Negative Plant-soil Feedback Research Articles

Host plant phylogeny and abundance predict root‐associated fungal community composition and diversity of mutualists and pathogens

Abstract Interactions between plants and their root‐associated fungi (RAF) may influence the relative abundance of tree species and determine forest community diversity. Such plant–soil feedbacks in turn depend on the degree to which spatial distance and phylogenetic relatedness of host trees structure pathogen and mutualist communities, but research detailing these aspects of RAF communities is lacking. Here, we characterize plant–RAF associations across a diverse plant community, focusing on the degree to which RAF communities are structured by spatial distance, host phylogenetic relatedness, and host abundance. We compare results for different functional groups, including both putative mutualists and pathogens, an aspect poorly examined hitherto. We collected roots at regular intervals along ten 50 m by 2 m transects, then used DNA barcoding to identify host plants, and characterize the associated fungal community. Variance partitioning was used to measure the relative contributions of host phylogenetic relatedness and spatial distance to explaining RAF community composition. A weighted linear regression was used to measure the correlation between host abundance and RAF diversity. Phylogenetic distance among hosts was a better predictor of RAF community composition than spatial distance, but this relationship was stronger for putative pathogens than for mutualists, suggesting that pathogens show stronger host preference than mutualists. Across all functional groups, RAF showed similar levels of spatial structure. Additionally, RAF communities of locally abundant plants were less diverse than RAF communities of rare plants. Synthesis. We found that RAF communities are structured by the phylogenetic relatedness of hosts and, to a lesser extent, by spatial distance, with pathogens showing stronger host preference than mutualists. Abundant hosts had less diverse RAF communities than rare hosts, which is notable because abundant plants tend to experience weaker negative plant–soil feedback. Going forward, mechanisms underlying the host abundance‐RAF diversity relationship warrant further investigation. Additionally, the survey approach presented here could be paired with experiments linking RAF community composition to plant recruitment.

Plant-soil feedback is shut down when nutrients come to town

Background and aimsThe concept of plant-soil feedback is increasingly used to explain plant community assembly processes. Soil nutrient availability can be expected to play a critical role on these processes. However, little is known about the effects of nutrient availability on feedback direction and strength.MethodsA plant-soil feedback experiment was performed with the grasses Anthoxanthum odoratum and Festuca rubra, and the forbs Leontodon hispidus and Plantago lanceolata, on soil with either low or high nutrient availability. Additionally, we tested if plant-soil feedback of the two forbs under these conditions changed by inoculation of the soil with spores of an arbuscular mycorrhizal fungus.ResultsIncreased nutrient availability neutralised plant-soil feedback based on shoot biomass independent of its negative or positive direction, whereas the effects on root biomass were either not altered or turned negative. Mycorrhizal fungi spore addition decreased negative feedback and increased positive feedback.ConclusionsOur results suggest that negative plant-soil feedback on low nutrient soil can be overcome with nutrient addition, and that positive soil biota associations on low nutrient soil may become superfluous with nutrient increase. We hypothesize that species-specific, microbial mediated plant community assembly processes occur in low rather than high nutrient environments.

Root traits and belowground herbivores relate to plant\u2013soil feedback variation among congeners

Plant–soil feedbacks contribute to vegetation dynamics by species-specific interactions between plants and soil biota. Variation in plant–soil feedbacks can be predicted by root traits, successional position, and plant nativeness. However, it is unknown whether closely related plant species develop more similar plant–soil feedbacks than more distantly related species. Where previous comparisons included plant species from distant phylogenetic positions, we studied plant–soil feedbacks of congeneric species. Using eight intra-continentally range-expanding and native Geranium species, we tested relations between phylogenetic distances, chemical and structural root traits, root microbiomes, and plant–soil feedbacks. We show that root chemistry and specific root length better predict bacterial and fungal community composition than phylogenetic distance. Negative plant–soil feedback strength correlates with root-feeding nematode numbers, whereas microbiome dissimilarity, nativeness, or phylogeny does not predict plant–soil feedbacks. We conclude that root microbiome variation among congeners is best explained by root traits, and that root-feeding nematode abundances predict plant–soil feedbacks.

