. Our study demonstrates that both resource availability and stoichiometry influence the effect of AMF on plant community productivity and diversity and suggests that AMF may increase the stability of plant communities under variations in the soil nutrients by increasing the stoichiometric homoeostasis of the plant community.     

Arbuscular mycorrhizal fungi alter plant community composition along a grazing gradient in Inner Mongolia Steppe

Arbuscular mycorrhizal fungi (AMF) have a significant influence on plant productivity and diversity in non-grazing grassland. However, the interactive effects between grazing intensity and AMF on plant community composition in natural grassland communities are not well known. We conducted a field experiment that manipulated AMF colonization and grazing intensity to study the impact of AMF suppression on plant community composition and nutrient status over 2 years (2015–2016) with contrasting rainfall levels. We found that AMF root colonization was significantly reduced by the application of the fungicide benomyl as a soil drench. Grazing intensity regulated plant community composition and aboveground biomass mainly by reducing the growth of Leymus chinensis over 2 years. AMF suppression increased the growth of Chenopodium glaucum, but it did not alter other plant species across all grazing intensities. The effects of AMF suppression on plant community composition changed along a grazing gradient considerably between years: AMF suppression increased the biomass of C. glaucum across all grazing intensities in 2015, but slightly increased it in 2016. Interactions between AMF suppression and grazing intensity altered the phosphorus concentration of Stipa grandis and Cleistogenes squarrosa in 2015 but not in 2016. AMF suppression decreased the shoot phosphorus content of L. chinensis but increased that of C. glaucum across all grazing intensities. Our results indicate that grazing intensity substantially alters aboveground community biomass and affects growth of dominant species; AMF by itself have limited effects on plant communities along a grazing gradient in typical steppe.     

漳江口红树林沉积物可培养真菌多样性

红树林真菌由于其独特的生态特征、多样性特点和丰富的新型生物活性代谢产物而引起了广泛的关注。本文以漳江口红树林保护区4个采样点的沉积物为研究对象,采用8种筛选培养基（RBM、PDA、CDA、Martin、YM、SDA、ISP2和R2A）分离可培养真菌,根据ITS序列分析对其进行物种鉴定,并利用多样性指数分析评价不同采样点真菌群落的差异。结果共分离到274株真菌,隶属于2门12纲23目39科52属,其中优势属为枝顶孢属（Acremonium）（20.8%）和青霉属（Penicillium）（11.3%）,其次是枝孢属（Cladosporium）（7.3%）和帚枝霉属（Sarocladium）（7.3%）。8种筛选培养基中,分离出真菌种属类型最丰富的培养基是RBM,其次是PDA。根据Shannon-Wiener多样性指数（H′）、Simpson优势度指数（D）、Magalef丰富度指数（R）和Pielou均匀度指数（J）的分析结果,在属的水平上,距海洋的位置越近,真菌的群落多样性越高,真菌分布越均匀。     

Arbuscular mycorrhizal fungi and plant symbiosis in a saline-sodic soil

The seasonality of arbuscular mycorrhizal (AM) fungi–plant symbiosis in Lotus glaber Mill. and Stenotaphrum secundatum (Walt.) O.K. and the association with phosphorus (P) plant nutrition were studied in a saline-sodic soil at the four seasons during a year. Plant roots of both species were densely colonized by AM fungi (90 and 73%, respectively in L. glaber and S. secundatum) at high values of soil pH (9.2) and exchangeable sodium percentage (65%). The percentage of colonized root length differed between species and showed seasonality. The morphology of root colonization had a similar pattern in both species. The arbuscular colonization fraction increased at the beginning of the growing season and was positively associated with increased P concentration in both shoot and root tissue. The vesicular colonization fraction was high in summer when plants suffer from stress imposed by high temperatures and drought periods, and negatively associated with P in plant tissue. Spore and hyphal densities in soil were not associated with AM root colonization and did not show seasonality. Our results suggest that AM fungi can survive and colonize L. glaber and S. secundatum roots adapted to extreme saline-sodic soil condition. The symbiosis responds to seasonality and P uptake by the host altering the morphology of root colonization.     

