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Can Fertilization of Soil Select Less Mutualistic Mycorrhizae?
Ecological Applications
Abstract
It has been noted previously that nutrient-stressed plants generally release more soluble carbohydrate in root exudates and consequently support more mycorrhizae than plants supplied with ample nutrients. Fertilization may select strains of vesicular-arbuscular mycorrhizal (VAM) fungi that are inferior mutualists if the same characteristics that make a VAM fungus successful in roots with a lowered carbohydrate content also reduce the benefits that the fungus provides a host plant. This two-phase study experimentally tests the hypothesis that fertilizing low-nutrient soil selects VAM fungi that are inferior mutualists. The first phase examines the effects of chemical fertilizers on the species composition of VAM fungal communities in long-term field plots. The second phase measures the effects of VAM fungal assemblages from fertilized and unfertilized plots on big bluestem grass grown in a greenhouse. The field results indicate that 8 yr of fertilization altered the species composition of VAM fungal communities. Relative abundance of Gi-gaspora gigantea, Gigaspora margarita, Scutellispora calospora, and Glomus occultum decreased while Glomus intraradix increased in response to fertilization. Results from the greenhouse experiment show that big bluestem colonized with VAM fungi from fertilized soil were smaller after 1 mo and produced fewer inflorescences at 3 mo than big bluestem colonized with VAM fungi from unfertilized soil. Fungal structures within big bluestem roots suggest that VAM fungi from fertilized soil exerted a higher net carbon cost on their host than VAM fungi from unfertilized soil. VAM fungi from fertilized soil produced fewer hyphae and arbuscules (and consequently provided their host with less inorganic nutrients from the soil) and produced as many vesicles (and thus provisioned their own storage structures at the same level) as fungi from unfertilized soil. These results support the hypothesis that fertilization selects VAM fungi that are inferior mutualists.
... In addition, the stress gradient hypothesis (SGH) predicts that beneficial interactions between species will be more prevalent in higher stress conditions, whereas more negative interactions will occur in low stress environments (Bertness and Callaway, 1994). For example, some pathogens are more virulent in wetter, less stressful conditions (Pratt and Mitchell, 1975;Gautam et al., 2013;Velásquez et al., 2018), and AMF can increase plant fitness when soil phosphorus is limited but can become parasitic when phosphorus is plentiful (Johnson, 1993). Therefore, plants may be more likely to form adaptive associations with microbes when their abiotic environment is more stressful, whereas F I G U R E 1 Predicted patterns of local (mal)adaptation both with and without microbes. ...
... For example, environmental stress can determine the proportion of pathogens and mutualists, with fewer pathogens tending to be present under higher abiotic stress (Lau and Lennon, 2012;Hernandez et al., 2021). In addition, previous work has found greater benefits to fitness in plants associating with microbes under greater nutrient limitation (Johnson, 1993;Hacquard et al., 2016;David et al., 2020;Fuggle et al., 2023), less water availability (Petipas et al., 2020;Basyal and Emery, 2021), higher salinity (Lumibao et al., 2022), and greater biotic stress (Bastías et al., 2022). Dry meadow populations may have exhibited strong local adaptation because selection strength is higher in more stressful (dry) environments (Parsons, 2005;Agrawal and Whitlock, 2010). ...
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.
... While these patterns may be mediated by the unique functional traits of rare species varying in phylogenetic relatedness, an alternative hypothesis may suggest that the characteristic low biomass of rare species is a function of ECM parasitism in low N environments. While these results are consistent with parasitic patterns in belowground fungi, as well as the functional equilibrium model of resource allocation under N enrichment (Johnson et al. 2008), it is also unusual to observe parasitism of mycorrhizal fungi in low N environments in which mutualisms are necessary for nutrient acquisition (Johnson 1993;Johnson et al. 2008). Consequently, the biomass of rare plant species may be representative of: 1) N-limitation; 2) allocation to belowground resources for limiting micronutrients; 3) the maintenance of valuable functional traits rather than biomass production in nutrient-lacking environments. ...