Domesticated tomatoes are more vulnerable to negative plant–soil feedbacks than their wild relatives

Abstract Domesticated plants can differ from their wild counterparts in the strength and outcome of species interactions, both above‐ and belowground. Plant–soil feedbacks influence plant success, and plant‐associated soil microbial communities can influence plant interactions with herbivores and their natural enemies, yet, it remains unclear if domestication has changed these relationships. To determine the effects of domestication on plant–soil interactions, we characterized soil microbial communities associated with various cultivars of domesticated tomato and some of its wild relatives. We measured the strength and direction of plant–soil feedbacks for domesticated and wild tomatoes, and the effects of soil on plant resistance to specialist herbivory by Manduca sexta, and the attraction of a parasitoid wasp, Cotesia congregata. Domesticated tomatoes and their wild relatives had negative plant–soil feedbacks, as conspecifics cultivated soil that negatively impacted performance of subsequent plants (longer germination time, lower biomass) than if they grew in non‐tomato soils. Significant variation existed among domesticated and wild tomato varieties in the strength of these feedbacks, ranging from neutral to strongly negative. For above‐ground plant biomass, tomato wild relatives were unaffected by growing in tomato‐conditioned soil, whereas domesticated tomatoes grew smaller in tomato soil, indicating effects of plant domestication. Overall, increased microbial biomass within the rhizosphere resulted in progressively less‐negative plant–soil feedbacks. Plant cultivars had different levels of resistance to herbivory by M. sexta, but this did not depend on plant domestication or soil type. The parasitoid C. congregata was primarily attracted to herbivore damaged plants, independent of plant domestication status, and for these damaged plants, wasps preferred some cultivars over others, and wild plants grown in tomato soil over wild plants grown in non‐tomato soil. Synthesis. These results indicate that crop tomatoes are more likely to show negative plant–soil feedbacks than wild progenitors, which could partially explain their sensitivity to monocultures in agricultural soils. Furthermore, cultivar‐specific variation in the ability to generate soil microbial biomass, independent of domestication status, appears to buffer the negative consequences of sharing the same soil. Last, soil legacies were relatively absent for herbivores, but not for parasitoid wasps, suggesting trophic level specificity in soil feedbacks on plant–insect interactions.

Steaming combined with biochar application eliminates negative plant-soil feedback for sanqi cultivation

Eliminating negative plant-soil feedback (NPSF) in agricultural production systems is the key step in maintaining soil health and sustainable crop production. Sanqi (Panax notoginseng) has been severely hampered by NPSF due to the build-up of soilborne pathogens and the accumulation of autotoxic ginsenosides. Steaming on soil followed by adding biochar was investigated. Greenhouse experiments demonstrated that consecutively cultivated soil treated with steaming (90 °C for 15 min) significantly increased seed germination rates and seedling survival rates of sanqi, but biochar amendment had no effects. The combination of steaming with biochar amendments (8 g L−1) was more effective on seed germination and seedling survival rates as well as increasing the biomass of sanqi than applying steaming alone. Steaming slightly degraded the content of autotoxic ginsenosides in the soil and completely eliminated the population of soilborne pathogens, including Ilyonectria spp. and Fusarium spp., in consecutively cultivated soil, and enhanced the release of nutritive substances into soil. Biochar showed a high adsorption capacity for autotoxic ginsenosides, and increased nutrient supplementation. Steaming plus biochar could effectively eliminate the negative effects of soilborne pathogens and autotoxicants to P. notoginseng. Four years of field studies further confirmed that consecutively cultivated soil could be successfully replanted after steaming plus biochar application. Thus, steaming plus biochar amendment was a promising practice for enhancing soil quality and health.

Plant interactions modulate root litter decomposition and negative plant-soil feedback with an invasive plant

Invasive plants often modify soil biota to benefit their own fitness over native species, and create a positive plant-soil feedback (PSF). Rhizosphere effect (RE), the alterations of decomposition processes by living roots and associated rhizodeposits, may act as a mechanism that contributes to the positive PSF via accelerating litter decomposition and subsequent soil nutrient turnover. However, the relative effects attributable to litter quality versus plant composition and edaphic conditions on RE remain unclear. A reciprocal translocation decomposition experiment was conducted in a screenhouse with root litter from an invasive (Flaveria bidentis) and a native species (Bidens maximowicziana) in two types of soils (one dominated by the invasive species and another by the native species) with four plantings (two monocultures, one mixture with both species, and one without living plants). We examined the performance of invasive and native species responses to different soils (PSF) and quantified the RE on litter decomposition. PSF and RE were dependent on whether focal species grew in the monocultures or mixture, and interspecific competition ameliorated negative PSF of F. bidentis when it competed with a weaker competitor. All RE values associated with B. maximowicziana litter under the monocultures were positive, whereas F. bidentis litter exhibited neutral to negative REs, mainly depending on the soil type examined. The RE values in the invaded soil were significantly greater than those in the native soil under the monocultures. However, mixture planting tended to decrease REs on decomposition, which in turn feedback to host plant by affecting soil N availability and microbial composition. Our study showed that innate differences between the two litters in terms of their decomposability and edaphic conditions are likely to exert a much greater impact in controlling litter decomposition. Importantly, rhizosphere-mediate belowground process has the potential to be an important contributor to plant-interaction outcomes.

