Effects Of Soil Biota Research Articles

Soil abiotic and biotic controls on plant performance during primary succession in a glacial landscape

Summary Plant–soil interactions are temporally dynamic in ways that are important for the development of plant communities. Yet, during primary succession, the degree to which changing soil characteristics (e.g. increasing nutrient availabilities) and developing communities of soil biota influence plant growth and species turnover is not well understood. We conducted a two‐phase glasshouse experiment with two native plant species and soils collected from three ages (early, mid‐ and late succession) of an actively developing glacial chronosequence ranging from approximately 5 to <100 years in age. The first phase assessed how soil abiotic characteristics and soil biota immediately influenced plant performance. In the second phase, we used conditioned soils to grow the same plant species and assessed how the direction and strength of plant–soil feedback (PSF) varied through succession. We used high and low nutrient and soil sterilization treatments to isolate effects of abiotic and biotic factors on plant performance and to assess how patterns of PSF related to soil nutrients. For both species, our experiments revealed that the initial effect of soil biota on plant performance depended on successional stage. In soils collected from early‐, mid‐ and late‐succession stages, we observed neutral to negative, negative and positive effects of soil biota on plant performance, respectively. Plant–soil feedback varied with successional stage and depended on whether microcosms were supplied with nutrients. Under low nutrient conditions, soil conditioning by plants, unrelated to initial soil biota, was an important mechanism for positive PSF. Under high nutrient conditions, PSFs differed by plant species in early‐ and mid‐succession soils, but were consistently neutral in late succession. Synthesis. Soil biota can affect the performance of pioneer plants over short gradients of ecosystem development, but the strength and direction of interactions depend on nutrient availability. Conceptual frameworks describing mechanisms of primary succession should acknowledge soil conditioning by previous plants as a potential mechanism of positive feedback.

Effects of soil biota influenced by long-term organic and chemical fertilizers on rice growth and resistance to insects

Effects of soil biota influenced by long-term organic and chemical fertilizers on rice growth and resistance to insects

Do exotic plants lose resistance to pathogenic soil biota from their native range? A test with Solidago gigantea.

Native plants commonly suffer from strong negative plant-soil feedbacks. However, in their non-native ranges species often escape from these negative feedbacks, which indicates that these feedbacks are generated by at least partially specialized soil biota. If so, introduced plants might evolve the loss of resistance to pathogens in their former native range, as has been proposed for the loss of resistance to specialized herbivores. We compared the magnitude of plant-soil feedbacks experienced by native and exotic genotypes of the perennial forb, Solidago gigantea. Feedbacks were assessed in soil collected across 14 sites sampled across the western part of Solidago's native range in the US. Both native and exotic genotypes of Solidago suffered consistently negative and broadly similar plant-soil feedbacks when grown in North American soil. Although there was substantial variation among soils from different sites in the strength of feedbacks generated, the magnitude of feedbacks generated by North American genotypes of S. gigantea were strongly correlated with those produced in the same soil by European genotypes. Our results indicate that Solidago experiences strong negative soil feedbacks in native soil and that introduced genotypes of Solidago have not lost resistance to these negative effects of soil biota. Both genotypic and landscape-level effects can be important sources of variation in the strength of plant-soil feedbacks.

Inhibitory effects of soil biota are ameliorated by high plant diversity

The idea that plant communities with high species diversity are more stable, productive, and resistant to invasion at small spatial scales has become an important ecological paradigm. Recently, the role of soil biota has emerged as a major driver of this relationship between plant species diversity and ecosystem function. In greenhouse experiments, we found that soil collected from experimentally constructed species-rich plant assemblages (that originally contained between 10 and 16 species) promoted the growth of 4 native target plant species more than soil from species-poor communities (that originally contained between 2 and 5 species). Sterilization of soils from species-poor communities improved the growth of these target species more than sterilization of soils from species-rich plant communities, indicative that inhibitory soil biota had greater negative impacts on plant growth in low versus high diversity soils. These results suggest that strong soil biota effects in soils do not simply accrue in experimental monocultures, but can occur in low diversity assemblages that are more realistic of what occurs in nature. Our findings suggest a mechanistic explanation for the diversity-productivity relationship, and further support the importance of inhibitory soil biota as significant contributors to spatial and temporal patterns of abundance in natural plant communities through negative plant-soil feedback.

