Shrub encroachment influences root traits and mycorrhization in subalpine grasslands

Shrub encroachment, a global phenomenon caused by land abandonment and shifts in traditional land use practices, is particularly prevalent in subalpine grasslands. This ecological shift is characterized by increased woodiness, which leads to changes in biogeochemical cycles and microbial composition. These changes in turn impact the soil's abiotic environment, particularly on carbon and nitrogen availability. While the influence of these changes on aboveground plant traits is well recognized, a substantial knowledge gap remains regarding their effects belowground. Understanding how shrub encroachment affects root morphological traits and mycorrhization is crucial, as they play a key role in nutrient uptake and transfer. This study focuses on the effects of shrub encroachment on root morphological traits and arbuscular mycorrhiza fungi (AMF) colonization at the levels of both herbaceous plants and of communities, i.e. including herbaceous and dwarf shrub plants, along a gradient of shrub encroachment in subalpine grasslands. We also aimed to describe the root economics space in encroached grasslands and to identify key soil changes correlated with changes in root traits. In herbaceous plants, shrub encroachment decreases AMF colonization and specific root length (SRL), and increases root tissue density (RTD). At the community level, AMF colonization, SRL, and RTD all decrease with shrub encroachment. Surprisingly, the observed root economics space at the community level does not follow the already established negative correlations of “do-it-yourself” strategies with high SRL and “outsourcing” strategies with increased root diameter and AMF colonization. Moreover, we observed a negative correlation between RTD and AMF. Our results highlight the importance of soil characteristics, specifically the carbon/nitrogen ratio (C:N) and soil pH, for changes in root traits. We conclude that shrub encroachment promotes the development of shorter and less dense roots and causes a decrease in AMF colonization through changes in the soil abiotic environment, such as an increase in C:N and a decrease in pH. This research provides valuable insights by expanding our understanding of belowground responses to shrub encroachment and highlights the importance of considering root traits in the broader context of ecosystem functioning.

Microplastics (MP) are recognized as a major pollutant in terrestrial environments, prompting concerns about their effects on plant–soil dynamics. Despite evidence of MP altering soil physicochemical properties, impacts on belowground root traits and arbuscular mycorrhizal (AM) fungi remain poorly explored. Existing research has mainly centred on a few model plant species, emphasizing root biomass, and often employs single polymer types and addition rates that surpass realistic scenarios. To investigate how environmentally relevant mixtures and concentrations of MPs impact plant growth, root trait expression and AM fungal colonization, we conducted a greenhouse experiment using six plant species chosen for their contrasting root life strategies; three species in the Amaryllidaceae family represented resource conservation root traits (Allium fistulosum (onion), Allium tuberosum (chive), Allium porrum (leek)), and three from the Solanaceae family, represented plants with resource acquisitive root traits (Solanum lycopersicum (tomato), Solanum melongena (eggplant), Capsicum annuum (pepper)). MP treatments consisted of control (0% MP), low (0.1% w/w) and high (1% w/w) MP additions, using an environmentally relevant MP mixture of weathered polymer types and shapes. We measured above and belowground biomass, average root trait expression (specific root length (SRL), average root diameter (D) and root tissue density (RTD)), AM fungal colonization, as well as intraspecific variability across MP addition treatments. We found that responses to environmentally relevant additions of MPs were species specific and not determined by root life‐strategy. MPs increased biomass in leek, eggplant and tomato, while decreasing AM fungal colonization in tomato. MP additions had no discernible impact on average root functional trait expression across species. However, the addition of MPs resulted in altered intraspecific variability in root traits and AM fungal colonization, indicating a mechanism for plant tolerance to MPs. To address the impacts of MPs on plant functioning, research needs to focus on environmentally relevant mixtures of MPs, considering various plant species' capacities to tolerate soil contamination and the potential for tipping points under real‐world conditions. Read the free Plain Language Summary for this article on the Journal blog.

