Nutrient-demanding species face less negative competition and plant-soil feedback effects in a nutrient-rich environment

Nutrient enrichment, propagule pressure, and herbivory interactively influence the competitive ability of an invasive alien macrophyte Myriophyllum aquaticum

Introduction

Freshwater ecosystems are susceptible to invasion by alien macrophytes due to their connectivity and various plant dispersal vectors. These ecosystems often experience anthropogenic nutrient enrichment, favouring invasive species that efficiently exploit these resources. Propagule pressure (reflecting the quantity of introduced individuals) and habitat invasibility are key determinants of invasion success. Moreover, the enemy release hypothesis predicts that escape from natural enemies, such as herbivores, allows alien species to invest more resources to growth and reproduction rather than defense, enhancing their invasive potential. Yet, the combined impact of propagule pressure, herbivory, and nutrient enrichment on the competitive dynamics between invasive alien macrophytes and native macrophyte communities is not well understood due to a paucity of studies.

Methods

We conducted a full factorial mesocosm experiment to explore the individual and combined effects of herbivory, nutrient levels, propagule pressure, and competition on the invasion success of the alien macrophyte Myriophyllum aquaticum into a native macrophyte community comprising Vallisneria natans, Hydrilla verticillata, and Myriophyllum spicatum. This setup included varying M. aquaticum densities (low vs. high, simulating low and high propagule pressures), two levels of herbivory by the native snail Lymnaea stagnalis (herbivory vs no-herbivory), and two nutrient conditions (low vs. high). Myriophyllum aquaticum was also grown separately at both densities without competition from native macrophytes.

Results

The invasive alien macrophyte M. aquaticum produced the highest shoot and total biomass when simultaneously subjected to conditions of high-density intraspecific competition, no herbivory, and low-nutrient availability treatments. Moreover, a high propagule pressure of M. aquaticum significantly reduced the growth of the native macrophyte community in nutrient-rich conditions, but this effect was not observed in nutrient-poor conditions.

Discussion

These findings indicate that M. aquaticum has adaptive traits enabling it to flourish in the absence of herbivory (supporting the enemy release hypothesis) and in challenging environments such as intense intraspecific competition and low nutrient availability. Additionally, the findings demonstrate that when present in large numbers, M. aquaticum can significantly inhibit the growth of native macrophyte communities, particularly in nutrient-rich environments. Consequently, reducing the propagule pressure of M. aquaticum could help control its spread and mitigate its ecological impact. Overall, these findings emphasize that the growth and impacts of invasive alien plants can vary across different habitat conditions and is shaped by the interplay of biotic and abiotic factors.

Seed germination demonstrates inter-annual variations in alkaline tolerance: a case study in perennial Leymus chinensis

BACKGROUND AND AIMS

The escalating issue of soil saline-alkalization poses a growing global challenge. Leymus chinensis is a perennial grass species commonly used in the establishment and renewal of artificial grasslands that is relatively tolerant of saline, alkaline, and drought conditions. Nonetheless, reduced seed setting rates limit its propagation, especially on alkali-degraded grassland. Inter-annual variations have an important effect on seed yield and germination under abiotic stress, and we therefore examined the effect of planting year on seed yield components of L. chinensis.

METHODS

We grew transplanted L. chinensis seedlings in pots for two (Y2), three (Y3), or four (Y4) years and collected spikes for measurement of seed yield components, including spike length, seed setting rate, grain number per spike, and thousand seed weight. We then collected seeds produced by plants from different planting years and subjected them to alkaline stress (25 mM Na2CO3) for measurement of germination percentage and seedling growth.

RESULTS

The seed setting rate of L. chinensis decreased with an increasing number of years in pot cultivation, but seed weight increased. Y2 plants had a higher seed setting rate and more grains per spike, whereas Y4 plants had a higher thousand seed weight. The effects of alkaline stress (25 mM Na2CO3) on seed germination were less pronounced for the heavier seeds produced by Y4 plants. Na2CO3 caused a 9.2% reduction in shoot length for seedlings derived from Y4 seeds but a 22.3% increase in shoot length for seedlings derived from Y3 seeds.