Plant-soil feedbacks promote coexistence and resilience in multi-species communities.

Both ecological theory and empirical evidence suggest that negative frequency dependent feedbacks structure plant communities, but integration of these findings has been limited. Here we develop a generic model of frequency dependent feedback to analyze coexistence and invasibility in random theoretical and real communities for which frequency dependence through plant-soil feedbacks (PSFs) was determined empirically. We investigated community stability and invasibility by means of mechanistic analysis of invasion conditions and numerical simulations. We found that communities fall along a spectrum of coexistence types ranging from strict pair-wise negative feedback to strict intransitive networks. Intermediate community structures characterized by partial intransitivity may feature “keystone competitors” which disproportionately influence community stability. Real communities were characterized by stronger negative feedback and higher robustness to species loss than randomly assembled communities. Partial intransitivity became increasingly likely in more diverse communities. The results presented here theoretically explain why more diverse communities are characterized by stronger negative frequency dependent feedbacks, a pattern previously encountered in observational studies. Natural communities are more likely to be maintained by strict negative plant-soil feedback than expected by chance, but our results also show that community stability often depends on partial intransitivity. These results suggest that plant-soil feedbacks can facilitate coexistence in multi-species communities, but that these feedbacks may also initiate cascading effects on community diversity following from single-species loss.

Continuous Monoculture Shapes Root and Rhizosphere Fungal Communities of Corn and Soybean in Soybean Cyst Nematode-Infested Soil

The rhizosphere effect shapes microbial communities around and within plant roots and may lead to the enrichment of fungi involved in both positive and negative plant-soil feedbacks. We used internal transcribed spacer 1 amplicon sequencing to investigate how continuous monoculture affects the fungal communities in the corn and soybean rhizosphere, rhizoplane, and root endosphere in a long-term crop rotation experiment where soils were infested with a major soybean pathogen, the soybean cyst nematode (SCN, Heterodera glycines). Community-level statistical analyses showed evidence of selective filtering and enrichment of fungi in and around corn and soybean roots and in SCN cysts. Patterns of relatedness between fungal communities in various agroecosystem compartments suggested that SCN cysts are colonized by soybean root endophytic fungi and that colonization of roots and cysts may aid proliferation of these fungi in the bulk soil agroecosystem compartment over time. Natural antagonists of the SCN such as nematode-trapping fungi and nematode endoparasites increased in relative abundance in the rhizosphere and root endosphere, respectively, over continuous soybean monoculture. In contrast, arbuscular mycorrhizal and plant-pathogenic fungi, several of which were negatively correlated with corn yield, increased in relative abundance over continuous corn monoculture. These results suggest the possibility of positive plant-soil feedbacks involving nematophagous fungi over continuous soybean monoculture and negative plant-soil feedbacks involving plant pathogens and some nonbeneficial arbuscular mycorrhizal fungi over continuous corn monoculture.

Carry-over effects of soil inoculation on plant growth and health under sequential exposure to soil-borne diseases

Background and aimsMost plant-soil feedback and inoculation studies are limited to one growth cycle. We examined the effects of inoculation with eight plant-conditioned soils on chrysanthemum during two sequential growth cycles. The plants were also exposed sequentially to soil diseases.MethodsIn cycle 1, plants were grown in sterile soil inoculated or not with plant-conditioned soils, and exposed or not to Pythium or root feeding nematodes. In cycle 2, new plants were grown in soil from cycle 1 or in new 100% sterile soil. Plants were exposed again to Pythium, or to soil with pathogens and nematodes collected from a commercial chrysanthemum greenhouse.ResultsAfter two cycles, effects of soil inoculation on plant growth were still present. Chrysanthemum exhibited a negative conspecific feedback response, but this was less strong in inoculated soils. Pythium or nematode addition did not affect plant growth. However, addition of pathogen-containing soil from the commercial greenhouse reduced plant growth in sterile soil but increased growth in plant-conditioned soils.ConclusionsInoculation with plant-conditioned soil can reduce the negative conspecific plant-soil feedback of chrysanthemum. Our study further advances our understanding of the temporal dynamics of conspecific and heterospecific plant-soil feedbacks, and how they interact with soil-borne diseases.

