Positive Plant-soil Feedbacks Research Articles

Magnesium Fertilizer-Induced Increase of Symbiotic Microorganisms Improves Forage Growth and Quality.

Magnesium (Mg) plays important roles in photosynthesis and protein synthesis; however, latent Mg deficiencies are common phenomena that can influence food quality. Nevertheless, the effects of Mg fertilizer additions on plant carbon (C):nitrogen (N):phosphorus (P) stoichiometry, an important index of food quality, are unclear and the underlying mechanisms unexplored. We conducted a greenhouse experiment using low-Mg in situ soil without and with a gradient of Mg additions to investigate the effect of Mg fertilizer on growth and stoichiometry of maize and soybean and also measure these plants' main symbiotic microorganisms: arbuscular mycorrhizal fungi (AMF) and rhizobium, respectively. Our results showed that Mg addition significantly improved both plant species' growth and also increased N and P concentrations in soybean and maize, respectively, resulting in low C:N ratio and high N:P ratio in soybean and low C:P and N:P ratios in maize. These results presumably stemmed from the increase of nutrients supplied by activation-enhanced plant symbiotic microorganisms, an explanation supported by statistically significant positive correlations between plant stoichiometry and plants' symbiotic microorganisms' increased growth with Mg addition. We conclude that Mg supply can improve plant growth and alter plant stoichiometry via enhanced activity of plant symbiotic microorganisms. Possible mechanisms underlying this positive plant-soil feedback include an enhanced photosynthetic product flow to roots caused by adequate Mg supply.

Response of rhizosphere soil microbial to Deyeuxia angustifolia encroaching in two different vegetation communities in alpine tundra

Deyeuxia angustifolia (Komarov) Y. L Chang is an herb species originating from the birch forests in the Changbai Mountain. Recently, this species has been found encroaching into large areas in the western slopes of the alpine tundra in the Changbai Mountain, threatening the tundra ecosystem. In this study, we systematically assessed the response of the rhizosphere soil microbial to D. angustifolia encroaching in alpine tundra by conducting experiments for two vegetation types (shrubs and herbs) by real-time PCR and Illumina Miseq sequencing methods. The treatments consisted of D. angustifolia sites (DA), native sites (NS, NH) and encroaching sites (ES, EH). Our results show that (1) Rhizosphere soil properties of the alpine tundra were significantly impacted by D. angustifolia encroaching; microbial nutrient cycling and soil bacterial communities were shaped to be suitable for D. angustifolia growth; (2) The two vegetation community rhizosphere soils responded differently to D. angustifolia encroaching; (3) By encroaching into both vegetation communities, D. angustifolia could effectively replace the native species by establishing positive plant-soil feedback. The strong adaptation and assimilative capacity contributed to D. angustifolia encroaching in the alpine tundra. Our research indicates that D. angustifolia significantly impacts the rhizosphere soil microbial of the alpine tundra.

Soil microbial communities in cucumber monoculture and rotation systems and their feedback effects on cucumber seedling growth

Loss of biodiversity caused by intensive agriculture is a major worldwide concern. Crop rotation can enhance crop productivity and increase soil microbial diversity. However, the functional significances of changes in soil microbial communities are poorly understood. Soil microbial communities from a cucumber monoculture system and the cucumber season of a two-year rotation system (tomato-celery-cucumber-Chinese cabbage) were analyzed by quantitative PCR and high throughput amplicon sequencing. Then, feedback effects of soil biota on cucumber seedlings were evaluated through soil sterilization, addition of soil inoculum, and application of bactericide and fungicide. Crop rotation increased cucumber yield and bacterial diversity, but decreased fungal diversity and abundance. The bacterial and fungal community compositions also differed between the two cropping systems. Abundances of potential plant pathogens and antagonistic microorganisms were lower while potential plant-growth-promoting microorganisms were higher in the rotation system. The overall effect of soil biota was positive on cucumber seedling growth, and was stronger in the rotation system. Increasing temporal plant diversity can change soil microbial communities and enhance crop productivity through positive plant-soil feedbacks mediated by soil biota.

