Differentiating between effects of invasion and diversity: impacts of aboveground plant communities on belowground fungal communities

Exotic plant species can affect soil microbial communities with the potential for community and ecosystem feedbacks. Yet, separating the effects of exotics from confounded changes in plant community diversity still remains a challenge. We focused on how plant diversity and native or exotic life history affected root fungi because of their significant roles in community and ecosystem processes. Specifically, we examined how fungi colonizing plant roots were affected by plant richness (one, two or four species) replicated across a range of plant community mixtures (natives, exotics, native-exotic mixtures). Fungal biomass inside roots was affected independently by plant richness and mixture, while root fungal community composition was affected only by plant richness. Extraradical networks also increased in size with plant richness. By contrast, plant biomass was a function of plant mixture, with natives consistently smaller than exotics and native-exotic mixtures intermediate. Plant invasions may have an impact on the belowground community primarily through their effects on diversity, at least in the short-term. Disentangling the effects of diversity and invasion on belowground microbial communities can help us to understand both the controllers of belowground resilience and mechanisms of successful colonization and spread of exotic plants.     

Legacy effects overwhelm the short-term effects of exotic plant invasion and restoration on soil microbial community structure,enzyme activities,and nitrogen cycling

Plant invasions can have substantial consequences for the soil ecosystem, altering microbial community structure and nutrient cycling. However, relatively little is known about what drives these changes, making it difficult to predict the effects of future invasions. In addition, because most studies compare soils from uninvaded areas to long-established dense invasions, little is known about the temporal dependence of invasion impacts. We experimentally manipulated forest understory vegetation in replicated sites dominated either by exotic Japanese barberry (Berberis thunbergii), native Viburnums, or native Vacciniums, so that each vegetation type was present in each site-type. We compared the short-term effect of vegetation changes to the lingering legacy effects of the previous vegetation type by measuring soil microbial community structure (phospholipid fatty acids) and function (extracellular enzymes and nitrogen mineralization). We also replaced the aboveground litter in half of each plot with an inert substitute to determine if changes in the soil microbial community were driven by aboveground or belowground plant inputs. We found that after 2 years, the microbial community structure and function was largely determined by the legacy effect of the previous vegetation type, and was not affected by the current vegetation. Aboveground litter removal had only weak effects, suggesting that changes in the soil microbial community and nutrient cycling were driven largely by belowground processes. These results suggest that changes in the soil following either invasion or restoration do not occur quickly, but rather exhibit long-lasting legacy effects from previous belowground plant inputs.     

The changing nature of plant–microbe interactions during a biological invasion

Invasive plant species can alter belowground microbial communities. Simultaneously, the composition of soil microbial communities and the abundance of key microbes can influence invasive plant success. Such reciprocal effects may cause plant–microbe interactions to change rapidly during the course of biological invasions in ways that either inhibit or promote invasive species growth. Here we use a space-for-time substitution to illustrate how effects of soil microbial communities on the exotic legume Vicia villosa vary across uninvaded sites, recently invaded sites, and sites invaded by V. villosa for over a decade. We find that soil microorganisms from invaded areas increase V. villosa growth compared to sterilized soil or live soils collected from uninvaded sites, likely because mutualistic nitrogen-fixing rhizobia are not abundant in uninvaded areas. Notably, the benefits resulting from inoculation with live soils were higher for soils from recently invaded sites compared to older invasions, potentially indicating that over longer time scales, soil microbial communities change in ways that may reduce the success of exotic species. These findings suggest that short-term changes to soil microbial communities following invasion may facilitate exotic legume growth likely because of increases in the abundance of mutualistic rhizobia, but also indicate that longer term changes to soil microbial communities may reduce the growth benefits belowground microbial communities provide to exotic species. Our results highlight the changing nature of plant–microbe interactions during biological invasions and illustrate how altered biotic interactions could contribute to both the initial success and subsequent naturalization of invasive legume species.     