Soil biota effects on local abundances of three grass species along a land-use gradient.

Biotic plant-soil interactions and land-use intensity are known to affect plant individual fitness as well as competitiveness and therefore plant-species abundances in communities. Therefore, a link between soil biota and land-use intensity on local abundance of plant species in grasslands can be expected. In two greenhouse experiments, we investigated the effects of soil biota from grassland sites differing in land-use intensity on three grass species that vary in local abundances along this land-use gradient. We were interested in those soil-biota effects that are associated with land-use intensity, and whether these effects act directly or indirectly. Therefore, we grew the three plant species in two separate experiments as single individuals and in mixtures and compared their performance. As single plants, all three grasses showed a similar performance with and without soil biota. In contrast, in mixtures growth of the species in response to the presence or absence of soil biota differed. This resulted in different soil-biota effects that tend to correspond with patterns of species-specific abundances in the field for two of the three species tested. Our results highlight the importance of indirect interactions between plants and soil microorganisms and suggest that combined effects of soil biota and plant-plant interactions are involved in structuring plant communities. In conclusion, our experiments suggest that soil biota may have the potential to alter effects of plant-plant interactions and therefore influence plant-species abundances and diversity in grasslands.

Separating the role of biotic interactions and climate in determining adaptive response of plants to climate change

Altered rainfall regimes will greatly affect the response of plant species to climate change. However, little is known about how direct effects of changing precipitation on plant performance may depend on other abiotic factors and biotic interactions. We used reciprocal transplants between climatically very different sites with simultaneous manipulation of soil, plant population origin, and neighbor conditions to evaluate local adaptation and possible adaptive response of four Eastern Mediterranean annual plant species to climate change. The effect of site on plant performance was negligible, but soil origin had a strong effect on fecundity, most likely due to differential water retaining ability. Competition by neighbors strongly reduced fitness. We separated the effects of the abiotic and biotic soil properties on plant performance by repeating the field experiment in a greenhouse under homogenous environmental conditions and including a soil biota manipulation treatment. As in the field, plant performance differed among soil origins and neighbor treatments. Moreover, we found plant species-specific responses to soil biota that may be best explained by the differential sensitivity to negative and positive soil biota effects. Overall, under the conditions of our experiment with two contrasting sites, biotic interactions had a strong effect on plant fitness that interacted with and eventually overrode climate. Because climate and biotic interactions covary, reciprocal transplants and climate gradient studies should consider soil biotic interactions and abiotic conditions when evaluating climate change effects on plant performance.

Effects of litter traits, soil biota, and soil chemistry on soil carbon stocks at a common garden with 14 tree species

