Positive Plant-soil Feedbacks Research Articles

Plant-soil-foliage feedbacks on seed germination and seedling growth of the invasive plant Ageratina adenophora.

Some exotic plants become invasive because they partially release from soil-borne enemies and thus benefit from positive plant-soil feedbacks (PSFs) in the introduced range. However, reports that have focused only on PSFs may exaggerate the invader's competitiveness. Here, we conducted three experiments to characterize plant-soil-foliage feedbacks, including mature leaves (ML), leaf litter (LL), rhizosphere soil (RS) and leaves plus soil (LS), on the early growth stages of the invasive plant Ageratina adenophora. In general, the feedbacks from aboveground (ML, LL) adversely affected A. adenophora by delaying germination time, inhibiting germination rate and reducing seedling growth. The increased invasion history exacerbated the adverse effects of LL and LS feedbacks on seedling growth. These adverse effects were partially contributed by more abundant fungi (e.g. Didymella) or/and more virulent fungi (e.g. Fusarium) developed in the aboveground part of A. adenophora during the invasion. Interestingly, the aboveground adverse effects can be weakened by microbes from RSs. Our novel findings emphasize the important role of aboveground feedbacks in the evaluation of plant invasiveness, and their commonness and significance remain to be explored in other invasive systems.

Soil Inoculation Steers Plant-Soil Feedback, Suppressing Ruderal Plant Species

Plant-soil feedbacks (PSFs) are important drivers of vegetation composition in nature. Whole-soil inoculations can help to steer plant-community assembly for nature restoration success. However, it is unclear how subsequent colonization by ruderal and late-successional plant species influences PSFs. Here we test the direction and strength of the PSFs for ruderal and target plant species on differently inoculated soils. We hypothesize that inoculation with late-successional field soil promotes positive and negative PSFs for late-successional and ruderal plants respectively. We conducted a glasshouse experiment with three plant-growth phases. First, we inoculated a common nutrient-rich ex-arable soil with either a heathland, grassland or an arable soil and grew mixtures of three ruderal and three late-successional target species in all soils. Subsequently, we divided the experimental units over four new pots and planted half with mixtures of three ruderal and half with mixtures of three late-successional plant species, to simulate local colonization by these species groups. After nine weeks, we removed the plants and replanted the pots with ruderal and late-successional species mixtures in a full factorial design to quantify the induced PSFs, after a further nine weeks of growth. We found that ruderal plants developed positive PSF on soils inoculated with arable soil and neutral feedback when soils were inoculated with grassland or heathland inoculum. The positive PSF was most pronounced for Myosotis arvensis, although all ruderal species showed the most positive PSFs on arable-inoculated soils. There was no significant effect of inoculation on the PSF of late-successional species. As a result of changing PSFs, the relative performance of ruderal and late-successional target species became more similar on soils inoculated with grassland or heathland inoculum, leading to higher evenness. We conclude that soil inoculation on nutrient-rich ex-arable soils can affect PSFs. Inoculation with grassland or heathland inoculum prevented ruderal species from developing positive PSF and shifted the competitive balance in favour of late-successional plants. Future studies need to address the longer-term dynamics of soil inoculation-induced shifts in PSFs, particularly because these are expected to develop over longer timescales in the slow-growing late-successional target species, as well as the impact of competitive plant-plant interactions.

Conditions of plant cultivation affect the differences in intraspecific plant-soil feedback between invasive and native dominants

Intraspecific plant-soil feedback (PSF), a process in which a plant species affects the soil and the modified soil affects further growth of the same plant species, has been proposed to be one of the mechanisms controlling plant dominance in the field, as well as plant invasiveness. However, very few studies compared the PSF of invasive species with native species of a similar level of dominance. In this study, we compared PSF of three pairs of invasive and native congeners with the same level of dominance in the field and with similar ecological requirements. Additionally, we assessed the role of conditions of plant cultivation for the PSF by growing the plants under four treatments (two moisture × two shading regimes).Both invasive and native dominants showed neutral to positive PSF for seedling establishment and negative PSF for biomass. Native species had more negative PSF for belowground biomass than invasive species. PSF for seedling establishment and aboveground biomass showed no overall difference between invasive and native dominants, although differences existed under some cultivating conditions. PSF for seedling establishment was affected by moisture with the response of individual genera reflecting their ecological requirements. PSF for aboveground biomass was affected by the interaction of moisture and shading and was most negative under the dry light treatment. PSF for aboveground biomass was negatively correlated to root biomass, indicating that higher allocation into roots under dry conditions might lead to intensified interactions with soil biota and thus more negative plant-soil feedback. Our study showed that cultivating conditions can significantly affect results of PSF experiments and pointed to the importance of including multiple measures of PSF in future experiments.