Soybean plants autoregulate to suppress excessive nodulation. It has been revealed recently that the autoregulation of various legumes controls both nodulation and arbuscular mycorrhizal (AM) fungal colonization. We investigated the involvement of autoregulation in the interaction between rhizobial nodulation and AM fungal colonization. We used a wild-type soybean cv. Enrei and its hypernodulating mutant Kanto100, defective in the autoregulation. We included four different treatments: an uninoculated control, inoculation with rhizobium Bradyrhizobium japonicum alone, inoculation with AM fungus Gigaspora rosea alone, and dual inoculation with rhizobium and AM fungus. In both Enrei and Kanto100, AM fungal colonization enhanced the weight and N2 fixation of nodules, suggesting that autoregulation of host plant is not involved in the stimulatory effect of AM fungal colonization on rhizobial nodulation. In plants with the AM fungus alone, the AM fungal colonization of Enrei was comparable to that of Kanto100. In plants with dual inoculation, however, this was significantly (P < 0.05) lower than in Kanto100. To confirm the control of AM fungal colonization by the autoregulation of host plant, a reciprocal grafting experiment was performed between Enrei and Kanto100. In plants with the AM fungus alone, AM fungal colonization was comparable among Enrei (shoot)/Enrei (root), Enrei/Kanto100, Kanto100/Enrei, and Kanto100/Kanto100 grafts. In plants with dual inoculation, however, AM fungal colonization of Enrei/Enrei and Enrei/Kanto100 grafts was significantly (P < 0.05) lower than that of Kanto100/Enrei and Kanto100/Kanto100. These results indicate that rhizobial nodulation suppresses AM fungal colonization, and the autoregulation of host plant, initiated by nodulation, is involved in this phenomenon.

Insect herbivory may influence arbuscular mycorrhizal (AM) fungal colonisation by changing plant chemistry, and these effects can vary from negative to positive. Yet the underlying mechanisms are unclear. We investigated AM fungal colonization of Chinese tallow tree (Triadica sebifera) after exposing its seedlings to four different foliar‐feeding insects at two levels of herbivory. We then examined the potential role of induced carbon allocation and secondary metabolites, particularly flavonoids, in AM fungal colonization. Light herbivory (ca. 10% leaf area removed) promoted early colonization by AM fungi whereas heavy herbivory (ca. 60% leaf area removed) decreased it. Root flavonoids were increased (in the short‐term) under light herbivory but decreased under heavy herbivory. Further, herbivore‐induced changes to root flavonoids were positively correlated with those of AM fungal colonisation. Moreover, quercitrin, one of the root‐secreted flavonoids, could mediate these AM fungal responses. Surprisingly, however, AM fungal colonization was not correlated with herbivore‐induced carbon allocation in roots or root exudates. We show that AM fungal responses to insect herbivory are at least partially flavonoid‐based and depend on herbivory intensity. Our work highlights the importance of secondary metabolites rather than carbon allocation in interactions between above‐ground herbivory and below‐ground mycorrhiza. Read the free Plain Language Summary for this article on the Journal blog.

BackgroundThe symbiotic interaction between arbuscular mycorrhizal fungi (AMF) and roots can change root traits, soil carbon - nitrogen processes and crop yield. However, the precise mechanisms by which AMF affect soil carbon economic strategies and crop yield remain unclear. A two - factor pot experiment was done with cotton. Factor 1 was nitrogen application (1.5, 1.0, 0.5 g·kg−1), Factor 2 was AMF treatment (colonization and non - colonization) to study relationships between AMF and root traits, nutrient strategies, yield.ResultsThe analysis of the root economic spectrum reveals that after inhibiting AMF colonization in roots, root nitrogen content (RNC), root intersection count (RIC), specific root length (SRL), root branching intensity (RBI), specific root area (SRA), and root tip count (RTC) adopting an acquisitive strategy, whereas AMF colonization and root diameter (RD) showed a conservative strategy. When AMF normally colonizes roots, AMF colonization, RNC, RBI and RTC exhibit conservative strategy, whereas SRA, RD, SOC and leaf nitrogen content (LNC) display an acquisitive strategy. Additionally, there is a non - linear relationship between root traits and seed - cotton yield. Notably, AMF colonization leads to variability in the relationships between SRA and yield, and between RTC and yield.ConclusionsUnder nitrogen reduction conditions, AMF colonization can enhance nitrogen acquisition by optimizing root characteristics (SRA and RBI), coordinating nitrogen metabolism between leaves and roots, and adjusting the soil carbon economic strategy. In addition, AMF hyphae will adopt a strategy of slowly acquiring nitrogen as a reward for plants, which is one of the key factors contributing to the observed differences between the trends in root morphology and seed - cotton yield.