Tree species interact with soil biota to impact soil organic carbon (C) pools, but it is unclear how this interaction is shaped by various ecological factors. We used multiple regression to describe how ~100 variables were related to soil organic C pools in a common garden experiment with 14 temperate tree species. Potential predictor variables included: (i) the abundance, chemical composition, and decomposition rates of leaf litter and fine roots, (ii) species richness and abundance of bacteria, fungi, and invertebrate animals in soil, and (iii) measures of soil acidity and texture. The amount of organic C in the organic horizon and upper 20 cm of mineral soil (i.e. the combined C pool) was strongly negatively correlated with earthworm abundance and strongly positively correlated with the abundance of aluminum, iron, and protons in mineral soils. After accounting for these factors, we identified additional correlations with soil biota and with litter traits. Rates of leaf litter decomposition, measured as litter mass loss, were negatively correlated with size of the combined soil organic C pool. Somewhat paradoxically, the combined soil organic C pool was also negatively related to the ratio of recalcitrant compounds to nitrogen in leaf litter. These apparent effects of litter traits probably arose because two independent components of litter “quality” were controlling different aspects of decomposition. Leaf litter mass loss rates were positively related with leaf litter calcium concentrations, reflecting greater utilization and depolymerization of calcium-rich leaf litter by earthworms and other soil biota, which presumably led to greater proportional losses of litter C as CO2 or dissolved organic C. The fraction of depolymerized and metabolized litter that is ultimately lost as CO2 is an inverse function of microbial C use efficiency, which increases with litter nutrient concentrations but decreases with concentrations of recalcitrant compounds (e.g. lignin); thus, high ratios of recalcitrant compounds to nitrogen in leaf litter likely caused a greater fraction of depolymerized litter to be lost as CO2. Existing conceptual models of soil C stabilization need to reconcile the effects of litter quality on these two potentially counteracting factors: rates of litter depolymerization and microbial C use efficiency.

Plant diversity, soil biota and resistance to exotic invasion

AbstractAimHigh species richness at small spatial scales can increase productivity and resist exotic invasion through complementary and selection effects. Recent evidence also suggests that soil biota can drive the increase in productivity caused by high species richness. Here, we take this a step farther and investigate whether soil biota can also affect how high species richness resists invasion.LocationMissoula, Montana, USA.MethodsTen native grassland species were used to create plant assemblages with either one species (monocultures) or 10 species (polycultures) in a common garden. Soils cultured by these assemblages were collected and either sterilized or not to examine the combined effects of species richness and soil biota on the growth and competitive ability of the 10 native species against the invader Bromus tectorum.ResultsLive soil from monocultures inhibited the growth of all native species as a group and native grasses as a functional group more than live soil from polycultures. Sterilization eliminated the negative effects of soil from monocultures but not from polycultures. Native species also competed with B. tectorum more successfully in live soil from polycultures than live soil from monocultures, and sterilization eliminated the competitive advantage of natives in live polyculture soil.Main conclusionsWe found that local plant species richness can affect soil biota in ways that can increase the competitive effects of natives against an aggressive exotic invader; thus, our results suggest a mechanism by which species diversity might provide resistance to exotic invasion.

Soil-borne pathogens restrict the recruitment of a subtropical tree: a distance-dependent effect.

The Janzen-Connell hypothesis suggests that density- and/or distance-dependent juvenile mortality driven by host-specific natural enemies can explain high species diversity in tropical forests. However, such density and distance effects may not occur simultaneously and may not be driven by the same mechanism. Also, reports of attempts to identify and quantify the differences between these processes in tropical forests are scarce. In a primary subtropical forest in China, we (1) experimentally examined the relative influence of the distance to parent trees vs. conspecific seedling density on mortality patterns in Engelhardia fenzelii, (2) tested the role of soil-borne pathogens in driving density- or distance-dependent processes that cause seedling mortality, and (3) inspected the susceptibilities of different tree species to soil biota of E. fenzelii and the effects of soil biota from different tree species on E. fenzelii. The results from these field experiments showed that distance- rather than density-dependent processes driven by soil pathogens strongly affect the seedling survival of this species in its first year. We also observed increased survival of a fungicide treatment for E. fenzelii seedlings in the parent soil but not for the seedlings of the other three species in the E. fenzelii parent soil, or for E. fenzelii seedlings in the parent soil of three other species. This study illustrates how the distance-dependent pattern of seedling recruitment for this species is driven by soil pathogens, a mechanism that likely restricts the dominance of this abundant species.