Nitrogen fertilization and its legacy have inconsistent and often negative effect on plant growth in undeveloped post mining soils

Nitrogen supplement by adding N-fertilizer and/or growing of N-fixing trees are considered as important stimulus for the development of early vegetation. We performed pot experiments to determine the responses of legumes (N-fixing) and grasses (non-N fixing) in respect to their biomass to N fertilization, legacy of N, and combination of these two in various ages of post mining soils. N legacy was achieved either by long-term mineral N fertilization (legacy of inorganic N) (experiment 1) or growing of N2 fixing plant (legacy of N-fixing) (experiment 2). The first pot experiment was conducted to test N fertilization and legacy of inorganic N and these two on growth of Festuca rubra and Lotus corniculatus when grown separately. Following this, plant-soil feedback was defined as positive or negative by comparing the second generation of plant biomass in soils with and without plants in the previous generation. In the second pot experiment, we compared the performance of P. compressa and L. corniculatus in the presence of legacy of N-fixing and N fertiliaztion when grown together, and at the same time, plant-soil feedback was determined.N fertilization increased the growth of grasses, but only increased biomass of legumes in young soils. N-legacy induced variable plant growth, in particular often decreased the biomass of legumes. Legacy of inorganic N was only beneficial for grasses rather than legumes in medium soils. When growing in the same pot, legacy of N-fixing showed positive effects on grasses, depending on soil age and N fertilization. Repeated growing of plants in the same soil mostly resulted in negative plant-soil feedback. Negative plant-soil feedback was not alleviated by N fertilization and legacy of inorganic N for both grasses and legumes when grown separately. However, growing grasses and legumes together can induce positive plant-soil feedback with no legacy of N-fixing.In conclusion, N addition often stimulates plant growth more than long-term N effect. Legacy of N tends to be only beneficial for non-N fixing plants in medium aged soils. While negative plant-soil feedback occurs for grasses and legumes both grown separately or together, legumes can induce positive plant-soil feedback when grown with grasses in no legacy of N-fixing soils.

Positive plant–soil feedback trigger tannin evolution by niche construction: A spatial stoichiometric model

Abstract Among plant traits, plant secondary metabolites such as tannins mediate plant–herbivore interactions but also have after‐life effects on litter decomposition and nutrient cycling. We propose that niche construction mechanisms based on positive plant–soil feedback (PSF) could influence the evolution of tannin production. By modelling the flow of nitrogen (N) and carbon (C) through plants and soil in a spatially explicit context, we explored the relative contribution of herbivory and positive PSF as drivers of tannin evolution. We assumed soil N to be contained in labile and recalcitrant compartments, the latter made of tannin–protein complexes from which plants can absorb nutrients via associations with mycorrhizal fungi. In infertile environments and for plants with low biomass turnover rates, we show that when tannins modify soil properties locally, positive PSF alone can drive their evolution. We further predict the existence of positive coevolutionary feedback between associations with mycorrhizal fungi with a decaying ability and tannins, possibly triggered by the evolution of the latter as protection against herbivores. In line with our theoretical results, empirical evidence suggest that tannins are mostly present in plants with low tissue turnover, associated with mycorrhizal fungi able to decay organic matter and inhabiting infertile environments. Synthesis. Our model proposes that the evolution of tannin production can be triggered by positive plant–soil feedback, provided that tannins promote the local N retention and that mycorrhizal fungi associated with plants are able to absorb N from tannin–protein complexes. In our model, tannin production evolves only in infertile ecosystems, in agreement with field observations. Our findings highlight that the strength of niche construction depends on the ecological context, hence that global ecological properties constrain local eco‐evolutionary dynamics.