外来植物火炬树(Rhus typhina L.)入侵对不同林型土壤性质的影响

生物入侵在世界范围内广泛发生,严重威胁当地生物多样性和生态系统稳定性。植物与土壤之间的相互作用在决定植物的竞争力以及分布格局中起着重要作用,是影响外来植物入侵力和生态系统可入侵性的一个重要方面。目前,有关研究已成为植被生态学与入侵生态学的研究热点。引自北美的外来植物火炬树(Rhus typhina L.)已成为我国北方主要的入侵木本植物之一。比较了火炬树单优林型、火炬树+刺槐(Robinia pseudoacacia L.)混交林、火炬树+麻栎(Quercus acutissima Carruth.)混交林、火炬树+银白杨(Populus alba L.)混交林4种不同林型的土壤微生物群落结构、土壤酶活性和土壤养分含量特征。结果表明:火炬树单优林土壤细菌、放线菌数量明显高于各混交林型,而真菌数量无显著差异;土壤酶活性方面,火炬树单优林脲酶、过氧化氢酶活性高,土壤磷酸酶活性低;火炬树的入侵显著提高了土壤全碳、全氮、全磷和硝态氮含量,同时明显降低了土壤铵态氮含量。硝态氮含量的增高可能与火炬树入侵造成土壤微生物群落组成变化、土壤硝化速率高有关;而火炬树入侵降低了土壤铵态氮含量,说明该物种可能更易于吸收利用铵态氮。以上研究结果表明,火炬树可以改变土壤生态系统的微生物群落组成和土壤酶活性并影响土壤相关营养元素循环,从而可能使其在与当地植物的竞争中获得优势,为自身的入侵创造有利条件。     

Cascading effects of belowground predators on plant communities are density‐dependent

Soil food webs comprise a multitude of trophic interactions that can affect the composition and productivity of plant communities. Belowground predators feeding on microbial grazers like Collembola could decelerate nutrient mineralization by reducing microbial turnover in the soil, which in turn could negatively influence plant growth. However, empirical evidences for the ecological significance of belowground predators on nutrient cycling and plant communities are scarce. Here, we manipulated predator density (Hypoaspis aculeifer: predatory mite) with equal densities of three Collembola species as a prey in four functionally dissimilar plant communities in experimental microcosms: grass monoculture (Poa pratensis), herb monoculture (Rumex acetosa), legume monoculture (Trifolium pratense), and all three species as a mixed plant community. Density manipulation of predators allowed us to test for density‐mediated effects of belowground predators on Collembola and lower trophic groups. We hypothesized that predator density will reduce Collembola population causing a decrease in nutrient mineralization and hence detrimentally affect plant growth. First, we found a density‐dependent population change in predators, that is, an increase in low‐density treatments, but a decrease in high‐density treatments. Second, prey suppression was lower at high predator density, which caused a shift in the soil microbial community by increasing the fungal: bacterial biomass ratio, and an increase of nitrification rates, particularly in legume monocultures. Despite the increase in nutrient mineralization, legume monocultures performed worse at high predator density. Further, individual grass shoot biomass decreased in monocultures, while it increased in mixed plant communities with increasing predator density, which coincided with elevated soil N uptake by grasses. As a consequence, high predator density significantly increased plant complementarity effects indicating a decrease in interspecific plant competition. These results highlight that belowground predators can relax interspecific plant competition by increasing nutrient mineralization through their density‐dependent cascading effects on detritivore and soil microbial communities.     

Exotic plant invasion alters chaparral ecosystem resistance and resilience pre- and post-wildfire