泰山丛枝菌根真菌群落结构特征

2007年对泰山植被根围内丛枝菌根(arbuscular mycorrhiza,AM)真菌群落组成、数量、分布及其与植物多样性的关系进行了研究。从泰山傲徕峰、黑龙潭库区等样地共分离出4属16种AM真菌:球囊霉属Glomus 9种、无梗囊霉属Acaulospora 4种、巨孢囊霉属Gigaspora 2种和盾巨孢囊霉属Scutellospora1种。其中,球囊霉属Glomus及聚球囊霉Glomus fasciculatum的孢子密度、相对多度、分布频度和重要值均最高,分别为泰山植被区根围内AM真菌优势属和优势种。各样地之间Sorenson相似系数在0.60和0.85之间。植被数量与孢子密度(r=0.80,p0.01)、植物种的丰富度与AM真菌种的丰富度(r=0.77,p0.01)以及与孢子密度(r=0.59,p0.01)均呈极显著正相关关系。研究结果表明植物多样性对于提高AM真菌多样性发挥极为重要的作用。     

Arbuscular mycorrhizal fungal communities in plant roots are not random assemblages

We investigated whether arbuscular mycorrhizal fungal (AMF) communities in plant roots are random subsets of the local taxon pool or whether they reflect the action of certain community assembly rules. We studied AMF small subunit rRNA gene sequence groups in the roots of plant individuals belonging to 11 temperate forest understorey species. Empirical data were compared with null models assuming random association. Distinct fungal species pools were present in young and old successional forest. In both forest types, the richness of plant-AMF associations was lower than expected by chance, indicating a degree of partner selectivity. AMF communities were generally not characteristic of individual plant species, but those associated with ecological groups of plant species - habitat generalists and forest specialists - were nonrandom subsets of the available pool of fungal taxa and differed significantly from each other. Moreover, these AMF communities were the least distinctive in spring, but developed later in the season. Comparison with a global database showed that generalist plants tend to associate with generalist AMF. Thus, the habitat range of the host and a possible interaction with season played a role in the assembly of AMF communities in individual plant root systems.     

Arbuscular mycorrhizal fungi enhance P uptake and alter plant morphology in the invasive plant Microstegium vimineum

Invasive plant species can interact with native soil microbes in ways that change how they use nutrients and allocate biomass. To examine whether Microstegium vimineum form symbiotic associations with arbuscular mycorrhizal fungi (AMF) and whether AMF mediate nutrient acquisition and growth of the plant, we conducted a field survey in Raleigh, NC and Hangzhou, China and two experiments in growth chambers. This is the first report that M. vimineum is mycorrhizal, with colonization rates of 47 and 21 % in its native and invaded range, respectively. In the growth chamber, addition of an AMF inoculum mixture significantly promoted M. vimineum biomass accumulation in both field and sterilized soils, particularly after 64 days of growth. Arbuscular mycorrhizal fungi also increased plant phosphorous (P) uptake but did not consistently affect total plant nitrogen (N) acquisition, leading to decreases in plant N:P ratios. More interestingly, AMF significantly altered plant morphology, increasing the number of stolons and aerial roots per individual (59 and 723 %), aerial roots per gram aboveground biomass (374 %) and aerial roots per stolon (404 %). Our results suggest that mycorrhizal enhancement of plant growth by stimulating tillering may serve as another mechanism by which M. vimineum can quickly take over new territory. Future studies on invasive plant-microbial interactions are needed to understand the mechanisms through which microbes contribute to the competitive ability of invasive plants.     