Removal of soil biota alters soil feedback effects on plant growth and defense chemistry.

Summary We examined how the removal of soil biota affects plant–soil feedback (PSF) and defense chemistry of Jacobaea vulgaris, an outbreak plant species in Europe containing the defense compounds pyrrolizidine alkaloids (PAs).Macrofauna and mesofauna, as well as fungi and bacteria, were removed size selectively from unplanted soil or soil planted with J. vulgaris exposed or not to above‐ or belowground insect herbivores. Wet‐sieved fractions, using 1000‐, 20‐, 5‐ and 0.2‐μm mesh sizes, were added to sterilized soil and new plants were grown. Sieving treatments were verified by molecular analysis of the inocula.In the feedback phase, plant biomass was lowest in soils with 1000‐ and 20‐μm inocula, and soils conditioned with plants gave more negative feedback than without plants. Remarkably, part of this negative PSF effect remained present in the 0.2‐μm inoculum where no bacteria were present. PA concentration and composition of plants with 1000‐ or 20‐μm inocula differed from those with 5‐ or 0.2‐μm inocula, but only if soils had been conditioned by undamaged plants or plants damaged by aboveground herbivores. These effects correlated with leaf hyperspectral reflectance.We conclude that size‐selective removal of soil biota altered PSFs, but that these PSFs were also influenced by herbivory during the conditioning phase.

Soil microbial communities alter conspecific and congeneric competition consistent with patterns of field coexistence in three Trifolium congeners

Abstract Coexistence and diversity in plant communities depend upon outcomes of plant competition. Competition and coexistence can be mediated by abiotic soil nutrient differences as well as by soil microbial communities. The latter effects occur through various mechanisms including negative plant–soil feedbacks, when plants foster the build‐up of specialized pathogenic microbes, which ultimately reduce conspecific, but not heterospecific, densities. Microbial mutualists can have generalized associations with host plants, and by associating with multiple species might affect coexistence by conferring different levels of benefit to hosts. We examined the effects of abiotic differences and soil microbial communities, including mutualistic nitrogen‐fixing rhizobial bacteria, on coexistence processes and asked whether these interactions inform patterns of co‐occurrence in natural communities. We measured plant–soil feedbacks in the greenhouse for three native Trifolium species that either highly co‐occurred or were spatially repulsed at our field site. Using size‐fractioned soil microbial inocula prepared from field‐collected soils, we explored the effect of soil microbes on nodulation and the outcome of competition (relative interaction intensity). We also examined the effects of soil origin (home vs. away soil) on the outcome of competition between species. Soil microbes had strong positive effects on plant growth and nodulation. Microbes in general reduced the strength of plant competition relative to competition in sterilized soil and altered the relative strength of interactions with conspecific vs. congeneric neighbours, which often occurred in ways predicted to enhance coexistence. In one pair of highly co‐occurring Trifolium, competition was strong in sterilized soils, but these species facilitated one other in the presence of the full microbial community. The net effect of these microbes in general reduced competition while also increasing performance over that in sterilized soil, suggesting a role for soil microbial mutualists in host coexistence. Synthesis. Our results demonstrate that interactions between plants and diverse soil communities can alter plant–plant interactions and plant–soil feedbacks. They may increase niche differences and equalize fitness differences in ways consistent with observed co‐occurrence in the field.