Perturbations such as wildfire and exotic plant invasion have significant impacts on soils, and the extent to which invaded soils are resistant or resilient to these disturbances varies by ecosystem type. Replacement of shrublands by herbaceous exotics pre- and post-wildfire may drastically alter soil chemical and biological properties for an unknown duration. We assessed above and belowground resistance and resilience to exotic plant invasion both before and after a chaparral wildfire. We hypothesized that exotic plant species would change chemical characteristics of chaparral soils by altering litter and microbial inputs, and that controlling exotics and seeding native species would restore chemical characteristics to pre-invaded conditions. We additionally hypothesized that exotic plant species would slow succession above- and belowground, as well as recovery of post-wildfire chaparral structure and function. Plant species composition and soil nutrient pools and cycling rates were evaluated in mature and invaded chaparral pre- and post-wildfire. Exotic plant species were weeded and native species were seeded to assess impacts of exotic competition on native species recovery. Invasion did not impact all soil characteristics before fire, but increased soil C/N ratio, pH, and N cycling rates, and reduced NO₃-N availability. After fire, invasives slowed succession above- and belowground. Removal of exotics and seeding natives facilitated succession and resulted in plant composition similar to uninvaded, post-wildfire chaparral. The chaparral ecosystem was not resistant to impacts of invasion as indicated by altered soil chemistry and C and N cycling rates; however, short-term restoration led to recovery of extractable nitrogen availability indicating resilience of chaparral soils. This suggests that the permanence of exotic plant species, once established, represents a greater ecological challenge than exotic plant impacts on soils.     

Punching above their weight: low-biomass non-native plant species alter soil properties during primary succession

Non‐native invasive plants can greatly alter community and ecosystem properties, but efforts to predict which invasive species have the greatest impacts on these properties have been generally unsuccessful. An hypothesis that has considerable promise for predicting the effects of invasive non‐native plant species is the mass ratio hypothesis (i.e. that dominant species exert the strongest effects). We tested this hypothesis using data from a four year removal experiment in which the presence of two dominant shrub species (one native and the other not), and subordinate plant species, were manipulated in factorial combinations over four years in a primary successional floodplain system. We measured the effects of these manipulations on the plant community, soil nutrient status and soil biota in different trophic levels of the soil food web. Our experiment showed that after four years, low‐biomass non‐native plant species exerted disproportionate belowground effects relative to their contribution to total biomass in the plant community, most notably by increasing soil C, soil microbial biomass, altering soil microbial community structure and increasing the abundance of microbial‐feeding and predatory nematodes. Low‐biomass, non‐native plant species had distinct life history strategies and foliar traits (higher foliar N concentrations and higher leaf area per unit mass) compared with the two dominant shrub species (97% of total plant mass). Our results have several implications for understanding species’ effects in communities and on soil properties. First, high‐biomass species do not necessarily exert the largest impacts on community or soil properties. Second, low‐biomass, inconspicuous non‐native species can influence community composition and have important trophic consequences belowground through effects on soil nutrient status or resource availability to soil biota. Our finding that low‐biomass non‐native species influence belowground community structure and soil properties more profoundly than dominant species demonstrates that the mass ratio hypothesis does not accurately predict the relative effects of different coexisting species on community‐ and ecosystem‐level properties.     

Plant litter effects on soil nutrient availability and vegetation dynamics: changes that occur when annual grasses invade shrub-steppe communities

Changes in the quantity and quality of plant litter occur in many ecosystems as they are invaded by exotic species, which impact soil nutrient cycling and plant community composition. Such changes in sagebrush-steppe communities are occurring with invasion of annual grasses (AG) into a perennial grass (PG) dominated system. We conducted a 5-year litter manipulation study located in the northern Great Basin, USA. Springtime litter was partially or completely removed in three communities with differing levels of invasion (invaded, mixed, and native) to determine how litter removal and litter biomass affected plant-available soil N and plant community composition. Litter biomass (prior to the removal treatment) was negatively correlated with plant-available N in the invaded community, but was positively correlated in the native community. Plant-available N had greater intra- and inter-annual fluctuations in the invaded compared to the mixed or native communities, but was not generally affected by removal treatments. Litter removal had negative effects on AG cover during a warm/dry year and negative effects on PG cover during a cool/wet year in the mixed community. Overall, the effectiveness of springtime litter manipulations on plant-available N were limited and weather dependent, and only removal treatments >75 % had effects on the plant community. Our study demonstrates how communities invaded by AGs have significantly increased temporal variability in nutrient cycling, which may decrease ecosystem stability. Further, we found that the ecological impacts from litter manipulation on sagebrush communities were dependent on the extent of AG invasion, the timing of removal, and seasonal precipitation.     

