Plant–soil feedbacks and the coexistence of competing plants

Plant–soil feedbacks can have important implications for the interactions among plants. Understanding these effects is a major challenge since it is inherently difficult to measure and manipulate highly diverse soil communities. Mathematical models may advance this understanding by making the interplay of the various processes affecting plant–soil interaction explicit and by quantifying the relative importance of the factors involved. The aim of this paper is to provide a complete analysis of a pioneering plant–soil feedback model developed by Bever and colleagues (J Ecol 85: 561–573, 1997; Ecol Lett 2: 52–62, 1999; New Phytol 157: 465–473, 2003) to fully understand the range of possible impacts of plant–soil feedbacks on plant communities within this framework. We analyze this model by means of a new graphical method that provides a complete classification of the potential effects of soil communities on plant competition. Due to the graphical character of the method, the results are relatively easy to obtain and understand. We show that plant diversity depends crucially on two key parameters that may be viewed as measures of the intensity of plant competition and the direction and strength of plant–soil feedback, respectively. Our analysis provides a formal underpinning of earlier claims that plant–soil feedbacks, especially when they are negative, may enhance the diversity of plant communities. In particular, negative plant–soil feedbacks can enhance the range of plant coexistence by inducing competitive oscillations. However, these oscillations can also destabilize plant coexistence, leading to low population densities and extinctions. In addition, positive feedbacks can allow locally stable forms of plant coexistence by inducing alternative stable states. Our findings highlight that the inclusion of plant–soil interactions may fundamentally alter the predictions on the structure and functioning of above-ground ecosystems. The scenarios presented in this study can be used to formulate hypotheses about the ways soil community effects may influence plant competition that can be tested with empirical studies. This will advance our understanding of the role of plant–soil feedback in ecological communities.     

Plant–soil feedback of native and range-expanding plant species is insensitive to temperature

Temperature change affects many aboveground and belowground ecosystem processes. Here we investigate the effect of a 5°C temperature increase on plant–soil feedback. We compare plant species from a temperate climate region with immigrant plants that originate from warmer regions and have recently shifted their range polewards. We tested whether the magnitude of plant–soil feedback is affected by ambient temperature and whether the effect of temperature differs between these groups of plant species. Six European/Eurasian plant species that recently colonized the Netherlands (non-natives), and six related species (natives) from the Netherlands were selected. Plant–soil feedback of these species was determined by comparing performance in conspecific and heterospecific soils. In order to test the effect of temperature on these plant–soil feedback interactions, the experiments were performed at two greenhouse temperatures of 20/15°C and 25/20°C, respectively. Inoculation with unconditioned soil had the same effect on natives and non-natives. However, the effect of conspecific conditioned soil was negative compared to heterospecific soil for natives, but was positive for non-natives. In both cases, plant–soil interactions were not affected by temperature. Therefore, we conclude that the temperature component of climate change does not affect the direction, or strength of plant–soil feedback, neither for native nor for non-native plant species. However, as the non-natives have a more positive soil feedback than natives, climate warming may introduce new plant species in temperate regions that have less soil-borne control of abundance.     

Spatial heterogeneity of plant–soil feedbacks increases per capita reproductive biomass of species at an establishment disadvantage

Plant–soil feedbacks have been widely implicated as a driver of plant community diversity, and the coexistence prediction generated by a negative plant–soil feedback can be tested using the mutual invasibility criterion: if two populations are able to invade one another, this result is consistent with stable coexistence. We previously showed that two co-occurring Rumex species exhibit negative pairwise plant–soil feedbacks, predicting that plant–soil feedbacks could lead to their coexistence. However, whether plants are able to reproduce when at an establishment disadvantage (“invasibility”), or what drivers in the soil might correlate with this pattern, are unknown. To address these questions, we created experimental plots with heterogeneous and homogeneous soils using field-collected conditioned soils from each of these Rumex species. We then allowed resident plants of each species to establish and added invader seeds of the congener to evaluate invasibility. Rumex congeners were mutually invasible, in that both species were able to establish and reproduce in the other’s resident population. Invaders of both species had twice as much reproduction in heterogeneous compared to homogeneous soils; thus the spatial arrangement of plant–soil feedbacks may influence coexistence. Soil mixing had a non-additive effect on the soil bacterial and fungal communities, soil moisture, and phosphorous availability, suggesting that disturbance could dramatically alter soil legacy effects. Because the spatial arrangement of soil patches has coexistence implications, plant–soil feedback studies should move beyond studies of mean effects of single patch types, to consider how the spatial arrangement of patches in the field influences plant communities.     