Phylogenetic relatedness, phenotypic similarity and plant–soil feedbacks

Summary Plant–soil feedbacks contribute to species invasions, the maintenance of biodiversity and climate change impacts on terrestrial ecosystems. Despite their far‐reaching importance, we lack a general understanding of the ecological and evolutionary determinants of plant–soil feedbacks. We conducted a large‐scale plant–soil feedback experiment using 49 co‐occurring plant species from southern Ontario, Canada, representing a wide phylogenetic range. We tested whether the effects of soil conditioning vary among these species and whether different focal species respond similarly to the same soil conditioning. Next, we investigated whether plant traits and soil feedbacks depend on phylogenetic similarity and which plant traits affect plant–soil feedbacks between pairs of plant species. Finally, we used our experimental results to test whether soil feedbacks affect co‐occurrence of species in the field. We found evidence of both strong positive and negative soil feedbacks between pairs of plant species. Our soil‐conditioning treatment explained nearly 20% of the variation in focal species performance. Phylogenetic relatedness and phenotypic similarity between plant species were unrelated to the strength of their soil feedback. However, numerous plant traits of the conditioning species influenced the strength of soil feedbacks on focal species, including specific leaf area and total above‐ground productivity. Trait differences between species were also predictive of plant–soil feedbacks, though for some pairs of species, increased trait differences were associated with positive plant–soil feedbacks and for others, trait differences were associated with negative plant–soil feedbacks. Plant species co‐occurrence in the field was related to their experimentally determined soil feedbacks but only for particular plant species. Synthesis. Our results illustrate how evolutionary history and phenotypic variation shape plant–soil feedbacks and highlight the need for trait‐based studies. Due to the unique evolutionary history of individual traits and the variability in their importance across all possible interacting species, we show that indices of overall phenotypic and phylogenetic relatedness are poor predictors of plant–soil feedbacks at large phylogenetic scales. We conclude that a detailed trait‐based approach can be used to predict plant–soil feedbacks, and laboratory measures of soil feedbacks can explain patterns of co‐occurrence in nature.

Competition overwhelms the positive plant-soil feedback generated by an invasive plant.

Invasive plant species can modify soils in a way that benefits their fitness more than the fitness of native species. However, it is unclear how competition among plant species alters the strength and direction of plant-soil feedbacks. We tested how community context altered plant-soil feedback between the non-native invasive forb Lespedeza cuneata and nine co-occurring native prairie species. In a series of greenhouse experiments, we grew plants individually and in communities with soils that differed in soil origin (invaded or uninvaded by L. cuneata) and in soils that were live vs. sterilized. In the absence of competition, L. cuneata produced over 60% more biomass in invaded than uninvaded soils, while native species performance was unaffected. The absence of a soil origin effect in sterile soil suggests that the positive plant-soil feedback was caused by differences in the soil biota. However, in the presence of competition, the positive effect of soil origin on L. cuneata growth disappeared. These results suggest that L. cuneata may benefit from positive plant-soil feedback when establishing populations in disturbed landscapes with few interspecific competitors, but does not support the hypothesis that plant-soil feedbacks influence competitive outcomes between L. cuneata and native plant species. These results highlight the importance of considering whether competition influences the outcome of interactions between plants and soils.

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.

Plant selection and soil legacy enhance long-term biodiversity effects.

Plant-plant and plant-soil interactions can help maintain plant diversity and ecosystem functions. Changes in these interactions may underlie experimentally observed increases in biodiversity effects over time via the selection of genotypes adapted to low or high plant diversity. Little is known, however, about such community-history effects and particularly the role of plant-soil interactions in this process. Soil-legacy effects may occur if co-evolved interactions with soil communities either positively or negatively modify plant biodiversity effects. We tested how plant selection and soil legacy influence biodiversity effects on productivity, and whether such effects increase the resistance of the communities to invasion by weeds. We used two plant selection treatments: parental plants growing in monoculture or in mixture over 8yr in a grassland biodiversity experiment in the field, which we term monoculture types and mixture types. The two soil-legacy treatments used in this study were neutral soil inoculated with live or sterilized soil inocula collected from the same plots in the biodiversity experiment. For each of the four factorial combinations, seedlings of eight species were grown in monocultures or four-species mixtures in pots in an experimental garden over 15 weeks. Soil legacy (live inoculum) strongly increased biodiversity complementarity effects for communities of mixture types, and to a significantly weaker extent for communities of monoculture types. This may be attributed to negative plant-soil feedbacks suffered by mixture types in monocultures, whereas monoculture types had positive plant-soil feedbacks, in both monocultures and mixtures. Monocultures of mixture types were most strongly invaded by weeds, presumably due to increased pathogen susceptibility, reduced biomass, and altered plant-soil interactions of mixture types. These results show that biodiversity effects in experimental grassland communities can be modified by the evolution of positive vs. negative plant-soil feedbacks of plant monoculture vs. mixture types.