Soil microbial communities adapt to genetic variation in leaf litter inputs

Plant genotypes can have important community‐ and ecosystem‐level effects. However, whether the extended phenotypes of plants feed back to influence the fitness of causal genotypes through soil processes remains unknown. We investigated whether aspen genotypes create distinct soil microbial communities that could potentially affect plant fitness. Using naturally occurring aspen stands in an old‐field system, we set up reciprocal litter transplants among ten genetically distinct aspen clones and tracked decomposition and changes in belowground nutrients and microbial communities for three years. We found that belowground microbial communities became adapted to process specific genotypes of aspen litter to the extent allowable by environment and litter chemistry. Belowground processes were driven by a combination of little quality and prior exposure to specific genotypes of litter. In general, litter from aspen genotypes native to the soil community decomposed more rapidly than did litter from foreign aspen genotypes (i.e. a home‐field advantage existed). While home‐field advantages have been documented to occur among litters of different species, we show that intraspecific variation can elicit similar, albeit weak, effects within a single species. Because rapid decomposition and nutrient cycling is likely to benefit fast‐growing, early‐successional species such as aspen, genotype‐mediated selection for soil microbial communities may feed back to positively affect plant fitness. In addition, belowground communities exhibited significant shifts in response to leaf litter inputs. When exposed to foreign litter, microbial communities changed to become more similar to the microbial community beneath the foreign litter's origin, indicating that belowground microbial communities are predictable given the genotype of the aboveground aspen clone.     

Effects of Exotic Plant Invasions on Soil Nutrient Cycling Processes

Although it is generally acknowledged that invasions by exotic plant species represent a major threat to biodiversity and ecosystem stability, little attention has been paid to the potential impacts of these invasions on nutrient cycling processes in the soil. The literature on plant–soil interactions strongly suggests that the introduction of a new plant species, such as an invasive exotic, has the potential to change many components of the carbon (C), nitrogen (N), water, and other cycles of an ecosystem. I have reviewed studies that compare pool sizes and flux rates of the major nutrient cycles in invaded and noninvaded systems for invasions of 56 species. The available data suggest that invasive plant species frequently increase biomass and net primary production, increase N availability, alter N fixation rates, and produce litter with higher decomposition rates than co-occurring natives. However, the opposite patterns also occur, and patterns of difference between exotics and native species show no trends in some other components of nutrient cycles (for example, the size of soil pools of C and N). In some cases, a given species has different effects at different sites, suggesting that the composition of the invaded community and/or environmental factors such as soil type may determine the direction and magnitude of ecosystem-level impacts. Exotic plants alter soil nutrient dynamics by differing from native species in biomass and productivity, tissue chemistry, plant morphology, and phenology. Future research is needed to (a) experimentally test the patterns suggested by this data set; (b) examine fluxes and pools for which few data are available, including whole-site budgets; and (c) determine the magnitude of the difference in plant characteristics and in plant dominance within a community that is needed to alter ecosystem processes. Such research should be an integral component of the evaluation of the impacts of invasive species.     

Interannual climate variability mediates changes in carbon and nitrogen pools caused by annual grass invasion in a semiarid shrubland

Exotic plant invasions alter ecosystem properties and threaten ecosystem functions globally. Interannual climate variability (ICV) influences both plant community composition (PCC) and soil properties, and interactions between ICV and PCC may influence nitrogen (N) and carbon (C) pools. We asked how ICV and non-native annual grass invasion covary to influence soil and plant N and C in a semiarid shrubland undergoing widespread ecosystem transformation due to invasions and altered fire regimes. We sampled four progressive stages of annual grass invasion at 20 sites across a large (25,000 km²) landscape for plant community composition, plant tissue N and C, and soil total N and C in 2013 and 2016, which followed 2 years of dry and wet conditions, respectively. Multivariate analyses and ANOVAs showed that in invasion stages where native shrub and perennial grass and forb communities were replaced by annual grass-dominated communities, the ecosystem lost more soil N and C in wet years. Path analysis showed that high water availability led to higher herbaceous cover in all invasion stages. In stages with native shrubs and perennial grasses, higher perennial grass cover was associated with increased soil C and N, while in annual-dominated stages, higher annual grass cover was associated with losses of soil C and N. Also, soil total C and C:N ratios were more homogeneous in annual-dominated invasion stages as indicated by within-site standard deviations. Loss of native shrubs and perennial grasses and forbs coupled with annual grass invasion may lead to long-term declines in soil N and C and hamper restoration efforts. Restoration strategies that use innovative techniques and novel species to address increasing temperatures and ICV and emphasize maintaining plant community structure—shrubs, grasses, and forbs—will allow sagebrush ecosystems to maintain C sequestration, soil fertility, and soil heterogeneity.     