Structure and ecological function of the soil microbiome affecting plant–soil feedbacks in the presence of a soil-borne pathogen

Interactions between plants and soil microbes are important for plant growth and resistance. Through plant–soil-feedbacks, growth of a plant is influenced by the previous plant that was growing in the same soil. We performed a plant–soil feedback study with 37 grass, forb and legume species, to condition the soil and then tested the effects of plant-induced changes in soil microbiomes on the growth of the commercially important cut-flower Chrysanthemum in presence and absence of a pathogen. We analysed the fungal and bacterial communities in these soils using next-generation sequencing and examined their relationship with plant growth in inoculated soils with or without the root pathogen, Pythium ultimum. We show that a large part of the soil microbiome is plant species-specific while a smaller part is conserved at the plant family level. We further identified clusters of plant species creating plant growth promoting microbiomes that suppress concomitantly plant pathogens. Especially soil inocula with higher relative abundances of arbuscular mycorrhizal fungi caused positive effects on the Chrysanthemum growth when exposed to the pathogen. We conclude that plants differ greatly in how they influence the soil microbiome and that plant growth and protection against pathogens is associated with a complex soil microbial community.     

Management,winter climate and plant–soil feedbacks on ski slopes: a synthesis

Owing to the increasing popularity of skiing and the upslope movement of the snow reliability line in mountain regions, more and more alpine environments are being turned into skiing areas, with strong impacts on ecosystem functions and biodiversity. Creation and management of ski slopes cause physical disturbance to soil and vegetation, while (artificial) snow supplements affect soil structure, chemistry, moisture and temperature regimes as well as shifts in snow season and growing season length. Vegetation–soil feedbacks may influence the outcome of these interactive effects on soil and vegetation, with possible consequences for soil erosion. Moreover, climate warming will lead to changing snow cover and duration, which will interact with ski slope management effects on soil and vegetation and its feedbacks. Based on a conceptual framework we review the main elements of these interactive effects on soil and vegetation on new and established ski slopes. We also set a research agenda with specific studies that could further advance our understanding of interacting ski slope management, winter climate, vegetation–soil feedbacks and ecosystem functioning. In such new investigations, alpine climate change ecology can probably learn much from the “experimental” disturbance and snow manipulations on ski slopes and vice versa.     

The role of plant–soil feedbacks and land-use legacies in restoration of a temperate steppe in northern China

Plant–soil feedbacks affect plant performance and plant community dynamics; however, little is known about their role in ecological restoration. Here, we studied plant–soil feedbacks in restoration of steppe vegetation after agricultural disturbance in northern China. First, we analyzed abiotic and biotic soil properties under mono-dominant plant patches in an old-field restoration site and in a ‘target’ steppe site. Second, we tested plant–soil feedbacks by growing plant species from these two sites on soils from con- and heterospecific origin. Soil properties generally did not differ between the old-field site and steppe site, but there were significant differences among mono-dominant plant patches within the sites. While soil species origin (i.e., the plant species beneath which the soil was collected) affected biomass of individual plant species in the feedback experiment, species-level plant–soil feedbacks were ‘neutral’. Soil site origin (old-field, steppe) significantly affected biomass of old-field and steppe species. For example, old-field species had higher biomass in old-field soils than in steppe soils, indicating a positive land-use legacy. However, soil site origin effects depended on the plant species beneath which the soils were collected. The predictive value of abiotic and biotic soil properties in explaining plant biomass differed between and within groups of old-field and steppe species. We conclude that the occurrence of positive land-use legacies for old-field species may retard successional replacement of old-field species by steppe species. However, high levels of idiosyncrasy in responses of old-field and steppe plant species to con- and heterospecific soils indicate interspecific variation in the extent to which soil legacies and plant–soil feedbacks control successional species replacements in Chinese steppe ecosystems.     