Plant-soil feedbacks: a comparative study on the relative importance of soil feedbacks in the greenhouse versus the field.

Interactions between plants and soil microorganisms influence individual plant performance and thus plant-community composition. Most studies on such plant-soil feedbacks (PSFs) have been performed under controlled greenhouse conditions, whereas no study has directly compared PSFs under greenhouse and natural field conditions. We grew three grass species that differ in local abundance in grassland communities simultaneously in the greenhouse and field on field-collected soils either previously conditioned by these species or by the general grassland community. As soils in grasslands are typically conditioned by mixes of species through the patchy and heterogeneous plant species' distributions, we additionally compared the effects of species-specific versus non-specific species conditioning on PSFs in natural and greenhouse conditions. In almost all comparisons PSFs differed between the greenhouse and field. In the greenhouse, plant growth in species-specific and non-specific soils resulted in similar effects with neutral PSFs for the most abundant species and positive PSFs for the less abundant species. In contrast, in the field all grass species tested performed best in non-specific plots, whereas species-specific PSFs were neutral for the most abundant and varied for the less abundant species. This indicates a general beneficial effect of plant diversity on PSFs in the field. Controlled greenhouse conditions might provide valuable insights on the nominal effects of soils on plants. However, the PSFs observed in greenhouse conditions may not be the determining drivers in natural plant communities where their effects may be overwhelmed by the diversity of abiotic and biotic above- and belowground interactions in the field.

Positive plant–soil feedbacks of the invasive Impatiens glandulifera and their effects on above‐ground microbial communities

SummaryImpatiens glandulifera is one of the most widespread invasive plant species in the UK. Although aspects of its biology are known, there is little information about its association with microbial communities, both above ground and below ground. Furthermore, it is unknown whether this species exhibits any form of plant–soil feedback (PSF), commonly seen in other invasive weeds. We conducted a PSF experiment, in which plants of I. glandulifera were grown in soil that supported the species and compared with plants grown in a control soil from the same locality. Soil nutrients were measured, and the soil and foliar microbial communities were assessed. Impatiens glandulifera grew larger and faster in conditioned soil compared with the control. Higher levels of phosphate were also found in conditioned soils. Arbuscular mycorrhizal fungal (AMF) colonisation was lower in conditioned soils, suggesting that I. glandulifera may rapidly alter AMF communities in invaded areas. PSFs had a significant effect on the foliar endophyte community, with clear separation of species between conditioned and control soils. These results show that I. glandulifera displayed a positive PSF and the PSF mechanism extended beyond the soil microbial community to affect foliar endophytes. The observed increase in endophytes in plants grown in conditioned soil could enhance resistance to herbivory, thus further accentuating the invasive properties of this species.

The invasive annual cheatgrass releases more nitrogen than crested wheatgrass through root exudation and senescence.

Plant-soil feedbacks are an important aspect of invasive species success. One type of feedback is alteration of soil nutrient cycling. Cheatgrass invasion in the western USA is associated with increases in plant-available nitrogen (N), but the mechanism for this has not been elucidated. We labeled cheatgrass and crested wheatgrass, a common perennial grass in western rangelands, with (15)N-urea to determine if differences in root exudates and turnover could be a mechanism for increases in soil N. Mesocosms containing plants were either kept moist, or dried out during the final 10days to determine the role of senescence in root N release. Soil N transformation rates were determined using (15)N pool dilution. After 75 days of growth, cheatgrass accumulated 30% more total soil N and organic carbon than crested wheatgrass. Cheatgrass roots released twice as much N as crested wheatgrass roots (0.11 vs. 0.05mgN kg(-1) soil day(-1)) in both soil moisture treatments. This occurred despite lower root abundance (7.0 vs. 17.3g dry root kg(-1) soil) and N concentration (6.0 vs. 7.6gN kg(-1) root) in cheatgrass vs. crested wheatgrass. We propose that increases in soil N pool sizes and transformation rates under cheatgrass are caused by higher rates of root exudation or release of organic matter containing relatively large amounts of labile N. Our results provide the first evidence for the underlying mechanism by which the invasive annual cheatgrass increases N availability and establishes positive plant-soil feedbacks that promote its success in western rangelands.