A glimpse into the past of land plants and of their mycorrhizal affairs: from fossils to evo‐devo

Mycorrhizal symbiosis, specifically arbuscular mycorrhiza, is one of Earth's oldest and most widespread symbiosis. Existing evidence suggests that plant species differ in their associations with mycorrhizal partners, with different species reported to be always (obligately mycorrhizal, OM), sometimes (facultatively mycorrhizal, FM) or never (non‐mycorrhizal, NM) associating with arbuscular mycorrhizal (AM) fungi and this plant reliance on AM fungi is called plant mycorrhizal status. However, very little is known about how host plant mycorrhizal status shapes the network topology of interacting AM fungi. Here, we use a standardized sampling scheme to test whether plant species with different mycorrhizal statuses differ in mean AM fungal hyphal colonization and various indices of the AM fungal networks such as nestedness rank and resource range. We collected the roots and rhizosphere soil of 19 plant species representing five families. Each plant species was sampled from three distinct habitats. We determined AM fungal colonization in the roots and AM fungal community composition in roots and rhizosphere soil using molecular methods. We found that previously reported NM plant species had lower mean AM fungal colonization than FM plant species, but no differences were found between FM and OM plant species. Network analyses indicated that AM fungal communities in the roots of FM plant species had higher nestedness rank and resource range than networks associated with OM plant species, suggesting that OM plant species are more generalist regarding partner selection and interact with a wider range of fungal partners. Our results suggest that plant mycorrhizal status conveys useful information about the characteristics of AM fungal interaction networks, revealing that plant species consistently associated with AM fungi are less selective towards their fungal partners. Read the free Plain Language Summary for this article on the Journal blog.

When enemies attack do plants get by with a little help from their friends?

Root and leaf traits are expected to converge on the plant economics spectrum (PES). Some studies have focused on correlation between specific root length (SRL) and specific leaf area (SLA), which reflect resource acquisition per invested mass in root and leaf, respectively. However, the results have been inconsistent amongst previous studies. We hypothesized that this discrepancy was due to overlooked variations in root traits depending on mycorrhizal types because SRL can be influenced by not only PES but also mycorrhizal types. To assess how mycorrhizal type inherently mediates the coordination of root and leaf traits, we determined the leaf and root traits of current-year seedlings of 33 species encompassing different leaf habits and mycorrhizal types, AM (arbuscular mycorrhizal) and ECM (ectomycorrhizal) species, grown under a common condition. Root and leaf traits correlated with the first axis of the principal component analysis, and this axis represented PES. Root diameter (RD) also correlated with the second axis, which differed between mycorrhizal types. Specific root length (SRL) and SLA were correlated positively to each other, but ECM species had higher SRL than AM species when compared at the same SLA. This was because (i) SRL is negatively related to root tissue density (RTD) and RD, (ii) RTD was negatively correlated with SLA and (iii) RD was smaller in ECM. Leaf and root traits are tightly coordinated with each other across species, but the relationship shifts between the mycorrhizal types.

There is little quantitative information about the relationship between root traits and the extent of arbuscular mycorrhizal fungi (AMF) colonization. We expected that ancestral species with thick roots will maximize AMF habitat by maintaining similar root traits across root orders (i.e., high root trait integration), whereas more derived species are expected to display a sharp transition from acquisition to structural roots. Moreover, we hypothesized that interspecific morphological differences rather than soil conditions will be the main driver of AMF colonization. We analyzed 14 root morphological and chemical traits and AMF colonization rates for the first three root orders of 34 temperate tree species grown in two common gardens. We also collected associated soil to measure the effect of soil conditions on AMF colonization. Thick-root magnoliids showed less variation in root traits along root orders than more-derived angiosperm groups. Variation in stele:root diameter ratio was the best indicator of AMF colonization within and across root orders. Root functional traits rather than soil conditions largely explained the variation in AMF colonization among species. Not only the traits of first order but the entire structuring of the root system varied among plant lineages, suggesting alternative evolutionary strategies of resource acquisition. Understanding evolutionary pathways in belowground organs could open new avenues to understand tree species influence on soil carbon and nutrient cycling.