Agroecosystem resilience is modified by management system via plant–soil feedbacks

Designing resilient cropping systems is essential to sustain agricultural production in the face of changing environmental and social pressures. However, the extent to which changes in farm management systems could alter resistance and resilience is largely unknown, especially in response to climate change. Plant and soil microbial community interactions are a vital component of functioning and resilient agroecosystems. The aim of our study was to use winter wheat (Triticum aestivum L.) and pea (Pisum sativum L.) plant–soil feedbacks (i.e. plant species-specific effects on soil biota and their impacts on subsequent plant growth) as a metric of system resilience and resistance to climate variability in three different farming management systems: 1) a chemical no-till system, 2) an USDA-certified organic system reliant on tillage and 3) an USDA-certified organic system that included sheep grazing with the overall goal of minimizing tillage intensity. Climate conditions soil experienced were ambient, warmer, and warmer and drier and were manipulated in the field using open-top chamber and rain-out shelters. Plant–soil feedbacks were negative for wheat and positive for pea but varied among farming management systems but were less sensitive to climate conditions. Plant–soil feedbacks were lower in magnitude in the tilled organic system indicating more resistance to the accumulation of pathogenic soil microbiota resulting from repeated cropping of wheat. However, recovery was lower when the crop was pea in the tilled organic indicating slower recovery and less resilience. Results indicate that while increases in crop diversity may promote more resilient agroecosystems, farming management will affect agroecosystem resilience.     

Local and regional abundance of exotic plant species on Mediterranean islands: are species traits important?

Aim We assess the importance of three relevant and readily obtainable life‐history traits (dispersal syndrome, stem height and growth form) and biogeographical origin (European vs. non‐European) on the local and regional abundance of over 400 exotic plant species across eight Mediterranean islands. Location The Mediterranean islands of Lesbos, Rhodes, Crete, Malta, Corsica, Sardinia, Majorca and Minorca. Methods We adopt two abundance criteria for each exotic species: the proportion of islands in which the species occurs (regional abundance), and a qualitative estimate of species abundance within each of five islands (local abundance). Subsequently, we assess the relationship between local and regional abundance, as well as the role of key life‐history traits on both regional and local abundance. These analyses were undertaken separately for the European exotics and the non‐European exotics. Results Only 10.9% of the species occur on more than four islands, and only four species are present on all eight islands. Both local and regional abundances were higher for the non‐European than the European species. Local and regional abundances were positively correlated, particularly for exotics with non‐European origins. Wind‐dispersed species tended to have higher regional abundance than species dispersed by other means but this trend only occurred for local abundance on two islands — Corsica and Majorca. Neither a species’ growth form nor its stem height explained trends in regional or local abundance. Conclusions Although wind‐dispersed exotics are more widespread in the Mediterranean, plant life‐history traits appear to play a lesser role in invasion success than area of biogeographical origin. In general, exotic species of non‐European origin were more abundant at both local and regional scales. Invasion patterns should be interpreted at both local and regional scales, but the stochastic nature of biological invasions may limit deterministic interpretations of invasion patterns, especially if islands are studied in isolation.     