Biotic and abiotic plant–soil feedback depends on nitrogen‐acquisition strategy and shifts during long‐term ecosystem development

Abstract Feedback between plants and soil is an important driver of plant community structure, but it remains unclear whether plant–soil feedback (PSF): (i) reflects changes in biotic or abiotic properties, (ii) depends on environmental context in terms of soil nutrient availability, and (iii) varies among plant functional groups. As soil nutrient availability strongly affects plant distribution and performance, soil chemical properties and plant nutrient‐acquisition strategies might serve as important drivers of PSF. We used soils from young and old stages of a long‐term soil chronosequence to represent sites where productivity is limited by nitrogen (N) and phosphorus (P) availability, respectively. We grew three N‐fixing and three non‐N‐fixing plant species in soils conditioned by co‐occurring conspecific or heterospecific species from each of these two stages. In addition, three soil treatments were used to distinguish biotic and abiotic effects on plant performance, allowing measurements of overall, biotic, and abiotic PSF. In young, N‐poor soils, non‐N‐fixing plants grew better in soils from N‐fixing plants than in their own soils (i.e., negative PSF). However, this difference was not only associated with improved abiotic conditions in soils from N‐fixing plants but also with changes in soil biota. By contrast, no significant PSF was observed for N‐fixing plants grown in young soils. Moreover, we did not observe any significant PSF for either N‐fixing or non‐N‐fixing plants growing in old, P‐impoverished soils. Synthesis. The direction and strength of plant‐soil feedback (PSF) varied among N‐acquisition strategies and soils differing in nutrient availability, with stronger plant‐soil feedback in younger, N‐poor soils compared to older, P‐impoverished soils. Our results highlight the importance of considering soil nutrient availability, plant‐mediated abiotic and biotic soil properties, and plant nutrient‐acquisition strategies when studying plant‐soil feedback, thereby advancing our mechanistic understanding of plant‐soil feedback during long‐term ecosystem development.

Relative importance of competition and plant-soil feedback, their synergy, context dependency and implications for coexistence.

Plants interact simultaneously with each other and with soil biota, yet the relative importance of competition vs. plant-soil feedback (PSF) on plant performance is poorly understood. Using a meta-analysis of 38 published studies and 150 plant species, we show that effects of interspecific competition (either growing plants with a competitor or singly, or comparing inter- vs. intraspecific competition) and PSF (comparing home vs. away soil, live vs. sterile soil, or control vs. fungicide-treated soil) depended on treatments but were predominantly negative, broadly comparable in magnitude, and additive or synergistic. Stronger competitors experienced more negative PSF than weaker competitors when controlling for density (inter- to intraspecific competition), suggesting that PSF could prevent competitive dominance and promote coexistence. When competition was measured against plants growing singly, the strength of competition overwhelmed PSF, indicating that the relative importance of PSF may depend not only on neighbour identity but also density. We evaluate how competition and PSFs might interact across resource gradients; PSF will likely strengthen competitive interactions in high resource environments and enhance facilitative interactions in low-resource environments. Finally, we provide a framework for filling key knowledge gaps and advancing our understanding of how these biotic interactions influence community structure.

Competition alters plant–soil feedbacks of two species in the Inner Mongolia Steppe, China

A dominant species of steppe in northern China, Stipa grandis, is gradually being replaced by S. krylovii due to climate change events. The aim of this study was to explore how conditioned soil, nitrogen (N) resource, and competitor influenced the responses of S. grandis and S. krylovii. We planted S. grandis and S. krylovii monoculture and mixture in each of four conditioned soils with two N treatments and analyzed which factor(s) significantly influenced the plant–soil feedbacks (PSFs), competitiveness, and N-addition effects of each species by general linear model univariate analysis. Competitor but not nitrogen significantly influenced PSFs of both S. grandis and S. krylovii. Compared with PSFs in monoculture, the negative PSFs of S. grandis were suppressed and PSFs of S. krylovii shifted from positive to negative in mixture. Competitor only significantly influenced the N-addition effects on S. krylovii; nitrogen only significantly influenced the competitiveness of S. krylovii. The highest competitiveness of S. krylovii was obtained in S. grandis conditioning soil with high N treatment. Compared with any of the other growth conditions, soil conditioned by S. grandis with high N treatment could increase S. krylovii’s competition superiority by increasing its competitiveness and N-addition effect.

