Do plant clumps constitute microbial hotspots in semiarid Mediterranean patchy landscapes?

Three semiarid Mediterranean patchy landscapes were investigated to test the existence of a microsite effect (i.e. plant canopy vs. inter-canopy) on soil microbial communities. Surface soil samples were independently taken from both microsites under naturally changing conditions of humidity and temperature through the year. In gypsiferous soils covered with a shrub steppe, improved physical and chemical soil properties were registered underneath the plant canopy, where the densest and most active microbial communities were also detected (e.g. microbial biomass C averaged 531 and 202 mg kg⁻¹ in canopy and inter-canopy areas, respectively). In calcareous perennial tussock grasslands, either growing on soils over limestones or alluvial deposits, the microsite effect was not so marked. Soil humidity, temperature and total organic C were homogeneously distributed over the landscape conditioning their uniform microbial activity under field moisture conditions (ATP content averaged 853 and 885 nmol kg⁻¹ in canopy and intercanopy areas, respectively). However, readily mineralizable C and microbial biomass C were preferentially accumulated in soils underneath the tussocks determining their larger potential microbial activity (e.g. C hydrolysis capacity under optimal conditions). In conclusion, plant clumps either functioned as microbial hotspots where enhanced microbially driven ecosystem processes took place or as microbial banks capable of undergoing a burst of activity under favourable climatic conditions. Our results provide experimental evidence of a non-patchy distribution of certain soil microbial properties in semi-arid Mediterranean patchy ecosystems.     

Tree species-mediated spatial patchiness of the composition of microbial community and physicochemical properties in the topsoils of a tropical montane forest

To evaluate the importance of plant-soil feedbacks in forest ecosystems, it is fundamental to understand the spatial range within which plant species control soil physicochemical and microbial properties. We investigated the spatial pattern of soil properties associated with canopy trees in a tropical montane forest on Mt. Kianbalu, Borneo. We analyzed soil physicochemical properties and microbial communities (biomarker lipid abundance) as a function of soil depth and distance from the tree trunk of a conifer (Dacrydium gracilis) or a broadleaf tree (Lithocarpus clementianus). The concentration of condensed tannins and fungi-to-bacteria were higher beneath Dacrydium than beneath Lithocarpus. Furthermore, carbon-degrading enzyme activities were lower beneath Dacrydium. These effects of the tree species were more distinct on soil properties beneath the tree crown than on those outside the tree crown. These effects appeared to be largely due to differences in litter chemistry, and the distinct set of soil properties formed corresponding to the above canopy crown. In conclusion, the species-rich forest on the tropical mountain contains spatially distinct units of soil properties associated with canopy trees, and this spatial pattern can influence ecosystem dynamics in the forest through plant-soil feedback effects.     

Organic acid exudation by mycorrhizal Andropogon virginicus L. (broomsedge) roots in response to aluminum

Elevated aluminum (Al) availability limits plant growth on acidic soils. Although this element is found naturally in soils, acidic conditions create an environment where Al solubility increases and toxic forms of Al impact plant function. Plant resistance to Al is often attributed to organic acid exudation from plant roots and the chelation of cationic Al in the rhizosphere. The association of arbuscular mycorrhizal (AM) fungi with the roots of plants may alleviate Al toxicity by altering soil Al availability or plant exposure through the binding of Al to fungal structures or through the influence of fungi on exudation from roots. Diverse communities of AM fungi are found in soil ecosystems and research suggests that AM fungi exhibit functional diversity that may influence plant performance under varying edaphic environments. In the present study, we evaluated acidic isolates of six AM species in their responses to Al. Andropogon virginicus (broomsedge), a warm-season grass that commonly grows in a range of stressful environments including acidic soils, was used as a plant host for Acaulospora morrowiae, Glomus claroideum, Glomus clarum, Glomus etunicatum, Paraglomus brasilianum, and Scutellospora heterogama. Fungal spores were germinated and exposed to 0 or 100 μM Al on filter paper in sand culture or were grown and exposed to Al in sand culture in association with A. virginicus. Short- and long-term responses to Al were evaluated using direct measurements of fungal spore germination, hyphal elongation, and measurements of A. virginicus colonization and plant growth as a phytometer of AM function in symbio. Spore germination and hyphal elongation varied among AM species in response to Al, but patterns were not consistent with the influences of these AM species on A. virginicus under Al exposure. Exposure to Al did not influence colonization of roots, although large differences existed in colonization among fungal species. Plants colonized by G. clarum and S. heterogama exhibited the least reduction in growth when exposed to Al, produced the highest concentrations of Al-chelating organic acids, and had the lowest concentrations of free Al in their root zones. This pattern provides evidence that variation among AM fungi in Al resistance conferred to their plant hosts is associated with the exudation of Al-binding organic acids from roots and highlights the role that AM fungal diversity may play in plant performance in acidic soil environments.     