Connecting Plant–Soil Feedbacks to Long‐Term Stability in a Desert Grassland

We found that plant–soil feedbacks between two dominant, congener grasses aligned with historical patterns of abundance fluctuation in a desert grassland. Specifically, in patches where the two species co-occurred with little abundance fluctuation in the historical data, we found negative plant–soil feedbacks indicative of stable coexistence in a field experiment. In patches where the relative abundance of the two species fluctuated throughout the historical record, we found neutral to positive plant–soil feedbacks. Our results reveal a novel connection between plant–microbe interactions and the temporal stability of plant species coexistence. Photographer: Anny Chung. Photo credit: Anny Chung. Photo credit: Anny Chung. These photographs illustrate the article “Connecting plant-soil feedbacks to long-term stability in a desert grassland” by Y. Anny Chung, Scott L. Collins, Jennifer A. Rudgers published in Ecology. https://doi.org/10.1002/ecy.2756

Maize varieties can strengthen positive plant-soil feedback through beneficial arbuscular mycorrhizal fungal mutualists.

Plant-soil feedback (PSF) describes the process whereby plant species modify the soil environment, which subsequently impacts the growth of the same or another plant species. Our aim was to explore PSF by two maize varieties (a landrace and a hybrid variety) and three arbuscular mycorrhizal fungi (AMF) species (Funneliformis mosseae, Claroideoglomus etunicatum, Gigaspora margarita, and the mixture). We carried out a pot experiment with a conditioning and a feedback phase to determine PSF with different species of AMF and with a non-mycorrhizal control. Sterilized soil was conditioned separately by each variety, with or without AMF; in the feedback phase, each soil community was used to grow each in its "home" soil and in the "away" soil. Plant performance was assessed as shoot biomass, phosphorus (P) concentration and P content, and fungal performance was assessed as mycorrhizal colonization and hyphal length density. Both maize varieties were differentially influenced by AMF in the conditioning phase. In the feedback phase, PSF was generally negative for non-mycorrhizal plants or when plants were colonized by G. margarita, whereas PSF was positive in the other three AMF treatments. When plants were grown on home soil, hyphal length density was larger than on away soil. We conclude that different maize varieties can strengthen positive plant-soil feedback for themselves through beneficial mutualists for themselves, but not across the maize varieties.

Transpiratory Rate, Biomass Production and Leaf Macronutrient Content of Different Plant Species Cultivated on a Regosol in the Brazilian Semiarid

The transpiratory rate, growth rate and biomass production of plant species from Fabaceae and Poaceae group were investigated in a long-term field study. We compared photosynthetic rate, stomatal conductance, leaf transpiration, internal CO2 concentration, water use efficiency, intrinsic instantaneous water use efficiency, instantaneous carboxylation efficiency, growth rate, biomass production, and leaf macronutrient content of eight different plant species in a long-term field experiment. We found the highest values for all studied variables with C. juncea, C. ochroleuca, C. ensiformis, D. lablab and M. pruriens. We also found significant differences among Fabaceae and Poaceae group for all studied variables. Our findings suggest that the plant species from Family Fabaceae present high transpiratory rate, fast-growth and better nutritional status than plant species from Family Poaceae, and these results highlight the importance of considering the long-term use of legume plants as green manure in order to create positive plant-soil feedback.

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.

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.

Soil micro‐organisms and competitive ability of a tussock grass species in a dry ecosystem