Density-dependency and plant-soil feedback: former plant abundance influences competitive interactions between two grassland plant species through plant-soil feedbacks

Backgrounds and aimsNegative plant-soil feedbacks (PSFs) are thought to promote species coexistence, but most evidence is derived from theoretical models and data from plant monoculture experiments.MethodsWe grew Anthoxanthum odoratum and Centaurea jacea in field plots in monocultures and in mixtures with three ratios (3:1, 2:2 and 1:3) for three years. We then tested in a greenhouse experiment the performance of A. odoratum and C. jacea in pots planted with monocultures and 1:1 mixtures and filled with live and sterile soils collected from the field plots.ResultsIn the greenhouse experiment, C. jacea produced less aboveground biomass in soil conditioned by C. jacea monocultures than in soil conditioned by A. odoratum monocultures, while the aboveground biomass of A. odoratum in general did not differ between the two monospecific soils. The negative PSF effect was greater in the 1:1 plant mixture than in plant monocultures for A. odoratum but did not differ for C. jacea. In the greenhouse experiment, the performance of C. jacea relative to A. odoratum in the 1:1 plant mixture was negatively correlated to the abundance of C. jacea in the field plot where the soil was collected from. This relationship was significant both in live and sterile soils. However, there was no relationship between the performance of A. odoratum relative to C. jacea in the 1:1 plant mixture in the greenhouse experiment and the abundance of A. odoratum in the field plots.ConclusionsThe response of a plant to PSF depends on whether the focal species grows in monocultures or in mixtures and on the identity of the species. Interspecific competition can exacerbate the negative plant-soil feedbacks compared to intraspecific competition when a plant competes with a stronger interspecific competitor. Moreover, the abundance of a species in mixed plant communities, via plant-soil feedback, negatively influences the relative competitiveness of that species when it grows later in interspecific competition, but this effect varies between species. This phenomenon may contribute to the coexistence of competing plants under natural conditions through preventing the dominance of a particular plant species.

The role of plant–soil feedbacks in stabilizing a reindeer‐induced vegetation shift in subarctic tundra

Abstract Herbivory can drive vegetation into different states of productivity and community composition, and these changes may be stable over time due to historical contingency effects. Interactions with abiotic and biotic soil components can contribute to such long‐term legacies in plant communities through stabilizing positive feedbacks. We studied the role of plant–soil feedbacks in maintaining vegetation changes caused by historical (~1350–1900 AD) reindeer herding in northern Sweden. These historical milking grounds (HMGs) consist of meadow plant communities formed in naturally nutrient‐poor heath or naturally nutrient‐rich shrub‐dominated vegetation and are still clearly visible in the landscape, a century after active use ceased. We selected two phytometer species: the forb Potentilla crantzii as representative of HMG vegetation, and the dwarf shrub Betula nana, as representative of control vegetation. We grew both species under glasshouse conditions on soils derived from replicated HMG and paired control plots, using live soils and sterilized (γ‐radiation)‐inoculated soils, to separate between biotic and abiotic soil effects. A net negative plant–soil feedback for B. nana biomass in its home (i.e., control) soil and a net positive feedback for P. crantzii in its home (i.e., HMG) soil in heath habitat was partly driven by the soil biotic community. However, abiotic differences in mineral nitrogen (N) concentrations between control and HMG soils were a stronger driver of differences in plant growth. Positive feedbacks maintaining a high mineral nutrient availability are thus important, especially in nutrient‐poor habitats. The positive plant responses to higher soil mineral N concentrations, combined with positive biotic plant–soil feedbacks, might shift the competitive balance in favour of typical HMG plant species, thereby contributing to stability of HMG plant communities. Our data indicate that herbivore‐driven changes in the interactions between plants and both biotic and abiotic components of the soil persist over long temporal scales. A plain language summary is available for this article.

Fertilizer N application rate impacts plant-soil feedback in a sanqi production system

Replant failure caused by negative plant-soil feedback (NPFS) in agricultural ecosystems is a critical factor restricting the development of sustainable agriculture. Soil nutrient availability has the capacity to affect plant-soil feedback. Here, we used sanqi (Panax notoginseng), which is severely threatened by NPSF, as a model plant to decipher the overall effects of nitrogen (N) rates on NPSF and the underlying mechanism. We found that a high rate of N at 450kgNha−1 (450N) aggravated the NPSF through the accumulation of pathogens in the soil compared with the optimal 250N. The increased N rates resulted in a significant increase in the soil electrical conductivity and available nitrogen but a decrease in the soil pH and C/N ratio. GeoChip 5.0 data demonstrated that these changed soil properties caused the soil to undergo stress (acidification, salinization and carbon starvation), as indicated by the enriched soil microbial gene abundances related to stress response and nutrition cycling (N, C and S). Accordingly, increased N rates reduced the richness and diversity of soil fungi and bacteria and eventually caused a shift in soil microbes from a bacterial-dominant community to a fungal-dominant community. In particular, the high 450N treatment significantly suppressed the abundance of copiotrophic bacteria, including beneficial genera Bacillus and Pseudomonas, thus weakening the antagonistic activity of these bacteria against fungal pathogens. Moreover, 450N application significantly enriched the abundance of pathogen pathogenicity-related genes. Once sanqi plants were grown in this N-stressed soil, their host-specific fungal pathogen Fusarium oxysporum significantly accumulated, which aggravated the process of NPSF. This study suggested that over-application of nitrogen is not beneficial for disease management or the reduction of fungicide application in agricultural production.