Abiotic drivers and plant traits explain landscape‐scale patterns in soil microbial communities

The controls on aboveground community composition and diversity have been extensively studied, but our understanding of the drivers of belowground microbial communities is relatively lacking, despite their importance for ecosystem functioning. In this study, we fitted statistical models to explain landscape‐scale variation in soil microbial community composition using data from 180 sites covering a broad range of grassland types, soil and climatic conditions in England. We found that variation in soil microbial communities was explained by abiotic factors like climate, pH and soil properties. Biotic factors, namely community‐weighted means (CWM) of plant functional traits, also explained variation in soil microbial communities. In particular, more bacterial‐dominated microbial communities were associated with exploitative plant traits versus fungal‐dominated communities with resource‐conservative traits, showing that plant functional traits and soil microbial communities are closely related at the landscape scale.     

The legacy of bacterial invasions on soil native communities

Soil microbial communities are often not resistant to the impact caused by microbial invasions, both in terms of structure and functionality, but it remains unclear whether these changes persist over time. Here, we used three strains of Escherichia coli O157:H7 (E. coli O157:H7), a species used for modelling bacterial invasions, to evaluate the resilience of the bacterial communities from four Chinese soils to invasion. The impact of E. coli O157:H7 strains on soil native communities was tracked for 120 days by analysing bacterial community composition as well as their metabolic potential. We showed that soil native communities were not resistant to invasion, as demonstrated by a decline in bacterial diversity and shifts in bacterial composition in all treatments. The resilience of native bacterial communities (diversity and composition) was inversely correlated with invader's persistence in soils (R² = 0.487, p < 0.001). Microbial invasions also impacted the functionality of the soil communities (niche breadth and community niche), the degree of resilience being dependent on soil or native community diversity. Collectively, our results indicate that bacteria invasions can potentially leave a footprint in the structure and functionality of soil communities, indicating the need of assessing the legacy of introducing exotic species in soil environments.     

Relationships among soil fertility,native plant diversity and exotic plant abundance inform restoration of forb‐rich eucalypt woodlands

Aim Water and nutrient availability are major limits to productivity in semi‐arid ecosystems; hence, ecological restoration often focuses on conserving or concentrating soil resources. By contrast, nutrient enrichment can promote invasion by exotic annuals, leading to restoration approaches that target reduction of soil nutrients. We aimed to explore potential biodiversity trade‐offs between these approaches by investigating relationships among soil nutrients, exotic annuals and native plant diversity and composition. In particular, we investigated the hypothesis that native plant diversity in semi‐arid to temperate woodlands reflects the productivity–diversity hypothesis, leading to hump‐backed relationships with soil nutrients such that (1) native plant diversity declines with increasing nutrient enrichment and (2) native diversity is limited at the lowest levels of soil fertility. Location Fragmented, long‐ungrazed Eucalyptus loxophleba subsp. loxophleba (York gum)–Acacia acuminata (jam) woodlands in the wheatbelt of South‐Western Australia. Methods We conducted stratified surveys of floristic composition and topsoil nutrient concentrations in 112 woodland patches. We used generalized linear models, structural equation models and ordinations to characterize relationships among soil nutrients, rainfall, exotic annuals and patch‐scale (100 m²) native plant composition and diversity. Results Patch‐scale native plant diversity declined strongly with increasing exotic abundance. This was partly related to elevated soil nutrient concentrations, particularly total nitrogen and available phosphorus. By contrast, there was little evidence for positive correlations between soil nutrients and native diversity, even at very low soil nutrient concentrations. Main conclusions Minimizing weed invasions is crucial for maximizing native plant diversity in E. loxophleba woodlands and could include nutrient‐depleting treatments without substantially compromising the functional capacity of soils to maintain native plant richness and composition. More broadly we emphasize that understanding relationships among ecosystem productivity, plant diversity and exotic invasions in the context of associated theoretical frameworks is fundamental for informing ecological restoration.     