Arbuscular mycorrhizal fungal community divergence within a common host plant in two different soils in a subarctic Aeolian sand area

There is rising awareness that different arbuscular mycorrhizal (AM) fungi have different autoecology and occupy different soil niches and that the benefits they provide to the host plant are dependent on plant-AM fungus combination. However, the role and community composition of AM fungi in succession are not well known and the northern latitudes remain poorly investigated ecosystems. We studied AM fungal communities in the roots of the grass Deschampsia flexuosa in two different, closely located, successional stages in a northern Aeolian sand area. The AM fungal taxa richness in planta was estimated by cloning and sequencing small subunit ribosomal RNA genes. AM colonization, shoot δ ¹³C signature, and %N and %C were measured. Soil microbial community structure and AM fungal mycelium abundance were estimated using phospholipid (PLFA) and neutral lipid (NLFA) analyses. The two successional stages were characterized by distinct plant, microbial, and fungal communities. AM fungal species richness was very low in both the early and late successional stages. AM frequency in D. flexuosa roots was higher in the early successional stage than in the late one. The AM fungal taxa retrieved belonged to the genera generally adapted to Arctic or extreme environments. AM fungi seemed to be important in the early stage of the succession, suggesting that AM fungi may help plants to better cope with the harsh environmental conditions, especially in an early successional stage with more extreme environmental fluctuations.     

Arbuscular mycorrhizal fungi reduce the differences in competitiveness between dominant and subordinate plant species

In grassland communities, plants can be classified as dominants or subordinates according to their relative abundances, but the factors controlling such distributions remain unclear. Here, we test whether the presence of the arbuscular mycorrhizal (AM) fungus Glomus intraradices affects the competitiveness of two dominant (Taraxacum officinale and Agrostis capillaris) and two subordinate species (Prunella vulgaris and Achillea millefolium). Plants were grown in pots in the presence or absence of the fungus, in monoculture and in mixtures of both species groups with two and four species. In the absence of G. intraradices, dominants were clearly more competitive than subordinates. In inoculated pots, the fungus acted towards the parasitic end of the mutualism–parasitism continuum and had an overall negative effect on the growth of the plant species. However, the negative effects of the AM fungus were more pronounced on dominant species reducing the differences in competitiveness between dominant and subordinate species. The effects of G. intraradices varied with species composition highlighting the importance of plant community to mediate the effects of AM fungi. Dominant species were negatively affected from the AM fungus in mixtures, while subordinates grew identically with and without the fungus. Therefore, our findings predict that the plant dominance hierarchy may flatten out when dominant species are more reduced than subordinate species in an unfavourable AM fungal relationship (parasitism).     

Arbuscular mycorrhizal fungal community composition associated with Juniperus brevifolia in native Azorean forest

The communities of glomeromycotan fungi (arbuscular mycorrhizal fungi, AMF) under native Juniperus brevifolia forest from two Azorean islands, Terceira and São Miguel, were compared, mainly by spore morphology, and when possible, by molecular analysis. Thirty-nine morphotypes were detected from 12 genera. Glomeromycotan fungal richness was similar in Terceira and São Miguel, but significantly different among the four fragments of native forest. Spore diversity and community composition differed significantly between the two islands. The less degraded island, Terceira, showed 10 exclusive morphotypes including more rare types, whereas the more disturbed forest on São Miguel showed 13 morphs, mostly of common types. Forests from Terceira were dominated by Acaulosporaceae and Glomeraceae. Whereas members of Acaulosporaceae, Glomeraceae and Ambisporaceae were most frequent and abundant in those from São Miguel. Spore abundance was greatest on Terceira, and correlated with soil chemical properties (pH), average monthly temperature and relative humidity.     

