Soil precipitation legacies influence intraspecific plant-soil feedback

Vegetative growth drives the negative effects of an invasive species on resident community diversity and is not limited by plant-soil feedbacks: A temporal assessment

Many pathways of invasion have been posited, but ecologists lack an experimental framework to identify which mechanisms are dominant in a given invasion scenario. Plant-soil feedbacks (PSFs) are one such mechanism that tend to initially facilitate, but over time attenuate, invasive species' impacts on plant diversity and ecosystem function. PSFs are typically measured under greenhouse conditions and are often assumed to have significant effects under field conditions that change over time. However, direct tests of PSFs effects in natural settings and their change over time are rare. Here we compare the role of PSFs with the effects of biomass in limiting the dominance of an invasive species and impacts on resident species diversity. We characterized the effects of the invader Bromus inermis (Leyss.) on native plant communities over time and measured changes in its conspecific PSFs and vegetative growth to understand their integrated effects on community diversity. To do so, we combined data from a 6-year field study documenting the rate and impacts of invasion with a short-term greenhouse experiment quantifying PSF as a function of time since invasion in the field. We found that the nature and strength of B. inermis PSFs did not change over time and were not mediated by soil microbial communities. Though PSFs impacted B. inermis reproduction, they did not sufficiently limit vegetative growth to diminish the negative impacts of B. inermis biomass on native species. B. inermis experienced the full strength of its negative PSFs immediately upon invasion, but they were ineffective at reducing B. inermis vigor to facilitate the recovery of the native plant community. We recommend that conservation efforts focus on limiting B. inermis vegetative growth to facilitate community recovery.

The soil microbiome affects patterns of local adaptation in an alpine plant under moisture stress

PREMISE

The soil microbiome plays a role in plant trait expression and fitness, and plants may be locally adapted or maladapted to their soil microbiota. However, few studies of local adaptation in plants have incorporated a microbial treatment separate from manipulations of the abiotic environment, so our understanding of microbes in plant adaptation is limited.

METHODS

Here we tested microbial effects on local adaptation in four paired populations of an abundant alpine plant from two community types, dry and moist meadow. In a 5-month greenhouse experiment, we manipulated source population, soil moisture, and soil microbiome and measured plant survival and biomass to assess treatment effects.

RESULTS

Dry meadow populations had higher biomass than moist meadow populations at low moisture, demonstrating evidence of local adaptation to soil moisture in the absence of microbes. In the presence of microbes, dry meadow populations had greater survival than moist meadow populations when grown with dry meadow microbes regardless of moisture. Moist meadow populations showed no signs of adaptation or maladaptation.

CONCLUSIONS

Our research highlights the importance of microbial mutualists in local adaptation, particularly in dry environments with higher abiotic stress. Plant populations from environments with greater abiotic stress exhibit different patterns of adaptation when grown with soil microbes versus without, while plant populations from less abiotically stressful environments do not. Improving our understanding of the role microbes play in plant adaptation will require further studies incorporating microbial manipulations.

Genetic relatedness can alter the strength of plant-soil interactions

PREMISE

Intraspecific variation may play a key role in shaping the relationships between plants and their interactions with soil microbial communities. The soil microbes of individual plants can generate intraspecific variation in the responsiveness of the plant offspring, yet have been much less studied. To address this need, we explored how the relatedness of seedlings from established clones of Solidago altissima altered the plant-soil interactions of the seedlings.

METHODS

Seedlings of known parentage were generated from a series of 24 clones grown in a common garden. Seedlings from these crosses were inoculated with soils from maternal, paternal, or unrelated clones and their performance compared to sterilized control inocula.

RESULTS

We found that soil inocula influenced by S. altissima clones had an overall negative effect on seedling biomass. Furthermore, seedlings inoculated with maternal or paternal soils tended to experience larger negative effects than seedlings inoculated with unrelated soils. However, there was much variation among individual crosses, with not all responding to relatedness.

CONCLUSIONS

Our data argue that genetic relatedness to the plant from which the soil microbial inoculum was obtained may cause differential impacts on establishing seedlings, encouraging the regeneration of non-kin adjacent to established clones. Such intraspecific variation represents a potentially important source of heterogeneity in plant-soil microbe interactions with implications for maintaining population genetic diversity.

Global patterns and drivers of plant-soil microbe interactions

Plant-soil feedback (PSF) is an important mechanism determining plant community dynamics and structure. Understanding the geographic patterns and drivers of PSF is essential for understanding the mechanisms underlying geographic plant diversity patterns. We compiled a large dataset containing 5969 observations of PSF from 202 studies to demonstrate the global patterns and drivers of PSF for woody and non-woody species. Overall, PSF was negative on average and was influenced by plant attributes and environmental settings. Woody species PSFs did not vary with latitude, but non-woody PSFs were more negative at higher latitudes. PSF was consistently more positive with increasing aridity for both woody and non-woody species, likely due to increased mutualistic microbes relative to soil-borne pathogens. These findings were consistent between field and greenhouse experiments, suggesting that PSF variation can be driven by soil legacies from climates. Our findings call for caution to use PSF as an explanation of the latitudinal diversity gradient and highlight that aridity can influence plant community dynamics and structure across broad scales through mediating plant-soil microbe interactions.