Distribution of ecosystem C and N within contrasting vegetation types in a semiarid rangeland in the Great Basin,USA

Semiarid sagebrush ecosystems are being transformed by wildfire, rangeland improvement techniques, and exotic plant invasions, but the effects on ecosystem C and N dynamics are poorly understood. We compared ecosystem C and N pools to 1 m depth among historically grazed Wyoming big sagebrush, introduced perennial crested wheatgrass, and invasive annual cheatgrass communities, to examine whether the quantity and quality of plant inputs to soil differs among vegetation types. Natural abundance δ¹⁵N isotope ratios were used to examine differences in ecosystem N balance. Sagebrush-dominated sites had greater C and N storage in plant biomass compared to perennial or annual grass systems, but this was predominantly due to woody biomass accumulation. Plant C and N inputs to soil were greatest for cheatgrass compared to sagebrush and crested wheatgrass systems, largely because of slower root turnover in perennial plants. The organic matter quality of roots and leaf litter (as C:N ratios) was similar among vegetation types, but lignin:N ratios were greater for sagebrush than grasses. While cheatgrass invasion has been predicted to result in net C loss and ecosystem degradation, we observed that surface soil organic C and N pools were greater in cheatgrass and crested wheatgrass than sagebrush-dominated sites. Greater biomass turnover in cheatgrass and crested wheatgrass versus sagebrush stands may result in faster rates of soil C and N cycling, with redistribution of actively cycled N towards the soil surface. Plant biomass and surface soil δ¹⁵N ratios were enriched in cheatgrass and crested wheatgrass relative to sagebrush-dominated sites. Source pools of plant available N could become ¹⁵N enriched if faster soil N cycling rates lead to greater N trace gas losses. In the absence of wildfire, if cheatgrass invasion does lead to degradation of ecosystem function, this may be due to faster nutrient cycling and greater nutrient losses, rather than reduced organic matter inputs.     

Effects of Plant Diversity,Functional Group Composition,and Fertilization on Soil Microbial Properties in Experimental Grassland

BackgroundLoss of biodiversity and increased nutrient inputs are two of the most crucial anthropogenic factors driving ecosystem change. Although both received considerable attention in previous studies, information on their interactive effects on ecosystem functioning is scarce. In particular, little is known on how soil biota and their functions are affected by combined changes in plant diversity and fertilization.Conclusions/SignificanceOur study highlights the role of plant species and functional group diversity as well as interactions between plant community composition and fertilizer application for soil microbial functions. Our results suggest soil microbial stoichiometry to be a powerful indicator of microbial functioning under N limited conditions. Although our results support the notion that plant diversity and fertilizer application independently affect microbial functioning, legume effects on microbial N limitation were superimposed by fertilization, indicating significant interactions between the functional composition of plant communities and nutrient inputs for soil processes.     

外来植物入侵对陆地生态系统地下碳循环及碳库的影响

生物入侵是当今全球性重大环境问题之一, 是全球变化的主要研究内容.评价外来植物入侵对于生态系统影响的研究多集中在地上部分,对于生态系统地下部分影响的研究相对较少.陆地生态系统地下部分对于生态系统过程的重要性之一体现在它处于生态系统碳分配过程的核心环节.入侵种通过影响群落凋落物的输入数量、质量以及输入时间,影响到对于土壤的碳输入,而入侵种与土著种根系的差异以及入侵种对微生物群落的影响是造成土壤呼吸强度发生变化的主要因素,前者土壤呼吸强度一般比后者高.多数研究表明外来植物入侵对生态系统地下碳循环和碳库产生影响,但由于入侵植物种类较多以及研究地点环境条件的不同,关于外来植物入侵对于土壤碳库和土壤有机碳矿化影响的研究结论并不统一.最后,提出了今后该研究领域应加强的一些建议和方向.     