Microbially mediated plant-soil feedbacks drive patterns of plant growth, competitive ability, succession, and community composition. Although rare plant species maintain unique functional traits that often facilitate negative feedbacks, there is not a consensus on the belowground drivers nor the effects of phylogenetic origin of previously plant-conditioned soil on aboveground traits associated with rare species. Using a common garden, we connect belowground fungal colonization to aboveground traits in species varying in rarity, and soil conditions varying in the phylogenetic relatedness of conditioning plant species, to demonstrate the mechanistic relationship between belowground ectomycorrhizal fungal (ECM) colonization and aboveground total plant biomass in 14 Eucalyptus species varying in their rarity status. Specifically, we found that while the rarest species displayed 88% less total biomass than common species, the rarest species also maintained 62% greater ECM colonization than common counterparts. Further, negative feedbacks resulted in reduced biomass coupled with positive feedbacks that resulted in increased ECM colonization that varied on the basis of phylogenetic relatedness. The rarest species decreased by 71% - 94% in total biomass but increased by 96% - 114% in ECM colonization in phylogenetically similar and distant soil compared to conspecific soil conditions. The effect size of ECM colonization directly affected the effect size of total biomass in phylogenetically distant conditions with a significant negative correlation (r^2 = -0.83) to show that biomass may be a function of ECM colonization acting differently among species varying in rarity. Consequently, rare plant species may utilize stronger associations with belowground mycorrhizal mutualists than common plant species, to facilitate geographic, competitive, and functional persistence, even while maintaining lower biomass.
... nutrient-poor soils), which cannot afford to be as selective in their fungal partnerships. These results are surprising considering the vast amount of studies reporting the reduction in benefits for plants provided by mycorrhizal fungi after fertilization (Hoeksema et al. 2010) and, furthermore, that mycorrhizal fungi could turn parasitic when fertilizers are applied (Johnson 1993;Peng et al. 2023) These disparities could indicate that the role of fertilizer as "filter agent" is dependent on the context, including plant and fungal species, as shown by Li et al. (2016) meta-analysis. In addition, it has been suggested that plants could discriminate mycorrhizal fungi in terms of the benefits they provide, and allocate more carbon to their preferred fungal partners (Wang et al. 2017). ...
Information regarding the relationship between fertilization, mycorrhizas, and plant growth is scattered for non-conventional productive plant species. We evaluated the effect of different substrates and fertilization treatments on growth and colonization by arbuscular mycorrhizas of young Berberis microphylla plants, a native Patagonian shrub with edible fruits. We conducted a greenhouse experiment based on two factors: substrate (conventional or native soil) and fertilization (no fertilization, organic fertilization, or inorganic fertilization). When plants were grown in conventional substrate, both fertilizers promoted growth, having the inorganic fertilizer a greater effect. The effect of both fertilizers was similar when plants were cultivated in native soil, and lesser than in conventional substrate. Plants grown in native soil were larger than those in conventional substrate when organic fertilizer or no fertilizer was applied, but this was reversed when inorganic fertilizer was applied. There was no mycorrhization on plants grown in conventional substrate. In native soil, mycorrhization was highest for non-fertilized plants (60.1%), followed by those with organic fertilization (40.4%), and lowest when inorganic fertilizer was applied (29.9%). The relative abundances of both vesicles and arbuscules showed the opposite tendency, having both their highest values in treatments with inorganic fertilizer. Mycorrhization was positively correlated with plant size, but only when fertilizers were applied. Based on our results, we hypothesized that fertilization reduce mycorrhization but select more beneficial mycorrhizal fungi. We concluded that organic fertilizers have a comparable effect to inorganic fertilizers in terms of promoting plant growth, accompanied by a lesser reduction of mycorrhization.
... The diversity of AMF was also reduced with a high soil available P, despite the fact that different plants contain different AM fungal communities (Ciccolini et al., 2016;Gosling et al., 2013;Van Geel et al., 2016;Verbruggen et al., 2012). In an eightyear field experiment, the relative abundance of Gigaspora gigantea, Gigaspora margarita, S. calospora, and Paraglomus occultum decreased, whereas R. irregularis increased in response to the application of mineral fertilizers (Johnson, 1993). Furthermore, AM fungal colonization was lower in the soil subjected to fertilization compared to the unfertilized soil. ...
