Mixed evidence for plant-soil feedbacks in forest invasions

Coordination of water use strategies and leaf economic traits in coexisting exotic and native woody species from evergreen and deciduous broadleaf forests

The leaf economics spectrum (LES) describes the covariation of traits relevant for carbon and nutrient economy in different plant species. However, much less is known about the correlation of LES with leaf water economy, not only because some woody species do not follow the rules, but also because they are rarely tested on the widespread, non-native, fast-growing trees. We hypothesized that fast-growing exotic species that spread on the fast side of the LES coordinate their water-use strategies (WUS) to maintain rapid growth, and that the pattern of coordination differs between evergreen and deciduous forests. Using 4 exotic and 4 native species from evergreen and deciduous broadleaf forests in China, we measured 17 traits of LES and WUS and analyzed their functional roles in different species groups. Our results suggest that LES plays a more important role in the coexistence of species within a community, while WUS contributes more to the distribution of species across different regions. The multidimensional coordination of LES and WUS could better explain the growth and distribution of different plant species and shed light on the coexistence of species from different forest types, especially fast-growing woody exotics.

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.

Changes in nitrogen and phosphorus availability driven by secondary succession in temperate forests shape soil fungal communities and function

The soil fungal community plays an important role in forest ecosystems and is crucially influenced by forest secondary succession. However, the driving factors of fungal community and function during temperate forest succession and their potential impact on succession processes remain poorly understood. In this study, we investigated the dynamics of the soil fungal community in three temperate forest secondary successional stages (shrublands, coniferous forests, and deciduous broad-leaved forests) using high-throughput DNA sequencing coupled with functional prediction via the FUNGuild database. We found that fungal community richness, α-diversity, and evenness decreased significantly during the succession process. Soil available phosphorus and nitrate nitrogen decreased significantly after initial succession occurred, and redundancy analysis showed that both were significant predictors of soil fungal community structure. Among functional groups, fungal saprotrophs and pathotrophs represented by plant pathogens were significantly enriched in the early-successional stage, while fungal symbiotrophs represented by ectomycorrhiza were significantly increased in the late-successional stage. The abundance of both saprotroph and pathotroph fungal guilds was positively correlated with soil nitrate nitrogen and available phosphorus content. Ectomycorrhizal fungi were negatively correlated with nitrate nitrogen and available phosphorus content and positively correlated with ammonium nitrogen content. These results indicate that the dynamics of fungal community and function reflected the changes in nitrogen and phosphorus availability caused by the secondary succession in temperate forests. The fungal plant pathogen accumulated in the early-successional stage and ectomycorrhizal fungi accumulated in the late-successional stage may have a potential role in promoting forest succession. These findings contribute to a better understanding of the response of soil fungal communities to secondary forest succession and highlight the importance of fungal communities during the successional process.

Effects of planting of two common crops, Allium fistulosum and Brassica napus, on soil properties and microbial communities of ginseng cultivation in northeast China

Background

Long-term cultivation of ginseng can cause severe crop disorders and soil sickness. Crop rotation is an effective agricultural management measure to improve soil sustainability and decrease pathogens. However, the suitable ginseng rotation system and the changes in soil microbial community and soil characteristics under the rotation system need to be further explored.

Methods

To explore suitable ginseng crop rotation systems and improve soil utilization, Allium fistulosum and Brassica napus were planted on ginseng cultivation soil for one year. The effects of the two crops on the chemical properties and enzyme activities of the ginseng cultivation soil were evaluated by chemical analysis. In addition, amplicon sequencing targeting 16 s rDNA genes of bacteria and ITS of fungi has been used to characterize the functional and compositional diversity of microbial communities.

Results

The results elucidated that the levels of available phosphorus (AP) and available potassium (AK) in the soil increased significantly after one year of cultivation for both crops and Allium fistulosum cultivation may also have reduced soil salinity. In addition, the effects of the two crops on the activities of key soil enzymes were different. Catalase (CAT), urease (URE), and acid phosphatase (A-PHO) activities were significantly reduced and sucrase (SUC), and laccase (LAC) activities were significantly increased after Allium fistulosum planting. While A-PHO activity was significantly increased and LAC activity was significantly decreased after Brassica napus planting. Allium fistulosum significantly reduced the abundance of soil fungal communities. The cultivation of Allium fistulosum and Brassica napus significantly altered the composition of soil bacterial and fungal communities, where changes in the abundance of dominant microorganisms, such as Ascomycota, and Mortierellomycota, etc., were closely related to soil chemistry and enzyme activity. Moreover, both significantly reduced the abundance of the pathogenic fungus Ilyonectria.

Conclusions

Our study clarified the effects of Allium fistulosum and Brassica napus on the microbial community and physicochemical properties of ginseng cultivated soil and provides a basis for the sustainable application of ginseng cultivation soil and the development of ginseng crop rotation systems.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-022-02592-0.

A quantitative synthesis of soil microbial effects on plant species coexistence

SignificanceUnderstanding the processes that maintain plant diversity is a key goal in ecology. Many previous studies have shown that soil microbes can generate stabilizing or destabilizing feedback loops that drive either plant species coexistence or monodominance. However, theory shows that microbial controls over plant coexistence also arise through microbially mediated competitive imbalances, which have been largely neglected. Using data from 50 studies, we found that soil microbes affect plant dynamics primarily by generating competitive fitness differences rather than stabilizing or destabilizing feedbacks. Consequently, in the absence of other competitive asymmetries among plants, soil microbes are predicted to drive species exclusion more than coexistence. These results underscore the need for measuring competitive fitness differences when evaluating microbial controls over plant coexistence.