Determinants of annual–perennial plant zonation across a salt–fresh marsh interface: a multistage assessment

Vegetation zonation patterns in coastal marshes are hypothesized to be the result of both physical stress and competitive interactions. How these patterns may be driven by these factors at different life history stages remains poorly understood. We investigated the relative importance of species tolerance (response to physical stress) and competitive ability in determining the distributions of two dominant marsh species across a salt–fresh marsh interface in the Yellow River Estuary, China. There is a steep gradient in salinity across this interface and Suaeda salsa, an annual, dominates the saline side of the interface, while Phragmites australis, a perennial species, dominates the freshwater side. Using a series of field transplants, we examined the roles of physical stress and competition in mediating this zonation at different life history stages. Suaeda salsa performed well in its home zone, but seedling emergence, seedling survival, adult survival and adult growth were significantly suppressed by competition in the freshwater P. australis zone. Emergence, survival and growth of P. australis were inhibited in the saline S. salsa zone, regardless of neighbor treatments, but it performed well in its home zone. The magnitude of the competitive effect on the performance of S. salsa differed among the life history stages. Competition from P. australis had a much stronger effect on S. salsa seedling emergence and adult growth than on seedling survival and adult survival. Our results reveal that both physical stress and competition contributed to the observed zonation patterns in this marsh system. However, for S. salsa, the effect of competition varied with life-history stage. Insight into these ecological processes is critical to understanding how the zonation pattern in the marsh system is formed and maintained.     

Effects of exotic-native species relationship on naturalization and invasion of exotic plant species北大核心CSCD

Aims: Darwin's naturalization conundrum describes the paradox that the relationship of exotic species to native residents could either promote or hinder invasion success through opposing mechanisms: niche pre-adaptation or competitive interactions. Previous Darwin's naturalization studies have showed invasion success could vary at stages, sites, and spatial and phylogenetic scales. Our objective was to assess the effects of exotic-native species relationship on invasion process of exotic plant species in China, where related research is still lacking. Methods: Generalized linear mixed models were used to examine relationship between exotic-native species relationship and performance of exotic species at different spatial scale (provincial, municipal and community) and invasion stages (naturalization, dispersal and invasion). At community scale, we measured environmental factors of communities we investigated to control the effect of habitat heterogeneity among them. Important findings: At the provincial and municipal scales, exotic species closely related to native flora were more likely to be naturalized and distributed, which is more consistent with the expectation of the pre-adaptation hypothesis. On the community scale, the exotic-native species relationship was not related to establishment and abundance of exotic species in the community. The results suggested that exotic species did not strongly compete with their close native relatives in communities, but were better adapted to areas where their close relatives had lived. Considering their high potential of naturalization and invasion, special attention should be paid to those exotic species that closely related to the native flora in the management of invasive species. © Editorial Office of Chinese Journal of Plant Ecology. All rights reserved.     

Pervasive and strong effects of plants on soil chemistry: a meta-analysis of individual plant ‘Zinke’ effects

Plant species leave a chemical signature in the soils below them, generating fine-scale spatial variation that drives ecological processes. Since the publication of a seminal paper on plant-mediated soil heterogeneity by Paul Zinke in 1962, a robust literature has developed examining effects of individual plants on their local environments (individual plant effects). Here, we synthesize this work using meta-analysis to show that plant effects are strong and pervasive across ecosystems on six continents. Overall, soil properties beneath individual plants differ from those of neighbours by an average of 41%. Although the magnitudes of individual plant effects exhibit weak relationships with climate and latitude, they are significantly stronger in deserts and tundra than forests, and weaker in intensively managed ecosystems. The ubiquitous effects of plant individuals and species on local soil properties imply that individual plant effects have a role in plant–soil feedbacks, linking individual plants with biogeochemical processes at the ecosystem scale.     