Arbuscular mycorrhizal and septate endophyte fungal associations in lycophytes and ferns of south India

We studied extent and type of arbuscular mycorrhizal (AM) and septate endophytic (SE) fungal associations in five lycophytes and 50 ferns collected from Eastern and Western Ghats regions. Of the 54 species and one variety (belonging to 31 genera) examined; 54 taxa had AM association and AM fungal structures were absent in Marsilea quadrifolia. This is the first report of AM and SE fungal status for 26 species each. Of the 55 taxa examined, AM morphology has been evaluated for the first time in 51 species. The hydrophytic fern Salvinia molesta was mycorrhizal and non-mycorrhizal at different sites. All the epiphytic and saxicolous species examined were mycorrhizal. The percentage of AM colonization ranged from 22.23 (Christella parasitica) to 82.20 (Adiantum lunulatum) in ferns and 53.46 (Selaginella bryopteris) to 84.34 (Selaginella sp.) in lycophytes. Epiphytic life-forms had the maximum average AM colonization levels, whereas aquatic life-forms had the minimum colonization levels. The percentage root length colonized by septate fungi ranged between 0.59 in Ophioglossum reticulatum and 16.36 in Pteris pellucida. The root length with AM and SE fungal structures as well as their total colonization significantly varied among the taxa examined. Most of the lycophytes and ferns had intermediate-type of AM morphology with a few exhibiting Paris-type. AM fungal spore numbers ranged from 1.0 (Angiopteris evecta, Pteridium aquilinum) to (Nephrolepis exaltata) 9.3 spores per 25 g soil and varied significantly among taxa. AM fungal spore morphotypes belonging to Claroideoglomus, Funneliformis, Glomus and Rhizophagus were recorded.     

Arbuscular mycorrhizal fungi alter phosphorus relations of broomsedge (Andropogon virginicus L.) plants

Broomsedge (Andropogon virginicus L.) is a dominant grass revegetating many abandoned coal-mined lands in West Virginia, USA. Residual soils on such sites are often characterized by low pH, low nutrients, and high aluminium. Experiments were conducted to assess the resistance of broomsedge to limited phosphorus (Pi) availability and to investigate the role that arbuscular mycorrhizal (AM) fungi play in aiding plant growth under low Pi conditions. Pregerminated mycorrhizal and non-mycorrhizal seedlings were grown in a sand-culture system with nutrient solutions containing Pi concentrations ranging from 10 to 100 microM for 8 weeks. Non-mycorrhizal plants exhibited severe inhibition of growth under Pi limitation (<60 microM). Colonization by AM fungi (combined Glomus clarum Nicolson & Schenck and Gigaspora gigantea (Nicol. & Gerd.) Gerd. & Trappe) greatly enhanced host plant growth at low Pi concentrations, but did not benefit growth when Pi was readily available (100 microM). In comparison to non-mycorrhizal plants, mycorrhizal plants had higher phosphorus use efficiency at low Pi concentrations and maintained nearly constant tissue nutrient concentrations across the gradient of Pi concentrations investigated. Manganese (Mn) and sodium (Na) accumulated in shoots of non-mycorrhizal plants under Pi limitation. Mycorrhizal plants exhibited lower instantaneous Pi uptake rates and significantly lower C(min) values compared to non-mycorrhizal plants. These patterns suggest that the symbiotic association between broomsedge roots and AM fungi effectively maintains nutrient homeostasis through changes in physiological properties, including nutrient uptake, allocation and use. The mycorrhizal association is thus a major adaptation that allows broomsedge to become established on infertile mined lands.     

Structural analysis of fouling community development in the damariscotta river estuary,maine

The development of the marine fouling community at a station in the Damariscotta River Estuary was studied in respect to current theories of ecological succession. This community may be divided into two subcomponents, the primary fouling assemblage, which attaches directly to the substratum, and the secondary foulers which live on and among the primary fouling assemblage. Structure of the total fouling community was examined using measures of diversity, evenness, and dominance, as well as the numbers of species and abundance.Multiple regression analysis was used to establish a hierarchical list of the factors influencing secondary fouling community structure. A definite successional trend was observed, although the length of the study precluded any observation of a climax community.