Abstract Drylands are among the most threatened ecosystems in the world because of the effects of global change and because they are not particularly resilient. Regeneration after disturbance is quite slow and secondary succession can be delayed to the point of nearly stop. This is exacerbated when the community is strongly dominated by a single species able to quickly respond after disturbance. One example of such monospecific dominance is the colonization of abandoned fields in SE Spain by an early successional and native tussock grass, Lygeum spartum, that seems to halt succession. Here we tested the competitive ability of Lygeum against Salsola oppositifolia, a shrub species that can be found interspersed with Lygeum in mid‐successional stages, and assessed how plant–soil interactions mediate the outcome of plant–plant competition. To do so, we sowed seeds and grew plants of Lygeum and Salsola in either intra‐ or interspecific competition under controlled conditions using sterile field soils inoculated with either live (i.e., with micro‐organisms) or sterile soil extracts from the understories of either Lygeum or Salsola. Soil nutrient content, seed germination rate, and shoot mass growth were determined after 5 months, and soil bacterial communities were characterized by sequencing. Lygeum soil micro‐organisms and soil properties, such as the high content of N and organic matter, enhanced seed germination rate of Lygeum individuals. By contrast, Salsola adults outperformed Lygeum when growing in interspecific competition. Synthesis. The enhanced competitive ability of Lygeum, which was mediated by soil micro‐organisms, may lead to complete dominance of Lygeum in the plant community right after abandonment of agricultural fields. However, when the plant community is already developed, Lygeum would be unable to enforce such dominance. We conclude that positive plant–soil feedbacks combined with certain plant traits such as clonal growth support the strong resilience of Lygeum and allow for its dominance in extreme habitats.

Mycorrhizal feedbacks generate positive frequency dependence accelerating grassland succession

Abstract Plant mutualists including arbuscular mycorrhizal (AM) fungi have been postulated as being important drivers of plant community diversity and succession. Late successional plants have been shown to be more responsive to AM fungi and more sensitive to AM fungal species identity, which could generate positive feedback and potentially accelerate succession. We test the effect of AM fungi on plant diversity and on frequency dependence predicted by positive plant–AM fungi feedback across a successional gradient. We created prairie mesocosms comprised of a majority of early successional, equal abundance, and a majority of late successional plant species. We inoculated these mesocosms and a field restoration experiment with 14 different communities of AM fungi from late successional prairies that varied in levels of species richness. Overall, we found that AM fungi increased plant diversity and this was driven by the response of late successional plant species to mycorrhizae. Our results indicate that AM fungal composition is more important than AM fungal diversity per se. We found that the effect of inoculation with a single species or groups of AM fungi depended on whether those fungi benefited late successional plant species. Early successional plants consistently exhibited negative frequency‐dependent growth regardless of fungal composition, while late successional plants demonstrated positive frequency‐dependent growth in our mesocosms—but only in the presence of beneficial AM fungal species. These results are consistent with positive plant–mycorrhizal feedbacks accelerating plant community successional trajectories once late successional plants establish. Mesocosm results were mirrored with field inoculation assays where we found that beneficial AM fungi facilitated late successional plant establishment. Synthesis. Our results provide support for beneficial arbuscular mycorrhizal fungi being a primary mechanism for positive plant–soil feedback driving plant community succession, as late successional seedlings grew faster and larger when their neighbours were also late successional plant species when they were associated with beneficial arbuscular mycorrhizal fungi. We found that this positive feedback thereby accelerated succession in mesocosms and in the field.

Leachate from fine root litter is more acidic than leaf litter leachate: A 2.5-year laboratory incubation

Some tree species increase fine root production under soil acidification, thus changing the balance of litter input from leaves and roots. Litter leaches a significant amount of acidic materials during its decomposition, which might facilitate soil acidification. In this context, we focused on dissolved organic matter (DOM) as the major component of acidic materials. We hypothesized that both the quality and quantity of DOM, which control its function (i.e., proton supply), differ between leaf and root litter. To test this hypothesis, we conducted a 2.5-year laboratory incubation experiment using fresh fine roots and fresh green leaves as litter of two coniferous species (Cryptomeria japonica and Chamaecyparis obtusa) and investigated the leachate pH and DOM composition based on the optical properties. After the early stage of decomposition when flash leaching of DOM converged, the amount of dissolved organic carbon (DOC) leached from roots increased again and leachate pH declined. In contrast, DOC concentrations continued to decrease in leaf leachates during the incubation period, and the pH decrease was not as striking as that of root leachates. Optical properties (ultraviolet visible absorption and fluorescence) of DOM revealed that humic-like substances in DOM played a central role in the acidic pH of root leachates. The total amount of protons released from roots of C. japonica and C. obtusa is about 13 and 18 times higher, respectively, than that from leaves. These results imply that the increase of fine root biomass may induce a positive plant–soil feedback in acidic soils, affecting soil biogeochemical functions of terrestrial ecosystems.