Predators in the plant-soil feedback loop: aboveground plant-associated predators may alter the outcome of plant-soil interactions.

Plant-soil feedback (PSF) can structure plant communities, promoting coexistence (negative PSF) or monodominance (positive PSF). At higher trophic levels, predators can alter plant community structure by re-allocating resources within habitats. When predator and plant species are spatially associated, predators may alter the outcome of PSF. Here, I explore the influence of plant-associated predators on PSF using a generalised cellular automaton model that tracks nutrients, plants, herbivores and predators. I explore key contingencies in plant-predator associations such as whether predators associate with live vs. senesced vegetation. Results indicate that plant-associated predators shift PSF to favour the host plant when predators colonise live vegetation, but the outcome of PSF will depend upon plant dispersal distance when predators colonise dead vegetation. I apply the model to two spider-associated invasive plants, finding that spider predators should shift PSF dynamics in a way that inhibits invasion by one forest invader, but exacerbates invasion by another.

The strength of negative plant-soil feedback increases from the intraspecific to the interspecific and the functional group level.

One of the processes that may play a key role in plant species coexistence and ecosystem functioning is plant–soil feedback, the effect of plants on associated soil communities and the resulting feedback on plant performance. Plant–soil feedback at the interspecific level (comparing growth on own soil with growth on soil from different species) has been studied extensively, while plant–soil feedback at the intraspecific level (comparing growth on own soil with growth on soil from different accessions within a species) has only recently gained attention. Very few studies have investigated the direction and strength of feedback among different taxonomic levels, and initial results have been inconclusive, discussing phylogeny, and morphology as possible determinants. To test our hypotheses that the strength of negative feedback on plant performance increases with increasing taxonomic level and that this relationship is explained by morphological similarities, we conducted a greenhouse experiment using species assigned to three taxonomic levels (intraspecific, interspecific, and functional group level). We measured certain fitness‐related aboveground traits and used them along literature‐derived traits to determine the influence of morphological similarities on the strength and direction of the feedback. We found that the average strength of negative feedback increased from the intraspecific over the interspecific to the functional group level. However, individual accessions and species differed in the direction and strength of the feedback. None of our results could be explained by morphological dissimilarities or individual traits. Synthesis. Our results indicate that negative plant–soil feedback is stronger if the involved plants belong to more distantly related species. We conclude that the taxonomic level is an important factor in the maintenance of plant coexistence with plant–soil feedback as a potential stabilizing mechanism and should be addressed explicitly in coexistence research, while the traits considered here seem to play a minor role.

Plant-soil feedbacks and root responses of two Mediterranean oaks along a precipitation gradient

Plant-soil feedbacks (PSFs) have been shown to be relevant drivers of forest community dynamics. However, few studies have explored variation of PSFs along environmental gradients. In a framework of climate change, there is a great need to understand how interactions between plants and soil microbes respond along climatic gradients. Therefore, we compared PSFs along a precipitation gradient in Mediterranean oak forests and included trait responses. Following the Stress Gradient Hypothesis (SGH), we expected less negative or even positive PSFs in the physically harsh dry end of our gradient and more negative PSFs in the wettest end. We grew Quercus ilex and Quercus suber acorns on soil inoculated with microbes sampled under adults of both species in six sites ranging in annual precipitation. After 4 months, we measured shoot biomass and allocation and morphological traits above and belowground. We found negative PSFs for Q. ilex independent of precipitation, whereas for Q. suber PSFs ranged from positive in dry sites to negative in wet sites, in agreement with the SGH. The leaf allocation showed patterns similar to shoot biomass, but belowground allocation and morphological traits revealed responses which could not be detected aboveground. We provide first evidence for context-dependent PSFs along a precipitation gradient. Moreover, we show that measuring root traits can help improve our understanding of climate-dependent PSFs. Such understanding helps to predict plant soil microbe interactions, and their role as drivers of plant community dynamics under ongoing climate change.