A global meta-analysis of the impacts of exotic plant species invasion on plant diversity and soil properties

Plant diversity and biogeochemical cycles are rapidly changing in response to exotic plant species invasion. However, there are conflicting conclusions regarding the quantification of such changes in the soil properties and plant diversity. Moreover, the relationships between soil properties and plant diversity are unclear. Here, a global meta-analysis was conducted on the impact of exotic species invasion on soil physicochemistry, microbial activity, and plant diversity using data from 123 published reports and 332 samples. Exotic species invasion significantly enhanced the soil pH, soil microbial activity, and soil nutrient content. The impact was more substantial for grass than for shrub and tree. Exotic species invasion did not significantly affect soil texture, but significantly reduced the plant diversity, richness, and evenness by 36.97%, 64.72%, and 47.21%, respectively. Soil pH, soil organic carbon, and total nitrogen were significantly correlated with plant diversity reduction. The response ratio of plant richness and evenness gradually increased with precipitation. However, the response ratio of phosphatase, microbial biomass nitrogen, microbial biomass phosphorus, total nitrogen, and soil moisture gradually decreased with precipitation. Overall, exotic species invasion significantly increased the soil nutrient content and soil microbial activity, but significantly decreased plant diversity. These effects were influenced by exotic species types and precipitation.

The effects of plant-soil feedback on invasion resistance are soil context dependent

There is growing interest in understanding the role that plant-soil feedbacks (PSFs) may play in invasion resistance. However, recent studies have shown that there is great uncertainty in explaining community patterns by PSF studies regarding invasions. This uncertainty may be partly because soils used for PSF studies are usually collected from open areas rather than natural communities, thus ignoring the effects of community contexts that may specifically influence the soil feedbacks of community residents to invaders. We performed a two-phase pot experiment to study the soil feedback initiated by ten co-occurring native and exotic species to a forest invader, Phytolacca americana, and the experiments were performed in forest soil and open area soil. The context-dependent mechanisms were further explored by studying different components of PSF. The results showed that natives and exotics had positive and negative effects on P. americana in the open area soil, respectively, but both had negative effects in the forest soil. Nutrient limitation was more important for the PSF in open area soil, whereas biotic factors were likely the primary mechanisms explaining the PSF in forest soil. Additionally, the litter-mediated allelopathy of dominant Quercus acutissima caused the strongest inhibition of the invader. These results suggest that native species can effectively resist invasion by producing negative PSF depending on the community context. Evidence that exotic species promote invasion through positive PSFs was not obtained. This study provided preliminary insights into the possibility of bridging PSF studies and community patterns.

The comprehensive changes in soil properties are continuous cropping obstacles associated with American ginseng (Panax quinquefolius) cultivation

This study aims to verify the time-variant feature of American ginseng (AG) continuous cropping obstacles and to explore the factors impeding continuous cropping. We verified the feature with a plant-soil feedback pot experiment and then investigated the factors by comparing the properties of control soils that had not been previously used for growing ginseng (CS) with those of soils with a 10-year-crop-rotation cycle following the growth of AG (RS). It's found that the survival rate of AG in RS was lower than that in CS. The RS had lower pH, available potassium content, and urease activity. Additionally, p-coumaric, p-hydroxybenzoic, vanillic, caffeic, and cinnamic acid levels were lower in RS than in CS, but salicylic acid levels showed the opposite pattern. RS had higher Rhodanobacter and lower Acidothermus, Sphingomonas relative abundances in bacterial community. It's also found that many bacteria were substantially correlated with phenolic acids and soil physiochemical properties. Results indicate that even after 10-year crop rotation, the negative effects of prior continuous cropping of AG has not been eliminated. The growth of AG can be affected negatively with deterioration of soil physicochemical properties and with lower levels of phenolic acids which promote pathogen reproduction. Probiotics reduction also weighs. Moreover, biotic factors are interrelated with abiotic ones. Therefore, it can be inferred that the comprehensive change of soil properties is the main obstacle for continuous cropping.

Plant-soil feedbacks under resource limitation may not contribute to the invasion by annual Asteraceae plants

Changes in resource availability can alter plant growth, the influence of plants on soil characteristics, and, ultimately, plant-soil feedback (PSF). Previous studies often show that invasive plants can outperform native plants under high but not low resource conditions. However, it remains unclear whether under low resource conditions, invaders can outperform natives in the long term by generating more positive or less negative PSFs. Using three non-native invasive and three non-invasive native annual Asteraceae plants, we conducted a two-phase pot experiment, where in the first, conditioning generation plants were grown to induce changes in soil characteristics, and in the second, bioassay generation plants were regrown to evaluate how they respond to these soils. Half of the pots received a nutrient addition treatment in the conditioning generation. We found significant species-specific effects of conditioning on most of the soil characteristics, and some soil characteristics were significantly correlated with bioassay generation biomass of a subset of species, but neither species nor invasive or native status affected bioassay generation biomass. All invasive species generated neutral PSFs across soil nutrient conditions. The native Emilia sonchifolia tended to condition the soil that favored its own growth more than others, and under low nutrient conditions, the native Eclipta prostrata conditioned the soil that disfavored its own growth more than others. These results indicate that invaders may not outperform natives through PSFs under low resource conditions, and increasing resource availability may change the types of PSFs for some native but not invasive plants.