Plant traits shape soil legacy effects on individual plant–insect interactions

Plant-mediated soil legacy effects can be important determinants of the performance of plants and their aboveground insect herbivores, but, soil legacy effects on plant–insect interactions have been tested for only a limited number of host plant species and soils. Here, we tested the performance of a polyphagous aboveground herbivore, caterpillars of the cabbage moth Mamestra brassicae, on twelve host plant species that were grown on a set of soils conditioned by each of these twelve species. We tested how growth rate (fast- or slow-growing) and functional type (grass or forb) of the plant species that conditioned the soil and of the responding host plant species growing in those soils affect the response of insect herbivores to conditioned soils. Our results show that plants and insect herbivores had lower biomass in soils that were conditioned by fast-growing forbs than in soils conditioned by slow-growing forbs. In soils conditioned by grasses, growth rate of the conditioning plant had the opposite effect, i.e. plants and herbivores had higher biomass in soils conditioned by fast-growing grasses, than in soils conditioned by slow-growing grasses. We show that the response of aboveground insects to soil legacy effects is strongly positively correlated with the response of the host plant species, indicating that plant vigour may explain these relationships. We provide evidence that soil communities can play an important role in shaping plant–insect interactions aboveground. Our results further emphasize the important and interactive role of the conditioning and the response plant in mediating soil–plant–insect interactions.     

Organic matter–microorganism–plant in soil bioremediation: a synergic approach

Bioremediation is a natural process, which relies on bacteria, fungi, and plants to degrade, break down, transform, and/or essentially remove contaminants, ensuring the conservation of the ecosystem biophysical properties. Since microorganisms are the former agents for the degradation of organic contaminants in soil, the application of organic matter (such as compost, sewage sludge, etc.), which increases microbial density and also provides nutrients and readily degradable organic matter (bioenhancement–bioaugmentation) can be considered useful to accelerate the contaminant degradation. Moreover, the organic matter addition, by means of the increase of cation exchange capacity, soil porosity and water-holding capacity, enhances the soil health and provides a medium satisfactory for microorganism activity. Plants have been also recently used in soil reclamation strategy both for their ability to uptake, transform, and store the contaminants, and to promote the degradation of organic contaminants by microbes at rhizosphere level. It is widely recognized that plant, through organic materials, nutrients and oxygen supply, produces a rich microenvironment capable of promoting microbial proliferation and activity.     

Effect of plant–soil feedbacks on the growth and competition of <Emphasis Type="Italic">Lactuca</Emphasis> species

Plant species generate specific soil communities that feedback on plant growth and competition. These feedbacks have been implicated in plant community composition and dispersion. We used Lactuca sativa and its wild progenitor Lactuca serriola to test the hypotheses that separate Lactuca species generate unique soil communities and that these soil communities differentially influence host, and neighboring, plant growth and competition. We grew each Lactuca in competition with the other, in sterile and non-sterile soils. We then examined the growth of each Lactuca species in sterile, non-sterile, and preconditioned soil. Finally, we used TRFLP techniques to explore whether the two Lactuca species generate significantly different bacterial communities in their rhizosphere soils. L. sativa proved to be the stronger competitor of the two species. However, sterilization increased the competitive effect of L. serriola background competitors. The growth experiment showed a significant effect on plant species, soil treatment, and the interaction of the two. Preconditioning soil caused reduced growth in both Lactuca species. Only L. serriola showed significantly increased growth in sterile soils. Our TRFLP analysis showed that the L. sativa soil community was significantly less diverse and that soil preconditioning had the largest impact on the community composition. These results show that Lactuca serriola’s rhizosphere communities generate a stronger negative feedback for plant growth than do the communities associated with L. sativa. Our study suggests that selection for plants that are able to grow in dense monoculture may have released Lactuca from species-specific negative soil feedbacks. This has important implications for both agriculture and the evolution of invasive plant species.     

Soil conditioning and plant–soil feedbacks affect competitive relationships between native and invasive grasses

Understanding how competition from invasive species and soil conditions individually and interactively affect native performance will increase knowledge of invasion dynamics and can be used to improve the success of restoration plans. This study, conducted in Reno NV, USA, uses a two-phase plant?Csoil feedback experiment coupled with a target-neighbor competition design to examine the individual and interactive effects of both soil conditions and invasive neighbors on native performance. Study species include invasive species (Bromus tectorum and Agropyron cristatum) and native species (Elymus elymoides and Pseudoroegneria spicata). Results indicate that both plant performance and competitive interactions were influenced by species-specific soil conditioning. Specifically, invasive B. tectorum generated a larger competitive effect on natives than invasive A. cristatum; however, only A. cristatum conditioned soil in a manner that increased competitive effects of conspecifics on natives. Native P. spicata was relatively unaffected by soil conditioning and conversely, E. elymoides was strongly affected by soil conditioning. Few previous studies have examined soil conditioning and the interaction of soil conditioning and neighbor effects that both are potentially important mechanisms in structuring plant communities and influencing plant invasion.     