Characterization and selection of arbuscular mycorrhizal fungal (AMF) taxa to design inocula tailored to meet a spectrum of needs is a crucial first step to achieve specific beneficial agronomic functions. Commonly, commercial microbial inocula are based on generalist single AM fungal taxa, having low genetic variability and not offering efficiency and stability when applied in agroecosystems. In this study, we investigated the AMF functional variability at inter-and intra-species levels by characterizing colonization traits, host growth, and mineral uptake of single-spore AM fungi isolated from soils with a fertility gradient. Nineteen single-spore cultures, showing high spore density and AMF colonization, were phylogenetically assigned to different isolates of 3 AMF species (i.e. Entrophospora claroidea, Funneliformis mosseae and Archaeospora trappei). A higher functional variability in infectivity and effectiveness was detected among isolates within AMF species (25 % of total variance) than among AMF species. Most of AMF isolates of F. mosseae have a better outcome in terms of plant growth, although with a performance gradient, while the isolates of E. claroidea showed a variable functional pattern, and those of A. trappei a less variable pattern. Overall, isolates originating from the soil of the conventional arable field with higher pH and phosphorous availability promoted the uptake of plant nutrients, while those originating from soils with higher SOM and plant diversity promoted plant growth. On the contrary, the infectivity traits of the AM fungi were more conserved, as they were not affected by the environmental parameters of the soils of origin. Finally, we highlighted that soil pH played an important role in shaping the pattern of AMF functionality. Boosting the isolation and cultivation of AMF taxa, originating from agricultural and natural soils, is shown to be a key step in exploiting AMF diversity and designing the new generation of microbial inoculants.
... Fungal taxa with high extraradical growth may be more effective in acquiring P (Hart & Reader, 2002;Maherali & Klironomos, 2007), although differences in function of the interfacial membranes may also play a role (Smith et al., 1994). P fertilization may favor AMF that preferentially develop inside the root, leading to suggestions of reduced mutualism (Johnson, 1993). However, it is possible that AMF provide other functions, such as pathogen protection, in more fertile soils (Lekberg & Waller, 2021). ...
Phosphorus (P) for carbon (C) exchange is the pivotal function of arbuscular mycorrhiza (AM), but how this exchange varies with soil P availability and among co‐occurring plants in complex communities is still largely unknown.
We collected intact plant communities in two regions differing c. 10‐fold in labile inorganic P. After a 2‐month glasshouse incubation, we measured ³²P transfer from AM fungi (AMF) to shoots and ¹³C transfer from shoots to AMF using an AMF‐specific fatty acid. AMF communities were assessed using molecular methods.
AMF delivered a larger proportion of total shoot P in communities from high‐P soils despite similar ¹³C allocation to AMF in roots and soil. Within communities, ¹³C concentration in AMF was consistently higher in grass than in blanketflower (Gaillardia aristata Pursh) roots, that is P appeared more costly for grasses. This coincided with differences in AMF taxa composition and a trend of more vesicles (storage structures) but fewer arbuscules (exchange structures) in grass roots. Additionally, ³²P‐for‐¹³C exchange ratios increased with soil P for blanketflower but not grasses.
Contrary to predictions, AMF transferred proportionally more P to plants in communities from high‐P soils. However, the ³²P‐for‐¹³C exchange differed among co‐occurring plants, suggesting differential regulation of the AM symbiosis.
... The diversity of AMF was also reduced with a high soil available P, despite the fact that different plants contain different AM fungal communities (Ciccolini et al., 2016;Gosling et al., 2013;Van Geel et al., 2016;Verbruggen et al., 2012). In an eightyear field experiment, the relative abundance of Gigaspora gigantea, Gigaspora margarita, S. calospora, and Paraglomus occultum decreased, whereas R. irregularis increased in response to the application of mineral fertilizers (Johnson, 1993). Furthermore, AM fungal colonization was lower in the soil subjected to fertilization compared to the unfertilized soil. ...
In conventional agriculture, benefits of arbuscular mycorrhizal (AM) symbiosis are beyond utilization for years due to faulty management practices. The use of chemical fertilizers and other agrochemicals and intensive tillage affect arbuscular mycorrhizal (AM) symbiosis negatively, hampering root colonization, spread of intra- and extraradical mycelia, spore germination, and functions. Some of the AMF species are extremely sensitive to fertilization and other cultivation practices to survive. The application of phosphorus fertilization reduces plant’s dependency on AMF due to higher P availability in rhizosphere and such practices also minimize the other mycorrhizal-mediated benefits to the host plants. In this regard, mycorrhizal friendly farm practices such as minimal use of chemical fertilizers, promotion of green pesticides along with restricted or no tillage, organic farming, and crop rotation may improve or restore the infective propagules of native AMF species. Introduction of AM inocula, especially commercial, is not always effective in all soil conditions and may lead to the alteration of native AM flora.