Bistable plant–soil dynamics and biogenic controls on the soil production function

AbstractSoil formation results from opposite processes of bedrock weathering and erosion, whose balance may be altered by natural events and human activities, resulting in reduced soil depth and function. The impacts of vegetation on soil production and erosion and the feedbacks between soil formation and vegetation growth are only beginning to be explored quantitatively. Since plants require suitable soil environments, disturbed soil states may support less vegetation, leading to a downward spiral of increased erosion and decline in ecosystem function. We explore these feedbacks with a minimal model of the soil–plant system described by two coupled nonlinear differential equations, which include key feedbacks, such as plant‐driven soil production and erosion inhibition. We show that sufficiently strong positive plant–soil feedback can lead to a ‘humped’ soil production function, a necessary condition for soil depth bistability when erosion is assumed to vary monotonically with vegetation biomass. In bistable plant–soil systems, the sustainable soil condition engineered by plants is only accessible above a threshold vegetation biomass and occurs in environments where the high potential rate of erosion exerts a strong control on soil production and erosion. Vegetation removal for agriculture reduces the stabilizing effect of vegetation and lowers the system resilience, thereby increasing the likelihood of transition to a degraded soil state. Copyright © 2016 John Wiley & Sons, Ltd.

Euonymus fortunei dominance over native species may be facilitated by plant–soil feedback

Positive plant–soil feedback (PSF) may be a mechanism of invader dominance, whereas PSF is often negative for native species. Previous work in Eastern deciduous forests of North America has shown that the invasive liana Euonymus fortunei participates in a net positive PSF with native groundcover Asarum canadense, indicating that PSF may contribute to invader dominance. However, to identify PSF as a general invasion driver for Euonymus, we must consider the average net pairwise feedback for multiple native–invasive species pairs, and compare this to the average net pairwise feedback amongst native–native pairs. Here, we test E. fortunei in net pairwise feedback against five native species, comparing native–invader feedback to feedback amongst natives over a gradient of light availability. PSF was on average neutral for invader–native pairs and on average negative for native–native pairs, indicating that Euonymus does not face the same constraints that limit the growth of native species. Because even neutral feedback can facilitate invasion, results indicate that PSF may facilitate invader dominance over a broad range of native functional groups and light conditions in Eastern deciduous forest.

The role of ammonium oxidizing communities in mediating effects of an invasive plant on soil nitrification

Invasive plants often benefit from changes that they impose on soil microbes via positive plant–soil feedback, but the mechanisms that underlie these changes, and the legacy of their effects, remain poorly quantified. We investigated the impacts of an invasive annual grass, Microstegium vimineum, on the structure and functioning of soil microbial communities in a multi-year, field-based common garden experiment. Given previous reports that M. vimineum can both elevate nitrification rates in soil and benefit from enhanced nitrate availability, we sought to answer the following questions: 1) Does M. vimineum alter the abundance or composition of soil nitrifying microbial communities (ammonia oxidizing archaea and bacteria, AOA and AOB, respectively)? 2) Are such effects reversible or do soil legacy effects persist after M. vimineum is no longer present? After three years, invaded plots had greater AOA abundances than uninvaded native dominated plots, as well as different AOA community structure. However, after seven years, and following a period of M. vimineum replacement by native plants in the invaded plots, AOA abundances and nitrification rates declined toward levels found in uninvaded plots. Collectively, our results suggest that while the impacts of M. vimineum invasions on nitrogen cycling likely relate to their association with AOA, these effects may not persist if M. vimineum declines over time and native plants and their associated microbes are able to re-establish.

Tree diversity modifies distance‐dependent effects on seedling emergence but not plant–soil feedbacks of temperate trees