CONCLUSIONS

Our findings demonstrate significant differences in seed yield components among three planting years of L. chinensis under pot cultivation in a finite space. Inter-annual variation in seed set may provide advantages to plants. Increased alkalinity tolerance of seed germination was observed for seeds produced in successive planting years.

Nitrogen addition affected the root competition in Cunninghamia lanceolata-Phoebe chekiangensis mixed plantation

Little is known about below-ground competition in mixed-species plantations under increasing nitrogen (N) deposition. This study aims to determine the effects of N addition on root competition in coniferous and broad-leaved species mixed plantations. A pot experiment was conducted using the coniferous species Cunninghamia lanceolata and the broad-leaved species Phoebe chekiangensis planted in mixed plantations with different competition intensities under N addition (0 or 45 kg N ha-1 yr-1). Biomass allocation, root morphology, root growth level, and competitive ability were determined after five months of treatment. Our findings indicated that root interactions in mixed plantations did not influence biomass allocation in either C. lanceolata or P. chekiangensis but promoted growth in C. lanceolata when no N was added. However, N addition decreased biomass accumulation in both species in the mixed plantation and had a negative effect on the root growth of C. lanceolata due to intensified competition. Addition of N increased the relative importance of root predatory competition in P. chekiangensis, and increased the allelopathic competitive advantage in C. lanceolata. This suggests that N addition causes a shift in the root competitive strategy from tolerance to competition. Overall, these findings highlight the significant impact that the addition of N can have on plant interactions in mixed plantations. Our results provide implications for the mechanisms of root competition in response to increasing atmospheric N deposition in mixed plantations.

Intransitivity in plant-soil feedbacks is rare but is associated with multispecies coexistence

Although plant-soil feedback (PSF) is being recognized as an important driver of plant recruitment, our understanding of its role in species coexistence in natural communities remains limited by the scarcity of experimental studies on multispecies assemblages. Here, we experimentally estimated PSFs affecting seedling recruitment in 10 co-occurring Mediterranean woody species. We estimated weak but significant species-specific feedback. Pairwise PSFs impose similarly strong fitness differences and stabilizing-destabilizing forces, most often impeding species coexistence. Moreover, a model of community dynamics driven exclusively by PSFs suggests that few species would coexist stably, the largest assemblage with no more than six species. Thus, PSFs alone do not suffice to explain coexistence in the studied community. A topological analysis of all subcommunities in the interaction network shows that full intransitivity (with all species involved in an intransitive loop) would be rare but it would lead to species coexistence through either stable or cyclic dynamics.

Females face more positive plant-soil feedback and intersexual competition under adequate nitrogen conditions compared to males in Populus cathayana

Plant-soil feedback (PSF) and competition influence plant performance, community structure and functions. However, how nutrient availability affects the interaction of PSF, sexual competition and coexistence in dioecious plants is poorly understood. In this study, the strengths of PSF and sexual competition, and their responses to nutrient availability were assessed in dioecious Populus cathayana using a garden experiment. We found that PSF reduced but did not eliminate the inequal sexual competition at low nitrogen (N) availability. Intersexual competition and nutrient limitation induced more negative PSF, which promoted sexual coexistence. PSF and competition were rather related to sexual dimorphism. Female plants experience more positive PSF and intersexual competition under adequate N conditions compared to males; the contrary was true with low N supply. Furthermore, the stability of root exudate networks and soil nutrient availability reflects the possibility of sexual coexistence regulated by PSF. Intersexual interaction promote more stable root exudate profiles and more saccharide secretion at low N supply. Meanwhile, the increased soil N and P mineralization in females with cultivated males explained the possible coexistence between females and males at low nutrient availability. Thus, these results indicate that soil biota can mitigate differences in sexual competitiveness and improve the stability of root exudate networks, consequently promoting sexual coexistence at low nutrient availability.