Habitat‐specific positive and negative effects of soil biota on seedling growth in a fragmented tropical montane landscape

Soil biota, in particular fungi and other microbes, are known to interactively influence plant community structure. However, soil biota effects that can be isolated in a greenhouse experiment may be overridden by other biotic and abiotic factors in the field. Here we conducted parallel greenhouse and field experiments and quantified how soil biota sampled at the habitat level affect the performance of potential host plants within and across neighboring habitat types (pastures, coffee plantations and forest fragments) in a montane region in Colombia. We hypothesized that the direction and strength of soil‐biota effects depend on the habitat where soil is sampled, focal plant's life history, and field environmental characteristics (soil nutrients, light). In a greenhouse experiment, we compared growth of 10 plant species with soil from home (where species typically occur) and foreign (where conspecific adults rarely occur) habitats, with or without soil sterilization. In the field, we conducted a reciprocal transplant experiment in which we suppressed soil fungi with the application of fungicide. In the greenhouse experiment, fast‐growing pasture grass and pioneer trees performed less well with live soil from their home, compared to foreign habitats, and such home disadvantage was reduced following soil sterilization. Home disadvantage associated with live soil biota was also detectable in the field experiment, although light conditions of grasslands and coffee plantations benefited growth of these fast‐growing species. In contrast, coffee and shade‐tolerant trees performed similarly or better with their home soils, and showed no response to soil biota suppression. Overall, the species‐and‐habitat specific soil biota effects detectable in the field experiment were similar in direction and relative strength to those from the greenhouse experiments. Our findings highlight the importance of habitat‐level plant–soil interactions and plant life history for the regeneration of natural forests and agricultural production in human‐modified landscapes.

Interactions with soil biota shift from negative to positive when a tree species is moved outside its native range

Studies evaluating plant-soil biota interactions in both native and introduced plant ranges are rare, and thus far have lacked robust experimental designs to account for several potential confounding factors. Here, we investigated the effects of soil biota on growth of Pinus contorta, which has been introduced from Canada to Sweden. Using Swedish and Canadian soils, we conducted two glasshouse experiments. The first experiment utilized unsterilized soil from each country, with a full-factorial cross of soil origin, tree provenance, and fertilizer addition. The second experiment utilized gamma-irradiated sterile soil from each country, with a full-factorial cross of soil origin, soil biota inoculation treatments, tree provenance, and fertilizer addition. The first experiment showed higher seedling growth on Swedish soil relative to Canadian soil. The second experiment showed this effect was due to differences in soil biotic communities between the two countries, and occurred independently of all other experimental factors. Our results provide strong evidence that plant interactions with soil biota can shift from negative to positive following introduction to a new region, and are relevant for understanding the success of some exotic forest plantations, and invasive and range-expanding native species.

Invasive plants escape from suppressive soil biota at regional scales

Summary A prominent hypothesis for plant invasions is escape from the inhibitory effects of soil biota. Although the strength of these inhibitory effects, measured as soil feedbacks, has been assessed between natives and exotics in non‐native ranges, few studies have compared the strength of plant–soil feedbacks for exotic species in soils from non‐native versus native ranges. We examined whether 6 perennial European forb species that are widespread invaders in North American grasslands (Centaurea stoebe, Euphorbia esula, Hypericum perforatum, Linaria vulgaris, Potentilla recta and Leucanthemum vulgare) experienced different suppressive effects of soil biota collected from 21 sites across both ranges. Four of the six species tested exhibited substantially reduced shoot biomass in ‘live’ versus sterile soil from Europe. In contrast, North American soils produced no significant feedbacks on any of the invasive species tested indicating a broad scale escape from the inhibitory effects of soil biota. Negative feedbacks generated by European soil varied idiosyncratically among sites and species. Since this variation did not correspond with the presence of the target species at field sites, it suggests that negative feedbacks can be generated from soil biota that are widely distributed in native ranges in the absence of density‐dependent effects. Synthesis. Our results show that for some invasives, native soils have strong suppressive potential, whereas this is not the case in soils from across the introduced range. Differences in regional‐scale evolutionary history among plants and soil biota could ultimately help explain why some exotics are able to occur at higher abundance in the introduced versus native range.