The intensity of intraspecific plant-soil feedbacks in alien Impatiens species depends on the environment

Plant-soil feedback, i.e. the relationship in which a plant affects the composition of the soil and such modified soil affects plant growth, is becoming an important concept for explaining plant invasiveness. Impatiens parviflora is one of the most widespread invasive plant species in Central Europe, but it is unknown whether this species exhibits any form of plant–soil feedback, previously seen in other invasive species. In this study, we examined intraspecific plant-soil feedback of I. parviflora and compared it to feedbacks of other three alien, but non-invasive, Impatiens species growing in Central Europe. Moreover, we studied the effect of environmental conditions on this feedback.The four species were studied in a two-phase feedback experiment. In the first phase, soil was conditioned by a species or left unconditioned (control). During the second phase, plants were grown in soil conditioned by the same species and in control, not cultivated soil, under four different environmental conditions (two levels of watering × two levels of shading).All the studied species showed positive plant-soil feedback in some types of environment indicating they can potentially become invasive. The feedbacks for total biomass and for root-shoot ratio were significantly affected by environmental conditions. Individual species and studied performance measures responded differently to individual treatments. In most cases, the feedback was changing from positive in optimal treatment to neutral and in I. balsamina to negative under some suboptimal treatments. No effect of the environmental conditions on the feedback for germination was observed.These results indicate that environmental conditions play an important role in plant-soil feedbacks. We show that some feedbacks detected in highly controlled experiments using just one type of environmental conditions might not exist in the field, and similarly some existing feedbacks might remain undetected. We therefore recommend using multiple environmental conditions or conditions closely resembling conditions in natural sites to increase the realism of the results.

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.

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.

Community-level plant-soil feedbacks explain landscape distribution of native and non-native plants.

Plant–soil feedbacks (PSFs) have gained attention for their potential role in explaining plant growth and invasion. While promising, most PSF research has measured plant monoculture growth on different soils in short‐term, greenhouse experiments. Here, five soil types were conditioned by growing one native species, three non‐native species, or a mixed plant community in different plots in a common‐garden experiment. After 4 years, plants were removed and one native and one non‐native plant community were planted into replicate plots of each soil type. After three additional years, the percentage cover of each of the three target species in each community was measured. These data were used to parameterize a plant community growth model. Model predictions were compared to native and non‐native abundance on the landscape. Native community cover was lowest on soil conditioned by the dominant non‐native, Centaurea diffusa, and non‐native community cover was lowest on soil cultivated by the dominant native, Pseudoroegneria spicata. Consistent with plant growth on the landscape, the plant growth model predicted that the positive PSFs observed in the common‐garden experiment would result in two distinct communities on the landscape: a native plant community on native soils and a non‐native plant community on non‐native soils. In contrast, when PSF effects were removed, the model predicted that non‐native plants would dominate all soils, which was not consistent with plant growth on the landscape. Results provide an example where PSF effects were large enough to change the rank‐order abundance of native and non‐native plant communities and to explain plant distributions on the landscape. The positive PSFs that contributed to this effect reflected the ability of the two dominant plant species to suppress each other's growth. Results suggest that plant dominance, at least in this system, reflects the ability of a species to suppress the growth of dominant competitors through soil‐mediated effects.

Plant-Soil Feedback Effects on Growth, Defense and Susceptibility to a Soil-Borne Disease in a Cut Flower Crop: Species and Functional Group Effects.