Negative plant–soil feedbacks can impair seedling survival near conspecific trees and may enhance forest diversity. However, the reciprocal aspect of this relationship, the influence of tree diversity on the strengths of plant–soil feedbacks is unknown. We examined whether tree diversity can affect plant–soil feedbacks of two common temperate tree species, beech (Fagus sylvatica) and sessile oak (Quercus petraea). Soils were collected from adjacent to beech and oak trees growing in a manipulative tree diversity field experiment and used as inocula in a greenhouse experiment to determine whether increasing tree diversity could moderate the strengths of plant–soil feedbacks. We also compared plant–soil feedback responses between Q. petraea and F. sylvatica to test the hypothesis that stronger negative feedbacks contribute to the relative rarity of Q. petraea. Negative plant–soil feedbacks on seedling emergence were observed for both tree species. Emergence increased with increasing distance of the soil inocula from established trees in the field, but only for inocula taken from the low‐diversity treatment (monocultures) and irrespective of the inoculum source (i.e., from conspecific vs. heterospecific trees). In contrast, tree diversity had no influence on the biomass responses of seedlings for either tree species. Quercus petraea seedlings experienced negative plant–soil feedbacks in their biomass responses which cascaded up to higher trophic levels, i.e., leaf pathogen infection. Fagus sylvatica biomass responses suggested positive plant–soil feedbacks, supporting the hypothesis that the abundance of this species is less limited by interactions with natural enemies. Our results suggest that distance‐dependent seedling emergence in temperate forests may depend on the diversity of tree communities and that plant–soil feedbacks may cascade up to higher trophic levels and change with the life history stage of the seedlings involved. An increased focus on the surrounding communities would help to judge the importance and context‐dependency of plant–soil feedbacks as a mechanism for stabilizing forest diversity.

Rhizosphere-mediated effects of the invasive grass Bromus tectorum L. and native Elymus elymoides on nitrogen cycling in Great Basin Desert soils

Background and aims There is evidence that the invasive grass Bromus tectorum can affect soil nitrogen (N) cycling, possibly leading to a positive plant-soil feedback. Rhizosphere priming of N mineralization could provide a mechanistic explanation for such a feedback. Methods We conducted a greenhouse study to isolate rhizosphere effects on N cycling by the invasive annual grass, Bromus tectorum L., and the native perennial grass, Elymus elymoides (Raf.) Swezey, in invaded and uninvaded soils. We compared the rhizosphere priming effect (RPE) on N mineralization by species and the distribution of N in various pools by planting treatment and soil type. Results B. tectorum had a negative RPE (�23 and �22 % in invaded and uninvaded soils, respectively), while E. elymoides had no significant RPE. B. tectorum was more competitive over E. elymoides in invaded compared to uninvaded soil. Conclusions B. tectorumhad a negative effect on soil N availability via root-mediated processes, even though its growth and competitiveness increased in invaded soils. Positive plant-soil feedback effects of B. tectorum may be mediated by aboveground inputs rather than belowground and/or depend on site-specific conditions.

Bacterial diversity amplifies nutrient‐based plant–soil feedbacks

Summary Plants foster diverse assemblages of bacteria in the rhizosphere serving important functions which may result in enhanced plant growth. Microbial diversity is increasingly recognized to shape the functionality of microbial communities. This leads to the assumption that there is a positive relationship between rhizosphere diversity and plant growth. Here we investigate how bacterial diversity affects the mineralization of organic matter and plant nutrient acquisition. We hypothesized that altered bacterial diversity will affect nitrogen mineralization, uptake by plants and ultimately plant growth. We set up a controlled model system with Arabidopsis thaliana colonized by defined assemblages of fluorescent pseudomonads, a well‐characterized plant‐beneficial rhizosphere taxon. The growth substrate contained casein as sole nitrogen source, making the plant nitrogen uptake dependant on breakdown by bacterial enzymes. Bacterial diversity was associated with a higher enzyme activity which increased nitrogen mineralization and enhanced plant growth. The effect of bacterial diversity on plant growth increased with time, pointing to a positive feedback between bacteria and plants: bigger plants associated with species‐rich bacterial communities supported more bacterial growth, which further enhanced the impact of bacteria on plant growth. We demonstrate that plant–soil feedbacks establish rapidly during one single growth season and that bacterial diversity modulates this interaction. Preserving soil microbial diversity therefore may improve positive plant–soil feedbacks and thereby plant growth.

Competition and soil resource environment alter plant-soil feedbacks for native and exotic grasses.