Responses of Soil Microbial Survival Strategies and Functional Changes to Wet-Dry Cycle Events

Soil microbial taxa have different functional ecological characteristics that influence the direction and intensity of plant-soil feedback responses to changes in the soil environment. However, the responses of soil microbial survival strategies to wet and dry events are poorly understood. In this study, soil physicochemical properties, enzyme activity, and high-throughput sequencing results were comprehensively anal0079zed in the irrigated cropland ecological zone of the northern plains of the Yellow River floodplain of China, where Oryza sativa was grown for a long period of time, converted to Zea mays after a year, and then Glycine max was planted. The results showed that different plant cultivations in a paddy-dryland rotation system affected soil physicochemical properties and enzyme activity, and G. max field cultivation resulted in higher total carbon, total nitrogen, soil total organic carbon, and available nitrogen content while significantly increasing α-glucosidase, β-glucosidase, and alkaline phosphatase activities in the soil. In addition, crop rotation altered the r/K-strategist bacteria, and the soil environment was the main factor affecting the community structure of r/K-strategist bacteria. The co-occurrence network revealed the inter-relationship between r/K-strategist bacteria and fungi, and with the succession of land rotation, the G. max sample plot exhibited more stable network relationships. Random forest analysis further indicated the importance of soil electrical conductivity, total carbon, total nitrogen, soil total organic carbon, available nitrogen, and α-glucosidase in the composition of soil microbial communities under wet-dry events and revealed significant correlations with r/K-strategist bacteria. Based on the functional predictions of microorganisms, wet-dry conversion altered the functions of bacteria and fungi and led to a more significant correlation between soil nutrient cycling taxa and environmental changes. This study contributes to a deeper understanding of microbial functional groups while helping to further our understanding of the potential functions of soil microbial functional groups in soil ecosystems.

Root endophytic bacterial and fungal communities in a natural hot desert are differentially regulated in dry and wet seasons by stochastic processes and functional traits

Dryland ecosystems experience seasonal cycles of severe drought and moderate precipitation. Desert plants may develop symbiotic relationships with root endophytic microbes to survive under the repeated wet and extremely dry conditions. Although community coalescence has been found in many systems, the colonization by functional microbes and its relationship to seasonal transitions in arid regions are not well understood. Here we examined root endophytic microbial taxa, and their traits in relation to their root colonization, during the dry and wet seasons in a hot desert of the southwestern United States. We used high-throughput DNA sequencing of 16S rRNA and internal transcribed spacer gene profiling of five desert shrubs, and analyzed the seasonal change in endophytic microbial lineages. Goodness of fit to the neutral community model in relationship to microbial traits was evaluated. In summer, Actinobacteria and Bacteroidia increased, although this was not genus-specific. For fungi, Glomeraceae selectively increased in summer. In winter, Gram-negative bacterial genera, including those capable of nitrogen fixation and plant growth promotion, increased. Neutral model analysis revealed a strong stochastic influence on endophytic bacteria but a weak effect for fungi, especially in summer. The taxa with higher frequency than that predicted by neutral model shared environmental adaptability and symbiotic traits, whereas the frequency of pathogenic fungi was at or under the predicted value. These results suggest that community assembly of bacteria and fungi is regulated differently. The bacterial community was affected by stochastic and deterministic processes via bacterial response to drought (response trait), beneficial effect on plants (effect trait), and likely stable mutualistic interactions with plants suggested by the frequency of nodule bacteria. For fungi, mycorrhizal fungi were selected by plants in summer. The regulation of beneficial microbes by plants in both dry and wet seasons suggests the presence of plant-soil positive feedback in this natural desert ecosystem.

Plant-soil feedback under drought: does history shape the future?

Plant-soil feedback (PSF) is widely recognised as a driver of plant community composition, but understanding of its response to drought remains in its infancy. Here, we provide a conceptual framework for the role of drought in PSF, considering plant traits, drought severity, and historical precipitation over ecological and evolutionary timescales. Comparing experimental studies where plants and microbes do or do not share a drought history (through co-sourcing or conditioning), we hypothesise that plants and microbes with a shared drought history experience more positive PSF under subsequent drought. To reflect real-world responses to drought, future studies need to explicitly include plant-microbial co-occurrence and potential co-adaptation and consider the precipitation history experienced by both plants and microbes.