Plant–soil biota interactions of an invasive species in its native and introduced ranges: Implications for invasion success

Biotic interactions of exotic plant species in their introduced ranges may differ from those of co-occurring plant species and from interactions in their native range. Interactions less negative (Enemy Release Hypothesis) or more positive (Enhanced Mutualist Hypothesis) compared to native plants may enhance invasive success and change exotic plant traits. We investigated plant–soil interactions of Chinese tallow (Triadica sebifera) from China (native range) and US (invasive range) populations along with US and China species within four co-occurring genera (Liquidambar, Ulmus, Celtis, and Platanus) grown in both ranges. Treatments included sterilized, home, away, active mix, and fungicide treated soils. The net effect of soil biota on Triadica performance was generally negative or had no effect in its native range but was always positive in its invasive range. In China, Triadica had higher survival and biomass in “away” (heterospecific) soils than in “home” (conspecific) soil, but in the US, only biomass was higher. The pattern of performance in home and away soils suggests a stronger negative effect of specific pathogens for Triadica in China (Enemy Release Hypothesis). No other species had a significant feedback in either range. Triadica mycorrhizal colonization was higher in the US than in China supporting the Enhanced Mutualist Hypothesis. On average, Triadica performance was similar for native and invasive populations. However, invasive populations had greater biomass in active mix than in sterilized or fungicide-treated soil and native populations had higher mycorrhizal colonization in home than in away soils or active mix. These population origin results suggest genetic differences in strength or specificity of mycorrhizal interactions. Overall, our results support both ecological and evolutionary variation in interactions with the soil community that may influence the invasion success of exotic plants.

A Review of Biochar and Soil Nitrogen Dynamics

Interest in biochar stems from its potential agronomic benefits and carbon sequestration ability. Biochar application alters soil nitrogen (N) dynamics. This review establishes emerging trends and gaps in biochar-N research. Biochar adsorption of NO3−, up to 0.6 mg g−1 biochar, occurs at pyrolysis temperatures >600 °C with amounts adsorbed dependent on feedstock and NO3− concentration. Biochar NH4+ adsorption depends on feedstock, but no pyrolysis temperature trend is apparent. Long-term practical effectiveness of inorganic-N adsorption, as a NO3− leaching mitigation option, requires further study. Biochar adsorption of ammonia (NH3) decreases NH3 and NO3− losses during composting and after manure applications, and offers a mechanism for developing slow release fertilisers. Reductions in NH3 loss vary with N source and biochar characteristics. Manure derived biochars have a role as N fertilizers. Increasing pyrolysis temperatures, during biochar manufacture from manures and biosolids, results in biochars with decreasing hydrolysable organic N and increasing aromatic and heterocyclic structures. The short- and long-term implications of biochar on N immobilisation and mineralization are specific to individual soil-biochar combinations and further systematic studies are required to predict agronomic and N cycling responses. Most nitrous oxide (N2O) studies measuring nitrous oxide (N2O) were short-term in nature and found emission reductions, but long-term studies are lacking, as is mechanistic understanding of reductions. Stable N isotopes have a role in elucidating biochar-N-soil dynamics. There remains a dearth of information regarding effects of biochar and soil biota on N cycling. Biochar has potential within agroecosystems to be an N input, and a mitigation agent for environmentally detrimental N losses. Future research needs to systematically understand biochar-N interactions over the long term.