Soil fungal and prokaryotic community structure exhibits differential short-term responses to timber harvest in the Pacific Northwest

Conventional clear-cut timber harvest is a widespread industrial practice across the Pacific Northwest;however,information regarding how these practices impact soil microbial community structure at the regional scale is limited.With evidence of consistent and substantial impact of harvest on soil microbial functional profiles across the region(despite a range of environmental conditions),the objective of this study was to determine the extent to which harvest also influences the structure of prokaryotic and fungal soil microbial communities,and how generalized these trends are throughout the geographic region.Paired soil samples were collected one year before and after harvest across nine second-growth Douglas-fir forests in the Pacific Northwest.Total community DNA was extracted from the soils,and high-throughput targeted gene sequencing of the 16 S r RNA gene for prokaryotes and the internal transcribed spacer(ITS)gene for fungi was performed.Alpha diversity was consistently and significantly higher after harvest;it was moderately so for fungal communities(+14.6%),but only marginally so for prokaryotic communities(+2.0%).Similarly,on average,a greater proportion of the variation in the community structure of fungi(20.1%)at each site was associated with forest harvest compared to that of prokaryotes(13.2%).Overall,the greatest influence of timber harvest on soil microbial communities appeared to be a relative depletion of ectomycorrhizal fungi,with a concomitant enrichment of saprotrophic fungi.Understanding the short-term responses of soil microbial communities across the region,particularly those of tree root-associated symbionts,may aid our understanding of the role soil microbial communities play in ecological succession.     

Methyl-mercury affects microbial activity and biomass,bacterial community structure but rarely the fungal community structure

Monomethyl-mercury is one of the most toxic compounds. Methylation of Hg usually appears under anoxic conditions. In Swiss forest soils, methyl-Hg concentrations of up to 3 μg kg⁻¹ soil dw have been observed, but the impact of methyl-Hg on soil microorganisms have rarely been examined so far. In this study, we investigated the effect of increasing concentrations of methyl-Hg (0, 5, 20, 90 μg kg⁻¹ soil dw) on the microbial communities in various forest soils differing in their physico-chemical properties. Experiments were conducted in microcosms under controlled conditions and the basal respiration (BR), the microbial biomass carbon (MBC) and the bacterial and fungal community structures using T-RFLP-profiling were investigated. BR was significantly affected by methyl-Hg. In general, the BR increased with increasing methyl-Hg concentrations, whereas the MBC was significantly reduced. Bacterial communities were more sensitive to methyl-Hg than fungal communities. In five out of seven soils, the bacterial community structures differed significantly between the treatments whereas the fungal communities did not. The impact of methyl-Hg on the soil bacterial communities was site specific. In one soil, a methyl-Hg concentration of already 5 μg kg⁻¹ soil dw significantly affected the relative abundance of 13% bacterial operational taxonomic units (OTU), whereas in other soils concentrations of even 90 μg kg⁻¹ soil dw rarely affected the abundance of OTUs. In this study, for the first time, the impact of methyl-Hg on soil bacterial and fungal communities in forest soils was assessed. We showed that its impact strongly depends on the physico-chemical conditions of the soil and that bacterial communities were more sensitive to methyl-Hg than fungi.     

Effect of invasive Acacia dealbata Link on soil microorganisms as determined by PCR-DGGE