外来植物入侵对土壤生物多样性和生态系统过程的影响

随着科学家对生态系统地下部分的重视,评价外来植物入侵对土壤生态系统的影响成为当前入侵生态学领域的研究热点之一。本文综述了外来植物入侵对土壤微生物、土壤动物以及土壤碳、氮循环动态影响的研究,并探讨了其影响机制。已有的研究表明,植物入侵对土壤生物多样性及相关生态系统过程的影响均存在不一致的格局,影响机制也是复杂多样的。外来植物与土著植物凋落物的质与量、根系特征、物候等多种生理生态特性的差异可能是形成格局多样性和影响机制复杂性的最主要原因。今后,加强多尺度和多生态系统的比较研究、机制性研究、生物多样性和生态系统过程的整合性研究及土壤生态系统对植物入侵的反馈研究是评价外来植物入侵对土壤生态系统影响的发展趋势。     

基于BIOLOG技术分析氮沉降和降水对土壤微生物功能多样性的影响

土壤微生物是有机物分解和养分循环的主要介质,因此在维持土壤的功能多样性和持续性方面发挥着关键作用。气候变化驱动因素会影响土壤微生物的生理活动,引起其群落结构和功能多样性的改变,并对生物地球化学循环和气候―生态系统反馈产生连锁效应,其中氮沉降和降水是全球气候变化的研究热点。土壤氮(N)的有效性有可能通过改变微生物的群落组成以调节微生物对降水变化的响应,但目前关于N沉降和降水及其交互作用对土壤微生物群落功能多样性的影响机制仍不清楚。为了准确预测未来气候条件下生态系统的功能状况,需要更好地了解土壤微生物对环境变化的响应。基于BIOLOG技术综述了氮沉降和降水变化及其交互作用对土壤微生物功能多样性影响的相关研究进展,可以为进一步研究全球气候变化背景下地下生态学的发展提供参考。另外,分析阐述了当前工作中存在的一些主要瓶颈,并对未来的研究热点进行了探讨和展望。     

Arbuscular Mycorrhizal Assemblages in Native Plant Roots Change in the Presence of Invasive Exotic Grasses

Plant invasions have the potential to significantly alter soil microbial communities, given their often considerable aboveground effects. We examined how plant invasions altered the arbuscular mycorrhizal fungi of native plant roots in a grassland site in California and one in Utah. In the California site, we used experimentally created plant communities composed of exotic (Avena barbata, Bromus hordeaceus) and native (Nassella pulchra, Lupinus bicolor) monocultures and mixtures. In the Utah semi-arid grassland, we took advantage of invasion by Bromus tectorum into long-term plots dominated by either of two native grasses, Hilaria jamesii or Stipa hymenoides. Arbuscular mycorrhizal fungi colonizing roots were characterized with PCR amplification of the ITS region, cloning, and sequencing. We saw a significant effect of the presence of exotic grasses on the diversity of mycorrhizal fungi colonizing native plant roots. In the three native grasses, richness of mycorrhizal fungi decreased; in the native forb at the California site, the number of fungal RFLP patterns increased in the presence of exotics. The exotic grasses also caused the composition of the mycorrhizal community in native roots to shift dramatically both in California, with turnover of Glomus spp., and Utah, with replacement of Glomus spp. by apparently non-mycorrhizal fungi. Invading plants may be able to influence the network of mycorrhizal fungi in soil that is available to natives through either earlier root activity or differential carbon provision compared to natives. Alteration of the soil microbial community by plant invasion can provide a mechanism for both successful invasion and the resulting effects of invaders on the ecosystem.