Help from under ground: soil biota facilitate knotweed invasion

Soil biota can be important drivers of plant community structure. Depending on the balance between antagonistic and mutualistic interactions, they can limit or promote the success of plant species. This is particularly important in the context of exotic plant invasions where soil biota can either increase the biotic resistance of habitats, or they can shift the balance between exotic and native plants towards the exotics and thereby greatly contribute to their dominance. Here, we explored the role of soil biota in the invasion success of exotic knotweed (Fallopia × bohemica), one of the world's most noxious invasive plants. We created artificial native plant communities that were experimentally invaded by knotweed, using a range of substrates where we manipulated different fractions of soil biota. We found that invasive knotweed benefited more from the overall presence of soil biota than any of the six native species. In particular the presence of the full natural soil biota strongly shifted the competitive balance in favor of knotweed. Soil biota promoted both regeneration and growth of the invader, which suggests that soil organisms may be important both in the early establishment of knotweed and possibly its later dominance of native communities. Addition of activated carbon to the soil made the advantage of knotweed disappear, which suggests that the mechanisms underlying the positive soil biota effects are chemically mediated. Our study demonstrates that soil organisms play a key role in the invasion success of exotic knotweed.

Increasing plant diversity effects on productivity with time due to delayed soil biota effects on plants

Plant diversity experiments in temperate grasslands have served as model system for exploring the consequences of biodiversity loss and understanding the relationship between biodiversity and ecosystem functioning. Recently, ecologists have emphasized that plant diversity effects on plant productivity increase with time; however, the mechanisms responsible for delayed ecosystem responses to diversity manipulations are poorly understood. The present paper briefly reviews temporal changes in grassland biodiversity experiments and proposes antagonistic and facilitative soil effects on plant performance as a mechanistic explanation for increasing plant diversity effects on plant productivity with time. We show that the mechanisms previously proposed to be responsible for the increasing influence of plant diversity effects with time rely in part or entirely on soil biota. Moreover, we show that significant plant diversity effects on soil organisms increase considerably with experimental duration and that the number of respective long-term studies is extremely low. We posit that in the long-term species-rich plant communities experience predominantly facilitative net effects by soil biota promoting plant community growth, such as decomposers, arbuscular mycorrhizal fungi and plant growth promoting rhizobacteria, whereas species-poor plant communities are subject to antagonistic net soil effects due to the accumulation of pathogens, such as nematodes. We therefore suggest that soil biota and their net effect on plants need to be considered in biodiversity experiments to understand changes in time of the effects of plant diversity on the productivity and functioning of plant communities.

Impact of Competition and Mycorrhizal Fungi on Growth of Centaurea stoebe, an Invasive Plant of Sand Dunes

Soil biota such as arbuscular mycorrhizal fungi (AMF) have been shown to increase invasive plant species success in a wide variety of systems by providing both direct and indirect benefits to the invader. For example, Centaurea stoebe invasion in the western US is at least partially due to AMF networks allowing Centaurea to parasitize some native plant species. Centaurea also invades sand dune systems of the northern Great Lakes region, which often have reduced or altered soil communities compared to other grasslands. In these habitats, AMF may play a different role in invasion success of this species. We conducted a greenhouse experiment to compare effects of soil biota and AMF on competitive interactions between Centaurea and two varieties of Ammophila breviligulata, a dominant native grass of Great Lakes sand dunes. We found that Centaurea growth was slowed by the presence of Ammophila competitors, while AMF had no direct or indirect effect on growth. Both Ammophila varieties were uninhibited by the presence of Centaurea. The commercially available Cape variety of Ammophila had more than twice the inhibitory effect of a native Michigan variety on Centaurea growth. It does not appear that Centaurea takes advantage of AMF networks in this dune system. Indeed, since Centaurea growth is actually reduced in direct competition with Ammophila, we suspect that invasion of dunes by this species is a result of disturbance. For land managers, planting Ammophila in open areas as part of a restoration plan may slow spread of Centaurea in this system.