Acacia dealbata Link is an Australian woody legume that has become a serious environmental problem in Northwest Spain where it forms dense monospecific patches modifying the structure of different native ecosystems and threatening native aboveground biodiversity. In spite of the dramatic changes observed in the vegetation of invaded sites little is known about the consequences of invasion for soil microorganisms. To investigate the effect of A. dealbata invasion on the structure of soil fungi and bacteria communities, samples were taken from invaded and non-invaded areas from three different ecosystems in Northwest Spain: pine forest, shrubland and grassland. In each ecosystem type, soil samples were taken in areas of native vegetation, areas invaded by A. dealbata and in the transition zone between native and invaded vegetation. Soil microorganisms were analyzed in the different samples by PCR-DGGE using general primers for eubacteria and fungi. Soil analyses were also performed to evaluate the effect of A. dealbata invasion on soil fertility.The invasion by A. dealbata consistently increased soil N, C, organic matter and exchangeable P content in the three studied ecosystems. A clear effect of the invasion on the overall structure of microorganism communities was only observed in the shrubland where soil fungal communities in the invaded and transition areas clustered together and apart from the native soil. Significant differences in soil microorganisms richness and diversity between invaded and not invaded soils were only found in the grassland. Grassland invasion by A. dealbata lead to a significant increase of bacterial richness and to a significant reduction in fungal richness and diversity. Our results show that although the changes on soil chemistry due to A. dealbata invasion are consistent among the studied ecosystems, the effect on soil microorganisms depends on the ecosystem type affected by the invasion.     

Soil bacterial and fungal communities along a soil chronosequence assessed by fatty acid profiling

Microbial communities are important components of terrestrial ecosystems. The importance of their diversity and functions for natural systems is well recognized. However, a better understanding of successional changes of microbial communities over long time scales is still required. In this work, the size and composition of microbial communities in soils of a deglaciation chronosequence at the Damma glacier forefield were studied by fatty acid profiling. Soil fatty acid concentrations clearly increased with soil age. The abundances of arbuscular mycorrhizal fungi (AMF), bacteria and other soil fungi, however, were more affected by abiotic soil parameters like carbon content and pH than by soil age. Analysis of ratios of the different microbial groups (AMF, fungi, bacteria) along the soil chronosequence indicated that: i) the ratios of AMF to bacteria and AMF to fungi decreased with soil age; and ii) the ratio of fungi to bacteria remained unchanged along the soil chronosequence. These two pieces of evidence suggest that the evolution of this ecosystem proceeds at an uneven pace over time and that the role of AMF is less important in older, more organic and acidified soils than in mineral soils. In contrast to other studies, no successional replacement of bacteria with fungi in more acidified and organic soil was observed.     

科尔沁沙地东南部植被类型变化对土壤磷素状况的影响

In the past 50 years, large areas of the Horqin sandy land were afforested to prevent desertification. Although the afforestation policy appears successful, many people now doubt whether it is suitable to plant trees with high density on the poor soils in semiarid regions. Little is known about the impacts of afforestation on the sandy soil properties, although the evaluation of these impacts is fundamental to judge the rationality of afforestation policy. Soil phosphorus （P） fractions, acid phosphomonoesterase activities, and other soil chemical properties were compared among five adjoining typical ecosystems on poor sandy soils in southeastern Horqin sandy land. The ecosystems studied are natural elm savanna, degraded grassland, Mongolian pine （Pinus sylvestris var. mongolica） plantation, Chinese pine （Pinus tabulaeformis） plantation, and mixed plantation of Mongolian pine and poplar （Populus simonii）. The results showed that organic P dominated soil P （47%-65%） was the principal source of available P. The degradation of elm savanna to grassland significantly reduced soil pH and resulted in an overall reduction in soil fertility, although slightly increased labile inorganic P. Grassland afforestation had no significant influence on soil pH, organic carbon, and total N but significantly reduced total P. Impacts of grassland afforestation on soil P fractions depended on tree species. Natural elm savanna had higher soil P conserving ability than artificial plantations. Therefore, with the aim of developing a sustainable ecosystem, we suggested that vegetations with low nutrient demand （particularly P） and efficient nutrient cycling would be more suitable for ecosystem restoration in the semiarid region.     

Structural and functional diversity of soil bacterial and fungal communities following woody plant encroachment in the southern Great Plains

In the southern Great Plains (USA), encroachment of grassland ecosystems by Prosopis glandulosa (honey mesquite) is widespread. Mesquite encroachment alters net primary productivity, enhances stores of C and N in plants and soil, and leads to increased levels of soil microbial biomass and activity. While mesquite’s impact on the biogeochemistry of the region is well established, it effects on soil microbial diversity and function are unknown. In this study, soils associated with four plant types (C₃ perennial grasses, C₄ midgrasses, C₄ shortgrasses, and mesquite) from a mesquite-encroached mixed grass prairie were surveyed to in an attempt to characterize the structure, diversity, and functional capacity of their soil microbial communities. rRNA gene cloning and sequencing were used in conjunction with the GeoChip functional gene array to evaluate these potential differences. Mesquite soil supported increased bacterial and fungal diversity and harbored a distinct fungal community relative to other plant types. Despite differences in composition and diversity, few significant differences were detected with respect to the potential functional capacity of the soil microbial communities. These results may suggest that a high level of functional redundancy exists within the bacterial portion of the soil communities; however, given the bias of the GeoChip toward bacterial functional genes, potential functional differences among soil fungi could not be addressed. The results of this study illustrate the linkages shared between above- and belowground communities and demonstrate that soil microbial communities, and in particular soil fungi, may be altered by the process of woody plant encroachment.     