Additive effects of exotic plant abundance and land-use intensity on plant–pollinator interactions

The continuing spread of exotic plants and increasing human land-use are two major drivers of global change threatening ecosystems, species and their interactions. Separate effects of these two drivers on plant–pollinator interactions have been thoroughly studied, but we still lack an understanding of combined and potential interactive effects. In a subtropical South African landscape, we studied 17 plant–pollinator networks along two gradients of relative abundance of exotics and land-use intensity. In general, pollinator visitation rates were lower on exotic plants than on native ones. Surprisingly, while visitation rates on native plants increased with relative abundance of exotics and land-use intensity, pollinator visitation on exotic plants decreased along the same gradients. There was a decrease in the specialization of plants on pollinators and vice versa with both drivers, regardless of plant origin. Decreases in pollinator specialization thereby seemed to be mediated by a species turnover towards habitat generalists. However, contrary to expectations, we detected no interactive effects between the two drivers. Our results suggest that exotic plants and land-use promote generalist plants and pollinators, while negatively affecting specialized plant–pollinator interactions. Weak integration and high specialization of exotic plants may have prevented interactive effects between exotic plants and land-use. Still, the additive effects of exotic plants and land-use on specialized plant–pollinator interactions would have been overlooked in a single-factor study. We therefore highlight the need to consider multiple drivers of global change in ecological research and conservation management.     

Intraspecific and interspecific pair-wise seedling competition between exotic annual grasses and native perennials: plant–soil relationships

Few studies have examined plant–soil relationships in competitive arenas between exotic and native plants in the western United States. A pair-wise competitive design was used to evaluate plant–soil relationships between seedlings of the exotic annual grasses Bromus tectorum and Taeniatherium caput-medusae and the native perennial grasses Elymus elymoides and Pseudoroegneria spicata. Two soils were tested: an arid soil (argid) occupied by E. elymoides and presently invaded by B. tectorum and a high elevation, high organic matter, soil (aquept) where none of the tested species would typically occur. Plant growth proceeded for 85 days at which time above-ground biomass and tissue nutrient concentrations were quantified. Soil also was collected from the rooting zone beneath each species and analyzed for various nutrient pools. The exotic species had significantly greater above-ground biomass than the natives and grew far better in the aquept soil than the argid soil. Growth of B. tectorum, and to some degree, T. caput-medusae was suppressed in intraspecific competition and enhanced, especially in the aquept soil, when competing with the natives. Although not significant, biomass of natives strongly trended downward when competing with the exotic grasses. Overall, concentrations of tissue nutrients were minimally affected by competition, but natives tended to be more negatively affected by competition with exotics. Except for phosphorus (P), all species had significantly greater nutrient concentrations when growing in the aquept soil compared to the argid soil. In both soils, exotics had significant greater tissue concentrations of manganese (Mn), magnesium (Mg), and iron (Fe), while natives had significantly greater nitrogen (N). Species affects on soil nutrient pools occurred mostly in the aquept soil with exotic species significantly decreasing pools of available N, potentially available N, and soil-solution pools of calcium (Ca²⁺), potassium (K⁺), and magnesium (Mg²⁺) relative to natives. Overall, the data suggest that, in the seedling state, B. tectorum is a superior competitor. Moreover, when the natives compete intra- or interspecifically, particularly in the aquept soil, availability of N and other nutrients in their rooting zone is consistently greater than when they compete interspecifically with the exotic grasses. These data suggest the exotics are able to co-opt nutrients in the rooting zone of the natives and perhaps gain a competitive advantage.