Soil biota effects on soil structure: Interactions between arbuscular mycorrhizal fungal mycelium and collembola

Soil aggregation is an important ecosystem process mediated by soil organisms. Collembola and arbuscular mycorrhizal (AM) fungi are major soil biota representing different functional groups, and are known as two key promoters of soil aggregation. Although several studies have experimentally demonstrated that AM fungi and, more recently, collembola affect soil structure, there is no study investigating how both soil organisms affect soil aggregation excluding the influence of plant roots, another important driver of soil aggregation. Considering the importance of AM fungi and collembola in terrestrial ecosystems, here we asked if both organisms have any influence on soil aggregation when roots are not present.In order to examine this question we conducted a completely factorial greenhouse study manipulating the presence of both collembola and AM fungi and excluded the roots of Plantago lanceolata using a 38 μm nylon screen compartment. We quantified soil aggregation as water stable soil aggregates in four size classes in the hyphal compartment and monitored a number of other explanatory variables, including AM (and non-AM) fungal soil hyphal length.The soil in the hyphal compartment showed greater soil aggregation with larger mean weight diameter when collembola were present, and a similar result was found in the presence of AM fungi, compared to control treatments. Moreover, combined presence of both AM fungi and collembola resulted in a non-additive increase of soil aggregation.Our study clearly indicated that collembola can enhance soil aggregation, that they can partially complement effects of AM fungi, and that these effects are independent of roots.

Microbial amelioration of crop salinity stress

The use of soil and irrigation water with a high content of soluble salts is a major limiting factor for crop productivity in the semi-arid areas of the world. While important physiological insights about the mechanisms of salt tolerance in plants have been gained, the transfer of such knowledge into crop improvement has been limited. The identification and exploitation of soil microorganisms (especially rhizosphere bacteria and mycorrhizal fungi) that interact with plants by alleviating stress opens new alternatives for a pyramiding strategy against salinity, as well as new approaches to discover new mechanisms involved in stress tolerance. Although these mechanisms are not always well understood, beneficial physiological effects include improved nutrient and water uptake, growth promotion, and alteration of plant hormonal status and metabolism. This review aims to evaluate the beneficial effects of soil biota on the plant response to saline stress, with special reference to phytohormonal signalling mechanisms that interact with key physiological processes to improve plant tolerance to the osmotic and toxic components of salinity. Improved plant nutrition is a quite general beneficial effect and may contribute to the maintenance of homeostasis of toxic ions under saline stress. Furthermore, alteration of crop hormonal status to decrease evolution of the growth-retarding and senescence-inducing hormone ethylene (or its precursor 1-aminocyclopropane-1-carboxylic acid), or to maintain source-sink relations, photosynthesis, and biomass production and allocation (by altering indole-3-acetic acid and cytokinin biosynthesis) seem to be promising target processes for soil biota-improved crop salt tolerance.

Effects of soil biota from different ranges onRobiniainvasion: acquiring mutualists and escaping pathogens

The net effects of soil biota on exotic invaders can be variable, in part, because net effects are produced by many interacting mutualists and antagonists. Here we compared mutualistic and antagonistic biota in soils collected in the native, expanded, and invasive range of the black locust tree, Robinia pseudoacacia. Robinia formed nodules in all soils with a broad phylogenetic range of N-fixing bacteria, and leaf N did not differ among the different sources of soil. This suggests that the global expansion of Robinia was not limited by the lack of appropriate mutualistic N-fixers. Arbuscular mycorrhizal fungi (AMF) from the native range stimulated stronger positive feedbacks than AMF from the expanded or invasive ranges, a biogeographic difference not described previously for invasive plants. Pythium taxa collected from soil in the native range were not more pathogenic than those from other ranges; however, feedbacks produced by the total soil biota were more negative from soils from the native range than from the other ranges, overriding the effects of AMF. This suggests that escape from other pathogens in the soil or the net negative effects of the whole soil community may contribute to superior performance in invaded regions. Our results suggest that important regional evolutionary relationships may occur among plants and soil biota, and that net effects of soil biota may affect invasion, but in ways that are not easily explained by studying isolated components of the soil biota.