Relationship between soil enzyme activities, nutrient cycling and soil fungal communities in a northern hardwood forest

Soil fungi are highly diverse and act as the primary agents of nutrient cycling in forests. These fungal communities are often dominated by mycorrhizal fungi that form mutually beneficial relationships with plant roots and some mycorrhizal fungi produce extracellular and cell-bound enzymes that catalyze the hydrolysis of nitrogen (N)- and phosphorus (P)- containing compounds in soil organic matter. Here we investigated whether the community structure of different types of mycorrhizal fungi (arbuscular and ectomycorrhizal fungi) is correlated with soil chemistry and enzyme activity in a northern hardwood forest and whether these correlations change over the growing season. We quantified these relationships in an experimental paired plot study where white-tailed deer (access or excluded 4.5 yrs) treatment was crossed with garlic mustard (presence or removal 1 yr). We collected soil samples early and late in the growing season and analyzed them for soil chemistry, extracellular enzyme activity and molecular analysis of both arbuscular mycorrhizal (AM) and ectomycorrhizal/saprotrophic fungal communities using terminal restriction fragment length polymorphism (TRFLP). AM fungal communities did not change seasonally but were positively correlated with the activities of urease and leucine aminopeptidase (LAP), enzymes involved in N cycling. The density of garlic mustard was correlated with the presence of specific AM fungal species, while deer exclusion or access had no effect on either fungal community after 4.5 yrs. Ectomycorrhizal/saprotrophic fungal communities changed seasonally and were positively correlated with most soil enzymes, including enzymes involved in carbon (C), N and P cycling, but only during late summer sampling. Our results suggest that fine scale temporal and spatial changes in soil fungal communities may affect soil nutrient and carbon cycling. Although AM fungi are not generally considered capable of producing extracellular enzymes, the correlation between some AM taxa and the activity of N acquisition enzymes suggests that these fungi may play a role in forest understory N cycling.     

吉林省玉米种植区土壤真菌群落多样性特征研究

为了探索吉林省玉米主要种植区土壤微生物的真菌群落多样性特征,采用Illumina MiSeq技术,对吉林省玉米主产区的72个土壤样品进行真菌群落多样性分析,通过土壤理化性质、酶活性特征、多样性指数、冗余分析(RDA)、偏最小二乘法(PL-SDA)等分析玉米种植区土壤微生物群落结构的特征及其与土壤理化性质的关系。结果表明,5种不同类型的土壤pH和有机质差异显著;偏碱性土壤的蔗糖酶、脲酶和碱性磷酸酶的活性较高;风沙土、黑钙土和黑土的真菌丰度显著高于白浆土和暗棕壤,而黑土的真菌多样性最高。所有土壤均以子囊菌门(Ascomycota)为优势菌门,粪壳菌纲(Sordariomycetes)为主要的优势菌纲,酵母菌属(Guehomyces)、青霉菌属(Penicillium)和腐殖霉属(Humicola)是主要的优势菌属。冗余分析结果表明,吉林省玉米种植区土壤pH和有机质是土壤真菌优势类群变化的主要驱动因子。上述的研究结果以期阐明东北黑土区土壤群落与土壤养分之间的关系,为黑土区退化土壤恢复提供理论参考。     

The influence of American Chestnut (Castanea dentata) on nitrogen availability, organic matter and chemistry of silty and sandy loam soils