Feedbacks between plants and soil biota are increasingly identified as key determinants of species abundance patterns within plant communities. However, our understanding of how plant-soil feedbacks (PSFs) may contribute to invasions is limited by our understanding of how feedbacks may shift in the light of other ecological processes. Here we assess how the strength of PSFs may shift as soil microbial communities change along a gradient of soil nitrogen (N) availability and how these dynamics may be further altered by the presence of a competitor. We conducted a greenhouse experiment where we grew native Stipa pulchra and exotic Avena fatua, alone and in competition, in soils inoculated with conspecific and heterospecific soil microbial communities conditioned in low, ambient and high N environments. Stipa pulchra decreased in heterospecific soil and in the presence of a competitor, while the performance of the exotic A. fatua shifted with soil microbial communities from altered N environments. Moreover, competition and soil microbial communities from the high N environment eliminated the positive PSFs of Stipa. Our results highlight the importance of examining how individual PSFs may interact in a broader community context and contribute to the establishment, spread and dominance of invaders.

Increased precipitation induces a positive plant-soil feedback in a semi-arid grassland

Background and Aims Given that plant growth is often water limited in drylands, it has been proposed that water seems to influence productivity by altering physiological/ metabolic responses and nutrient availability in short term. It is unclear, however, whether water mediates a positive plant-soil feedback and whether the feedback drives variations in plant productivity. Methods A 4-year field experiment was performed to examine the effects of water and nitrogen (N) addition on nutrient concentrations in soil and plant, nutrient resorption and potential return, in a temperate grassland in northern China. Results Water addition enhanced plant N and phosphorus (P) concentrations but reduced plant N and P resorption efficiency, leading to the increased potential N and P return to soil via litterfall. Enhanced nutrient potential return likely contributed to an increase of plant productivity in the following year. These “fertilization effects” caused by water addition were similar to those by N addition. Conclusions Our study suggests that the positive plantsoil feedback induced by increased precipitation may have a role in water-induced increases in productivity, and highlights the “fertilization effect” of water addition in a semiarid grassland in short term.

Edaphic factors and feedback do not limit range expansion of an exotic invasive plant

Dog-strangling vine (Vincetoxicum rossicum, Apocynaceae) is an exotic invasive plant from Europe that is increasingly invasive in southern Ontario, Canada. The species is highly adaptive to light, soil moisture, and temperature. V. rossicum grows on calcareous soils in its native range. However, although this is currently consistent in its invaded range, its potential to spread to northern Ontario, with more acidic soils, is currently unknown. We tested the ability of V. rossicum to grow in soil from northern Ontario in comparison with southern Ontario soil. We also grew Solidago canadensis (Asteraceae), an ubiquitous and abundant native species that co-occurs with V. rossicum. We grew both species in soil collected from 26 sites in northern and southern Ontario. Soils were then used in a feedback study with both species to determine whether V. rossicum’s invasion success could be explained, at least partially, by positive plant–soil feedback. We found that biomass of V. rossicum was not significantly different between soil types but S. canadensis had lower biomass in the northern than in southern Ontario soil. Soil feedback was neutral for both species, regardless of soil type. We conclude that the spread of V. rossicum into areas of central/northern Ontario will not be limited by soil type and that plant–soil feedback may not be an important factor driving V. rossicum’s invasion.

The invasive annual cheatgrass increases nitrogen availability in 24-year-old replicated field plots.

Previous studies comparing invaded and non-invaded sites suggest that cheatgrass (Bromus tectorum L.) causes soil N cycling to increase. Unfortunately, these correlative studies fail to distinguish whether cheatgrass caused the differences in N cycling, or if cheatgrass simply invaded sites where N availability was greater. We measured soil C and N concentrations and net and gross N-cycling rates on 24-year-old replicated field plots in a sagebrush-steppe ecosystem that had been plowed, fumigated, and seeded to different plant communities in 1984. Laboratory assays of soil collected throughout the soil profiles (0-60 cm) showed that soil NO3 (-), organic C and N, and net N mineralization, net nitrification, and soil respiration rates were all greater beneath cheatgrass than in sagebrush-perennial grass plots. In surface soils (0-10 cm), field and lab assays on five sampling dates during 2 years showed gross N mineralization, net N mineralization, and net nitrification rates were all faster beneath cheatgrass than in sagebrush-perennial grass plots. Modeling analyses based on soil respiration and gross N-cycling rates suggest that cheatgrass provides soil microbes with lower C:N substrates and that this could explain the faster N-cycling rates beneath cheatgrass. This is the first long-term replicated field study to conclusively show that cheatgrass created greater soil organic N pool sizes and stimulated N-cycling rates compared to similar-aged stands of sagebrush and native perennial grasses. Increased N-cycling rates may represent a positive plant-soil feedback that promotes continued dominance by cheatgrass, even in the absence of soil disturbance or fire.