Short-term plant-soil feedback experiment fails to predict outcome of competition observed in long-term field experiment

Mounting evidence suggests that plant-soil feedbacks (PSF) may determine plant community structure. However, we still have a poor understanding of how predictions from short-term PSF experiments compare with outcomes of long-term field experiments involving competing plants. We conducted a reciprocal greenhouse experiment to examine how the growth of prairie grass species depended on the soil communities cultured by conspecific or heterospecific plant species in the field. The source soil came from monocultures in a long-term competition experiment (LTCE; Cedar Creek Ecosystem Science Reserve, MN, USA). Within the LTCE, six species of perennial prairie grasses were grown in monocultures or in eight pairwise competition plots for 12 years under conditions of low or high soil nitrogen availability. In six cases, one species clearly excluded the other; in two cases, the pair appeared to coexist. In year 15, we gathered soil from all 12 soil types (monocultures of six species by two nitrogen levels) and grew seedlings of all six species in each soil type for 7 weeks. Using biomass estimates from this greenhouse experiment, we predicted coexistence or competitive exclusion using pairwise PSFs, as derived by Bever and colleagues, and compared model predictions to observed outcomes within the LTCE. Pairwise PSFs among the species pairs ranged from negative, which is predicted to promote coexistence, to positive, which is predicted to promote competitive exclusion. However, these short-term PSF predictions bore no systematic resemblance to the actual outcomes of competition observed in the LTCE. Other forces may have more strongly influenced the competitive interactions or critical assumptions that underlie the PSF predictions may not have been met. Importantly, the pairwise PSF score derived by Bever et al. is only valid when the two species exhibit an internal equilibrium, corresponding to the Lotka-Volterra competition outcomes of stable coexistence and founder control. Predicting the other two scenarios, competitive exclusion by either species irrespective of initial conditions, requires measuring biomass in uncultured soil, which is methodologically challenging. Subject to several caveats that we discuss, our results call into question whether long-term competitive outcomes in the field can be predicted from the results of short-term PSF experiments.

The role of plant-soil feedback in long-term species coexistence cannot be predicted from its effects on plant performance

BACKGROUND

Despite many studies on the importance of competition and plants' associations with mutualists and pathogens on plant performance and community organization, the joint effects of these two factors remain largely unexplored. Even less is known about how these joint effects vary through a plant's life in different environmental conditions and how they contribute to the long-term coexistence of species.

METHODS

We investigated the role of plant-soil feedback (PSF) in intra- and interspecific competition, using two co-occurring dry grassland species as models. A two-phase PSF experiment was used. In the first phase, soil was conditioned by the two plant species. In the second, we assessed the effect of soil conditioning, competition and drought stress on seedling establishment, plant growth in the first and second vegetation season, and fruit production. We also estimated effects of different treatments on overall population growth rates and predicted the species' potential coexistence.

RESULTS

Soil conditioning played a more important role in the early stages of the plants' life (seedling establishment and early growth) than competition. Specifically, we found strong negative intraspecific PSF for biomass production in the first year in both species. Although the effects of soil conditioning persisted in later stages of plant's life, competition and drought stress became more important. Surprisingly, models predicting species coexistence contrasted with the effects on individual life stages, showing that our model species benefit from their self-conditioned soil in the long run.

CONCLUSIONS

We provide evidence that the effects of PSF vary through plants' life stages. Our study suggests that we cannot easily predict the effects of soil conditioning on long-term coexistence of species using data only on performance at a single time as commonly done in PSF studies. We also show the importance of using as realistic environmental conditions as possible (such as drought stress experienced in dry grasslands) to draw reasonable conclusions on species coexistence.