Plants can influence the soil they grow in, and via these changes in the soil they can positively or negatively influence other plants that grow later in this soil, a phenomenon called plant–soil feedback. A fascinating possibility is then to apply positive plant–soil feedback effects in sustainable agriculture to promote plant growth and resistance to pathogens. We grew the cut flower chrysanthemum (Dendranthema X grandiflora) in sterile soil inoculated with soil collected from a grassland that was subsequently conditioned by 37 plant species of three functional groups (grass, forb, legume), and compared it to growth in 100% sterile soil (control). We tested the performance of chrysanthemum by measuring plant growth, and defense (leaf chlorogenic acid concentration) and susceptibility to the oomycete pathogen Pythium ultimum. In presence of Pythium, belowground biomass of chrysanthemum declined but aboveground biomass was not affected compared to non-Pythium inoculated plants. We observed strong differences among species and among functional groups in their plant–soil feedback effects on chrysanthemum. Soil inocula that were conditioned by grasses produced higher chrysanthemum above- and belowground biomass and less leaf yellowness than inocula conditioned by legumes or forbs. Chrysanthemum had lower root/shoot ratios in response to Pythium in soil conditioned by forbs than by grasses. Leaf chlorogenic acid concentrations increased in presence of Pythium and correlated positively with chrysanthemum aboveground biomass. Although chlorogenic acid differed between soil inocula, it did not differ between functional groups. There was no relationship between the phylogenetic distance of the conditioning plant species to chrysanthemum and their plant–soil feedback effects on chrysanthemum. Our study provides novel evidence that plant–soil feedback effects can influence crop health, and shows that plant–soil feedbacks, plant disease susceptibility, and plant aboveground defense compounds are tightly linked. Moreover, we highlight the relevance of considering plant–soil feedbacks in sustainable horticulture, and the larger role of grasses compared to legumes or forbs in this.

Spatial Distribution Patterns of Root-Associated Bacterial Communities Mediated by Root Exudates in Different Aged Ratooning Tea Monoculture Systems.

Positive plant–soil feedback depends on beneficial interactions between roots and microbes for nutrient acquisition; growth promotion; and disease suppression. Recent pyrosequencing approaches have provided insight into the rhizosphere bacterial communities in various cropping systems. However; there is a scarcity of information about the influence of root exudates on the composition of root-associated bacterial communities in ratooning tea monocropping systems of different ages. In Southeastern China; tea cropping systems provide the unique natural experimental environment to compare the distribution of bacterial communities in different rhizo-compartments. High performance liquid chromatography–electrospray ionization–mass spectrometry (HPLC–ESI–MS) was performed to identify and quantify the allelochemicals in root exudates. A high-throughput sequence was used to determine the structural dynamics of the root-associated bacterial communities. Although soil physiochemical properties showed no significant differences in nutrients; long-term tea cultivation resulted in the accumulation of catechin-containing compounds in the rhizosphere and a lowering of pH. Moreover; distinct distribution patterns of bacterial taxa were observed in all three rhizo-compartments of two-year and 30-year monoculture tea; mediated strongly by soil pH and catechin-containing compounds. These results will help to explore the reasons why soil quality and fertility are disturbed in continuous ratooning tea monocropping systems; and to clarify the associated problems.

Plant-soil feedbacks in declining forests: implications for species coexistence.

Plant-soil feedbacks (PSFs) play a relevant role as drivers of species abundance, coexistence, and succession in plant communities. However, the potential contribution of PSFs to community dynamics in changing forest ecosystems affected by global change drivers is still largely unexplored. We measured the direction, strength and nature (biological vs. chemical) of PSFs experienced by coexisting tree species in two types of declining Quercus suber forests of southwestern Spain (open woodland vs. closed forest) invaded by the exotic soil pathogen Phytophthora cinnamomi. To test PSFs in a realistic community context, we focused not only on individual PSFs (i.e., comparing the growth of a tree species on conspecific vs. heterospecific soil) but also calculated net-pairwise PSFs by comparing performance of coexisting tree species on their own and each other's soils. We hypothesized that the decline and death of Q.suber would alter the direction and strength of individual and net-pairwise PSFs due to the associated changes in soil nutrients and microbial communities, with implications for recruitment dynamics and species coexistence. In support of our hypothesis, we found that the decline of Q.suber translated into substantial alterations of individual and net-pairwise PSFs, which shifted from mostly neutral to significantly positive or negative, depending on the forest type. In both cases however the identified PSFs benefited other species more than Q.suber (i.e., heterospecific positive PSF in the open woodland, conspecific negative PSF in the closed forest). Our results supported PSFs driven by changes in chemical soil properties (mainly phosphorus) and arbuscular mycorrhizal fungi, but not in pathogen abundance. Overall, our study suggests that PSFs might reinforce the loss of dominance of Q.suber in declining forests invaded by P.cinnamomi by promoting the relative performance of non-declining coexisting species. More generally, our results indicate an increase in the strength of net PSFs as natural forests become disturbed by global change drivers (e.g., invasive species), suggesting an increasingly important role of PSFs in forest community dynamics in the near future.