American chestnut trees once dominated vast areas of deciduous forest in eastern North America, but the exotic chestnut blight almost eliminated the species from the region. Introduction of blight-resistant American chestnut hybrids will probably start in the next decade after many years of tree breeding. What were the historic effects of chestnut on forest soils, and what changes may follow reintroduction of hybrid chestnuts? A site in southern Wisconsin provided an opportunity to examine the effect of chestnut trees on soil properties. At this site, 600 km northwest of chestnut's historic distribution, naturalized chestnuts have spread throughout an intact mixed-species forest from nine planted trees. The site contains soil developed on a silty loess-mantled ridge that abuts sandier hillslopes, allowing the effects of individual chestnuts to be examined on two soil types. I sampled and analyzed forest floor and mineral soils beneath canopies of individual American chestnuts and the surrounding mixed-species deciduous forest on fine-silt and sandy-loam soil types. On sandy loam soils, total soil carbon (C) and nitrogen (N), inorganic N and net mineralization and nitrification rates were 10–17% higher beneath chestnut canopies compared to soils beneath mixed-species deciduous forest. The pool of total soil N beneath chestnut canopies was positively related to the silt content of the sandy loam soils. In contrast, there were no differences between properties of chestnut canopy and mixed-species deciduous forest soils on the fine silt texture class. On sandy loam soil conditions common throughout the pre-blight distribution of American chestnut, soil biogeochemical processes differ beneath individual chestnut trees relative to a diverse mixture of deciduous species. These findings suggest that widespread chestnut reintroduction has the potential to alter both stand- and watershed-scale processes.     

Influence of repeated prescribed burning on incorporation of C from cellulose by forest soil fungi as determined by RNA stable isotope probing

Repeated prescribed burning is frequently used as a forest management tool and can influence soil microbial diversity and activity. Soil fungi play key roles in carbon and nutrient cycling processes and soil fungal community structure has been shown to alter with increasing burning frequency. Such changes are accompanied by changes to soil carbon and nitrogen pools, however, we know little regarding how repeated prescribed burning alters functional diversity in soil fungal communities. We amended soil with ¹³C-cellulose and used RNA stable isotope probing to investigate the effect of biennial repeated prescribed burning over a 34-year period on cellulolytic soil fungi. Results indicated that repeated burning altered fungal community structure. Moreover, fungal community structure and diversity in ¹²C and ¹³C fractions from the unburned soil were not significantly different from each other, while those from the biennial burned soils differed from each other. The data indicate that fewer active fungi in the biennially burned soil incorporated ¹³C from the labelled cellulose and that repeated prescribed burning had a significant impact on the diversity of an important functional group of soil fungi (cellulolytic fungi) that are key drivers of forest soil decomposition and carbon cycling processes.     

Response of the soil fungal community to multi-factor environmental changes in a temperate forest

Both environmental and climatic changes are known to influence soil microbial biomes in terrestrial ecosystems. However, there are limited data defining the interactive effects of multi-factor environmental disturbances, including N-deposition, precipitation, and air temperature, on soil fungal communities in temperate forests. A 3-year outdoor pot experiment was conducted to examine the temporal shifts of soil fungal communities in a temperate forest following N-addition, precipitation and air temperature changes. The shifts in the structure and composition of soil fungal communities were characterized by denaturing gradient gel electrophoresis and DNA sequencing. N-addition regimen induced significant alterations in the composition of soil fungal communities, and this effect was different at both higher and lower altitudes. The response of the soil fungal community to N-addition was much stronger in precipitation-reduced soils compared to soils experiencing enhanced precipitation. The combined treatment of N-addition and reduced precipitation caused more pronounced changes in the lower altitude versus those in the higher one. Certain fungal species in the subphylum Pezizomycotina and Saccharomycotina distinctively responded to N fertilization and soil water control at both altitudes. Redundancy discrimination analysis showed that changes in environmental factors and soil physicochemical properties explained 43.7% of the total variability in the soil fungal community at this forest ecosystem. Variations in the soil fungal community were significantly related to the altitude, soil temperature, total soil N content (TN) and pH value (P < 0.05). We present evidence for the interactive effects of N-addition, water manipulation and air temperature to reshape soil fungal communities in the temperate forest. Our data could provide new insights into predicting the response of soil micro-ecosystem to climatic changes.     