Legacies of precipitation influence primary production in Panicum virgatum

Precipitation is a key driver of primary production worldwide, but primary production does not always track year-to-year variation in precipitation linearly. Instead, plant responses to changes in precipitation may exhibit time lags, or legacies of past precipitation. Legacies can be driven by multiple mechanisms, including persistent changes in plant physiological and morphological traits and changes to the physical environment, such as plant access to soil water. We used three precipitation manipulation experiments in central Texas, USA to evaluate the magnitude, duration, and potential mechanisms driving precipitation legacies on aboveground primary production of the perennial C₄ grass, Panicum virgatum. Specifically, we performed a rainout shelter study, where eight genotypes grew under different precipitation regimes; a transplant study, where plants that had previously grown in a rainout shelter under different precipitation regimes were moved to a common environment; and a mesocosm study, where the effect of swapping precipitation regime was examined with a single genotype. Across these experiments, plants previously grown under wet conditions generally performed better than expected when exposed to drought. Panicum virgatum exhibited stronger productivity legacies of past wet years on current-year responses to drought than of past dry years on current-year responses to wet conditions. Additionally, previous year tiller counts, a proxy for meristem availability, were important in determining legacy effects on aboveground production. As climate changes and precipitation extremes-both dry and wet-become more common, these results suggest that populations of P. virgatum may become less resilient.

Drought legacy effects on root morphological traits and plant biomass via soil biota feedback

Drought causes soil feedback effects on plant performance. However, how the linkages between conditioned soil biota and root traits contribute to explain plant-soil feedback (PSF) as a function of drought is unknown. We utilized soil inoculum from a conditioning experiment where grassland species grew under well-watered and drought conditions, and their soil fungi were analyzed. Under well-watered conditions, we grew 21 grassland species with those inocula from either conspecific or heterospecific soils. At harvest, plant biomass and root traits were measured. Negative PSF (higher biomass in heterospecific than in conspecific soils) was predominant, and favored in drought-conditioned soils. Previous drought affected the relationship between root traits and fungal groups. Specific root surface area (SRSA) was higher in heterospecific than in conspecific droughted soils and was linked to an increase in saprotroph richness. Overall, root diameter was higher in conspecific soils and was linked to mutualist and pathogen composition, whereas the decrease of root : shoot in heterospecific soils was linked to pathogenic fungi. Drought legacy affects biomass and root morphological traits via conditioned soil biota, even after the drought conditions have disappeared. This provides new insights into the role that soil biota have modulating PSF responses to drought.

Drought legacies and ecosystem responses to subsequent drought

Climate change is expected to increase the frequency and severity of droughts. These events, which can cause significant perturbations of terrestrial ecosystems and potentially long-term impacts on ecosystem structure and functioning after the drought has subsided are often called 'drought legacies'. While the immediate effects of drought on ecosystems have been comparatively well characterized, our broader understanding of drought legacies is just emerging. Drought legacies can relate to all aspects of ecosystem structure and functioning, involving changes at the species and the community scale as well as alterations of soil properties. This has consequences for ecosystem responses to subsequent drought. Here, we synthesize current knowledge on drought legacies and the underlying mechanisms. We highlight the relevance of legacy duration to different ecosystem processes using examples of carbon cycling and community composition. We present hypotheses characterizing how intrinsic (i.e. biotic and abiotic properties and processes) and extrinsic (i.e. drought timing, severity, and frequency) factors could alter resilience trajectories under scenarios of recurrent drought events. We propose ways for improving our understanding of drought legacies and their implications for subsequent drought events, needed to assess the longer-term consequences of droughts on ecosystem structure and functioning.

Changes in precipitation patterns can destabilize plant species coexistence via changes in plant-soil feedback

Climate change can alter species coexistence through changes in biotic interactions. By describing reciprocal interactions between plants and soil microbes, plant-soil feedback (PSF) has emerged as a powerful framework for predicting plant species coexistence and community dynamics, but little is known about how PSF will respond to changing climate conditions. Hence, the context dependency of PSF has recently gained attention. Water availability is a major driver of all biotic interactions, and it is expected that precipitation patterns will change with ongoing climate change. We tested how soil water content affects PSF by conducting a full factorial pairwise PSF experiment using eight plant species common to southeastern United States coastal prairies under three watering treatments. We found coexistence-stabilizing negative PSF at drier-than-average conditions shifted to coexistence-destabilizing positive PSF under wetter-than-average conditions. A simulation model parameterized with the experimental results supports the prediction that more positive PSF accelerates the erosion of diversity within communities while decreasing the predictability in plant community composition. Our results underline the importance of considering environmental context dependency of PSF in light of a rapidly changing climate.

Residence time determines invasiveness and performance of garlic mustard (Alliaria petiolata) in North America

While biological invasions have the potential for large negative impacts on local communities and ecological interactions, increasing evidence suggests that species once considered major problems can decline over time. Declines often appear driven by natural enemies, diseases or evolutionary adaptations that selectively reduce populations of naturalised species and their impacts. Using permanent long-term monitoring locations, we document declines of Alliaria petiolata (garlic mustard) in eastern North America with distinct local and regional dynamics as a function of patch residence time. Projected site-specific population growth rates initially indicated expanding populations, but projected population growth rates significantly decreased over time and at the majority of sites fell below 1, indicating declining populations. Negative soil feedback provides a potential mechanism for the reported disappearance of ecological dominance of A. petiolata in eastern North America.