Plant-soil-enzyme C-N-P stoichiometry and microbial nutrient limitation responses to plant-soil feedbacks during community succession: A 3-year pot experiment in China

Studying plant-soil feedback (PSF) can improve the understanding of the plant community composition and structure; however, changes in plant-soil-enzyme stoichiometry in response to PSF are unclear. The present study aimed to analyze the changes in plant-soil-enzyme stoichiometry and microbial nutrient limitation to PSF, and identify the roles of nutrient limitation in PSF. Setaria viridis, Stipa bungeana, and Bothriochloa ischaemum were selected as representative grass species in early-, mid-, and late-succession; furthermore, three soil types were collected from grass species communities in early-, mid-, and late-succession to treat the three successional species. A 3-year (represents three growth periods) PSF experiment was performed with the three grasses in the soil in the three succession stages. We analyzed plant biomass and plant-soil-enzyme C-N-P stoichiometry for each plant growth period. The plant growth period mainly affected the plant C:N in the early- and late- species but showed a less pronounced effect on the soil C:N. During the three growth periods, the plants changed from N-limited to P-limited; the three successional species soils were mainly limited by N, whereas the microbes were limited by both C and N. The plant-soil-enzyme stoichiometry and plant biomass were not significantly correlated. In conclusion, during PSF, the plant growth period significantly influences the plant-soil-microbial nutrient limitations. Plant-soil-enzyme stoichiometry and microbial nutrient limitation cannot effectively explain PSF during succession on the Loess Plateau.

The temporal development of plant-soil feedback is contingent on competition and nutrient availability contexts

Strength and direction of plant-soil feedback (PSF), the reciprocal interactions between plants and soil, can change over time and have distinct effects on different life stages. PSF and its temporal development can also be modified by external biotic and abiotic factors such as competition and resource availability, yet most PSF research is conducted in simple experimental settings without considering temporal changes. Here I have studied the effect of different competitive settings (intraspecific, interspecific, and no competition) and nutrient addition on the magnitude and direction of biomass-based PSF (performance in conspecific relative to heterospecific inoculum) across 46 grassland species, estimated at the 4th, 10th, and 13th month of the response phase. I also examined whether conspecific inoculum had a long-term effect on plant survival at the 36th month, and whether biomass-based PSF may predict survival-based PSF effects. PSF pooled across all treatments and time points was negative, but a significant overall temporal trend or differences among competitive settings were missing. PSF developed unimodally for interspecific competition across the three time points, whereas it declined gradually in case of intraspecific and no competition. Nutrient addition attenuated negative biomass-based PSF and eliminated negative effects of conspecific inoculum on survival. Interspecific differences in biomass-based PSF were related to survival-based PSF, but only after nutrient addition. This study demonstrates that PSF is dynamic and modulated by external abiotic and biotic factors. PSF research should consider the temporal dynamics of focal communities to properly estimate how PSF contributes to community changes, preferably directly in the field.

Soil Microbes Generate Stronger Fitness Differences than Stabilization among California Annual Plants

AbstractSoil microorganisms influence a variety of processes in plant communities. Many theoretical and empirical studies have shown that dynamic feedbacks between plants and soil microbes can stabilize plant coexistence by generating negative frequency-dependent plant population dynamics. However, inferring the net effects of soil microbes on plant coexistence requires also quantifying the degree to which they provide one species an average fitness advantage, an effect that has received little empirical attention. We conducted a greenhouse study to quantify microbially mediated stabilization and fitness differences among 15 pairs of annual plants that co-occur in southern California grasslands. We found that although soil microbes frequently generate negative frequency-dependent dynamics that stabilize plant interactions, they simultaneously generate large average fitness differences between species. The net result is that if the plant species are otherwise competitively equivalent, the impact of plant-soil feedbacks is to often favor species exclusion over coexistence, a result that becomes evident only by quantifying the microbially mediated fitness difference. Our work highlights that comparing the stabilizing effects of plant-soil feedbacks to the fitness difference they generate is essential for understanding the influence of soil microbes on plant diversity.