降水变化和氮沉降对荒漠草原土壤丛枝菌根真菌群落结构的影响

降水变化和氮沉降是影响植物、微生物和土壤环境变化的两个重要方面。尽管丛枝菌根（AM）真菌在陆地生态系统中起着至关重要的作用，但人们对降水变化和氮添加如何交互影响AM真菌群落仍知之甚少。本研究以短花针茅荒漠草原为研究对象，采用裂区设计，主区为自然降雨(CK)、增雨30%(W)和减雨30%(R)三个水分梯度，副区为0(N0)，30(N30)，50(N50)和100(N100) kg?hm-2?a-1 四个氮素梯度共12个处理，通过高通量测序分析了土壤中AM真菌群落的多样性和组成。结果发现，水分处理对土壤AM真菌的Alpha多样性有促进作用，氮素处理抑制了土壤AM真菌的Alpha多样性，水分增加和氮素添加的交互作用促进了AM真菌的Alpha多样性增加，并改变了土壤AM真菌群落组成。水分和氮素刺激了各功能型植物生物量的增加，氮添加使多年生杂草和半灌木、小半灌木生物量显著增加，多年生禾草生物量显著减少。此外，多型孢子菌科的相对丰度与一二年生植物和半灌木、小半灌木生物量呈显著正相关，一二年生植物和半灌木、小半灌木生物量在氮添加和增雨处理下增加。本研究证明了AM真菌群落在短期气候变化下的稳定性。此外，AM真菌在科水平上的丰度与各生活型植物地上生物量的相关性证明了地上和地下生态系统的连通性。     

Shifting microbial community structure across a marine terrace grassland chronosequence, Santa Cruz, California

Changes in the biomass and structure of soil microbial communities have the potential to impact ecosystems via interactions with plants and weathering minerals. Previous studies of forested long-term (1000s - 100,000s of years) chronosequences suggest that surface microbial communities change with soil age. However, significant gaps remain in our understanding of long-term soil microbial community dynamics, especially for non-forested ecosystems and in subsurface soil horizons. We investigated soil chemistry, aboveground plant productivity, and soil microbial communities across a grassland chronosequence (65,000-226,000 yrs old) located near Santa Cruz, CA. Aboveground net primary productivity (ANPP) initially increased to a maximum and then decreased for the older soils. We used polar lipid fatty acids (PLFA) to investigate microbial communities including both surface (<0.1 m) and subsurface (≥0.2 m) soil horizons. PLFAs characteristic of Gram-positive bacteria and actinobacteria increased as a fraction of the microbial community with depth while the fungal fraction decreased relative to the surface. Differences among microbial communities from each chronosequence soil were found primarily in the subsurface where older subsurface soils had smaller microbial community biomass, a higher proportion of fungi, and a different community structure than the younger subsurface soil. Subsurface microbial community shifts in biomass and community structure correlated with, and were likely driven by, decreasing soil P availability and Ca concentrations, respectively. Trends in soil chemistry as a function of soil age led to the separation of the biological (≤1 m depth) and geochemical (>1 m) cycles in the old, slowly eroding landscape we investigated, indicating that this separation, commonly observed in tropical and subtropical ecosystems, can also occur in temperate climates. This study is the first to investigate subsurface microbial communities in a long-term chronosequence. Our results highlight connections between soil chemistry and both the aboveground and belowground parts of an ecosystem.     

Impact of inorganic nitrogen additions on microbes in biological soil crusts

Many studies have shown that changes in nitrogen (N) availability affect the diversity and composition of soil microbial community in a variety of terrestrial systems, but less is known about the responses of microbes specific to biological soil crusts (BSCs) to increasing N additions. After seven years of field experiment, the bacterial diversity in lichen-dominated crusts decreased linearly with increasing inorganic N additions (ambient N deposition; low N addition, 3.5 g N m⁻² y⁻¹; medium N addition, 7.0 g N m⁻² y⁻¹; high N addition, 14.0 g N m⁻² y⁻¹), whereas the fungal diversity exhibited a distinctive pattern, with the low N-added crust containing a higher diversity than the other crusts. Pyrosequencing data revealed that the bacterial community shifted to more Cyanobacteria with modest N additions (low N and medium N) and to more Actinobacteria and Proteobacteria and much less Cyanobacteria with excess N addition (high N). Our results suggest that soil pH, together with soil organic carbon (C), structures the bacterial communities with N additions. Among the fungal communities, the relative abundance of Ascomycota increased with modest N but decreased with excess N. However, increasing N additions favored Basidiomycota, which may be ascribed to increases in substrate availability with low lignin and high cellulose contents under elevated N conditions. Bacteria/fungi ratios were higher in the N-added samples than in the control, suggesting that the bacterial biomass tends to dominate over that of fungi in lichen-dominated crusts after N additions, which is especially evident in the excess N condition. Because bacteria and fungi are important components and important decomposers in BSCs, the alterations of the bacterial and fungal communities may have implications in the formation and persistence of BSCs and the cycling and storage of C in desert ecosystems.     