Summary Root, stem and leaf traits are thought to be functionally coordinated to maximize the efficiency of acquiring and using limited resources. However, evidence is mixed for consistent whole‐plant trait coordination among woody plants, and we lack a clear understanding of the adaptive value of root traits along soil resource gradients. If fine roots are the below‐ground analogue to leaves, then low specific root length (SRL) and high tissue density should be common on infertile soil. Here, we test the prediction that root, stem and leaf traits and relative growth rate respond in unison with soil fertility gradients. We measured fine root, stem and leaf traits and relative growth rate on individual seedlings of 66 tree species grown in controlled conditions. Our objectives were (i) to determine whether multiple root traits align with growth rate, leaf and stem traits and with each other and (ii) to quantify the relationships between community‐weighted mean root traits and two strong soil fertility gradients that differed in spatial extent and community composition. At the species level, fast growth rates were associated with low root and stem tissue density and high specific leaf area. SRL and root diameter were not clearly related to growth rate and loaded on a separate principal component from the plant economic spectrum. At the community level, growth rate was positively related to soil fertility, and root tissue density (RTD) and branching were negatively related to soil fertility. SRL was negatively related and root diameter was positively related to soil fertility on the large‐scale gradient that included ectomycorrhizal angiosperms. Synthesis. Root, stem and leaf tissue traits of tree seedlings are coordinated and influence fitness along soil fertility gradients. RTD responds in unison with above‐ground traits to soil fertility gradients; however, root traits are multidimensional because SRL is orthogonal to the plant economic spectrum. In contrast to leaves, trees are not constrained in the way they construct fine roots: plants can construct high or low SRL roots of any tissue density. High RTD is the most consistent below‐ground trait that reflects adaptation to infertile soil.

1. Below-ground plant functional traits regulate plant–soil interactions and may therefore strongly influence ecosystem responses to global change. Despite this, knowledge of how fine-root functional traits vary among plant species and along environmental gradients has lagged far behind our understanding of above-ground traits. 2. We measured species- and community-level root and leaf trait responses for 50 temperate rain forest species from 28 families of ferns, woody and herbaceous angiosperms and conifers, along a soil chronosequence in New Zealand that exhibits a strong gradient in soil nutrient availability. Relationships among species traits (both above- and below-ground) and their distribution along the chronosequence were tested using phylogenetic generalized least-squares regression to account for plant relatedness. 3. Distinctive root trait syndromes were observed; they were closely linked to species' distribution along the chronosequence. Species growing in the strongly P-limited late stages of the chronosequence had relatively high specific root length (SRL), thin root diameter, high root tissue density, high levels of root branching and low root nutrient concentrations compared to intermediate stages. Species on the youngest site also had high SRL, but had low root tissue density, thick root diameter and high root nutrient concentrations. 4. Species root and leaf nutrient concentrations were positively correlated, reflecting the strong underlying gradient in soil fertility. In contrast, the relationship between SRL and SLA was more complex; there was a weak positive correlation between SRL and SLA, but this conflicted with stronger patterns of increasing SRL and declining SLA with increasing site age. 5. Community-averaged trait values calculated using presence/absence data showed similar trends to the species-level patterns. In contrast, community averages calculated using species abundance-weighted data showed weaker relationships with site age, particularly for morphological traits. This suggests that much of the variation in morphological traits between sites was driven by shifts in the presence of subordinate or 'rare' species rather than by changes in the dominant species. 6. Synthesis. Our study demonstrates co-ordinated species- and community-level changes in root traits along a soil chronosequence. These results highlight the influence of soil nutrition on plant functional traits and contribute to our understanding of the drivers of community assembly in a changing environment.

Symbiotic interactions between arbuscular mycorrhizal (AM) fungi and male papaya plants: Its status, role and implications

Plant sexual reproduction is an energetic process since it requires large amounts of resource allocation for flower and fruit production. Therefore, the establishment of a mutualism with arbuscular mycorrhizal (AM) fungi can increase the plant’s reproductive success. Acaena elongata is a native weed and a plant species indicator of anthropogenic perturbations in the understory of Abies religiosa forests of the Magdalena river basin in Mexico City. The aim of this work was to assess whether: i) there are temporal and spatial variations in AM fungal root colonization in A. elongata, ii) AM fungal root colonization is linked to A. elongata flower and fruit production patterns, and iii) AM fungal root colonization, spore abundance and richness, and A. elongata reproductive phenology are related to soil variables. Eight plots were established with five randomly selected individuals of A. elongata. Over the course of 1 year, the A. elongata reproductive phenology, environmental temperature, and relative soil moisture were registered monthly. Additionally, root samplings for AM fungal colonization and soil for determining the AM fungal spore abundance and richness were carried out during the rainy and dry seasons. The mean total AM fungal root colonization percentage was higher in the dry season (78 ± S.E. 11.09%) than in the rainy season (71 ± S.E. 13.2%); A. elongata showed reproductive structures throughout the year, with negative significant correlations between colonization and some phenological phases during the rainy season. Flower and fruit production as well as AM fungal root colonization were related to variations in the abiotic variables in each season and population. This study contributes to the understanding of the relationship between AM fungi and a plant species indicator of anthropogenic perturbations.

Comparison of arbuscular mycorrhizal and dark septate endophyte fungal associations in soils irrigated with pulp and paper mill effluent and well-water