In situ dynamics of soil fungal communities under different genotypes of potato, including a genetically modified cultivar

Fungi are key to the functioning of soil ecosystems, and exhibit a range of interactions with plants. Given their close associations with plants, and importance in ecosystem functioning, soil-borne fungi have been proposed as potential biological indicators of disturbance and useful agents in monitoring strategies, including those following the introduction of genetically modified (GM) crops. Here we report on the impact of potato crop varieties, including a cultivar that was genetically modified for its starch quality, on the community composition of the main phyla of fungi in soils, i.e. Ascomycota, Basidiomycota and Glomeromycota in rhizosphere and bulk soil. Samples were collected at two field sites before sowing, at three growth stages during crop development and after the harvest of the plants, and the effects of field site, plant growth stage and plant cultivar (genotype) on fungal community composition assessed using three phylum-specific T-RFLP profiling strategies and multivariate statistical analysis (NMDS ordinations with ANOSIM test). In addition, fungal biomass, arbuscular mycorrhizal colonization of roots and activities of extracellular fungal enzymes (laccases, Mn-peroxidases and cellulases) involved in degradation of lignocelluloses-rich organic matter were determined. Fungal community compositions, densities and activities were observed to differ significantly between the rhizosphere and bulk soil. The most important factors determining fungal community composition and functioning were plant growth stage for the rhizosphere communities and location and soil properties for the bulk soil communities. The basidiomycetes were the most numerous fungal group in the bulk soils and in the rhizosphere of young plants, with a shift toward greater ascomycete numbers in the rhizosphere at later growth stages. There were no detectable differences between the GM cultivar and its parental cultivar in terms of influence on fungal community structure of function. Fungal community structure and functioning of both GM- and parental cultivars fell within the range of other cultivars at most sampling moments.     

Shrubs influence arbuscular mycorrhizal fungi communities in a semi-arid environment

Interactions between arbuscular mycorrhizal fungi (AMF) and plants are essential components of ecosystem functioning; however, they remain poorly known in dry ecosystems. We examined the relationship between seven shrub species and their associated AMF community in a semi-arid plant community in southern Spain. Soil characteristics and plant physiological status were measured and related to AMF community composition and genetic diversity by multivariate statistics. We found differences in AMF communities in soils under shrubs and in gaps among them, whereas no differences were detected among AMF communities colonizing roots. Soil nutrients content drove most of the spatial variations in the AMF community and genetic diversity. AMF communities were more heterogeneous in fertile islands with low nitrogen-to-phosphorus ratio and vice versa. AMF genetic diversity increased in soils limited by phosphorus and with high soil organic matter content, while AMF genetic diversity increased in roots growing in soil not limited by phosphorus. Overall, we could not find a clear link between plant performance and the associated AMF community. Our findings show that different shrub species generate islands of fertility which differ in nutrient content and, therefore, support different AMF communities, increasing AMF diversity at the landscape level.     

Differential effect of arbuscular mycorrhizal fungal communities from ecosystems along management gradient on the growth of forest understorey plant species

Arbuscular mycorrhizal fungi (AMF) are important functional components of ecosystems. Although there is accumulating knowledge about AMF diversity in different ecosystems, the effect of forest management on diversity and functional characteristics of AMF communities has not been addressed. Here, we used soil inoculum representing three different AM fungal communities (from a young forest stand, an old forest stand and an arable field) in a greenhouse experiment to investigate their effect on the growth of three plant species with contrasting local distributions - Geum rivale, Trifolium pratense and Hypericum maculatum. AM fungal communities in plant roots were analysed using the terminal restriction fragment length polymorphism (T-RFLP) method. The effect of natural AMF communities from the old and young forest on the growth of studied plant species was similar. However, the AMF community from the contrasting arable ecosystems increased H. maculatum root and shoot biomass compared with forest inocula and T. pratense root biomass compared to sterile control. According to ordination analysis AMF inocula from old and young forest resulted in similar root AMF communities whilst plants grown with AM fungi from arable field hosted a different AMF community from those grown with old forest inocula. AMF richness in plant roots was not related to the origin of AMF inoculum. G. rivale hosted a significantly different AM fungal community to that of T. pratense and H. maculatum. We conclude that although the composition of AM fungal communities in intensively managed stands differed from that of old stands, the ecosystem can still offer the ‘symbiotic service’ necessary for the restoration of a characteristic old growth understorey plant community.