Microbially mediated invasion is an emerging science that aims to account for the role of microbial agents in facilitating or preventing biological invasion. Progress has been made in identifying possible scenarios occurring when microbes can transmit either from native to invasive hosts (`symbiont spillback') or from invasive to native hosts (`symbiont spillover'). For example, the presence of pre-existing mycorrhizal networks in the soil may facilitate plant invasion, and invaders are more likely to be successful if native species are infected with parasites co-introduced with their invasive hosts. However, a unifying theoretical framework to contextualize these individual scenarios and explore the consequences of microbial transmission between hosts along a continuum of beneficial to harmful host-microbial interactions is lacking. Here, we present and analyse such a framework. We discuss interesting scenarios emerging from our analysis and multiple pathways through which microbes can facilitate (or prevent) host invasion, microbial invasion, and the invasion of both hosts and their co-introduced microbes. Our framework provides a new, cohesive and intuition-enhancing perspective on microbially driven dynamics and the way in which the subtleties of the relationships between hosts and microbes affect invasion outcomes.
summaryRoot colonization by vesicular-arbuscular (VA) mycorrhizal fungi is often suppressed by phosphorus (P) additions; however, nitrogen (N) additions have been reported to both stimulate and suppress root colonization. The objective of this research was to examine colonization of Allium cepa L. roots at a range of P and N concentrations in order to clarify the effects of these nutrients on root colonization. In the first experiment, conducted under N-limiting conditions, P addition had no effect on root colonization by Glontus etunicatum Becker & Gerdemann and increased root colonization by Gigaspora margarita Gerdemann and Trappe. In the second experiment, conducted under N-sufficient conditions, P addition suppressed colonization by G. etunicatum. In the third experiment, P and N additions were such that tissue P:N ratios ranged from 0–03 (P limiting) to 017 (N limiting). No relationship was found between P:N ratio and root colonization for two isolates of G. etunicatum. Root colonization was not affected by P addition when plants were deficient in N but, when N was sufficient, P addition suppressed root colonization. These results demonstrate that plant N stress affects the resistance of the host root to colonization by VA mycorrhizal fungi.
We compared the effects of phosphorus (P) supply on the formation of vesicular-arbuscular (VA) mycorrhizas in Trifolium subterraneum L. by Gigaspora calospora (Nicol. & Gerd.) Gerd. and Glomus fasciculatum (Thaxter sensu Gerd.) Gerd & Trappe.
For both fungi, increasing the supply of P decreased the percentage of root length infected. At low P, this decrease was associated with a reduction in the growth of the fungi inside the root. At higher P, the growth of the fungi outside the root was also reduced. These detrimental effects of P on the growth of both mycorrhizal fungi were correlated with reductions in the concentrations of soluble carbohydrates in root extracts and root exudates but were not correlated with the concentrations of free amino-nitrogen in extracts or exudates.
The percentage of root length infected by G. calospora was decreased to a greater extent than that of G. fasciculatum with increasing P supply. This could partially be explained by the lower concentrations of soluble carbohydrates in the roots of plants inoculated with G. calospora. Additionally, the percentage of root length infected by G. calospora appeared to be more sensitive to reductions in the supply of soluble carbohydrates than that of G. fasciculatum.
Growth responses to inoculation were dependent on the species of mycorrhizal fungus and the P level. Enhancements and depressions in the growth of tops could be explained in terms of increased and decreased P uptake.
S ummary
Six species of vesicular‐arbuscular mycorrhizal fungi originally isolated from orchard soils were evaluated at low phosphorus (approx. 30 mg kg ⁻¹ ) and high phosphorus (approx. 180 mg kg ⁻¹ ) levels for root colonization and effects on growth of apple (Malus domestica Borkh.) seedlings. Fungi tested were Gigaspora calospora (Nicol. & Gerd.) Gerd. & Trappe, Glomus mosseae (Nicol. & Gerd.) Gerd. & Trappe, Glomus maculosum sp. ined., Glomus manihotis Howeler, Sieverding & Schenck, Glomus bitunicatum sp. ined., and two isolates of Glomus occultum Walker. Trees grew larger and had higher leaf P concentrations at high soil P than at low soil P regardless of mycorrhizas. At low P, fungal species differed in percentage root length colonized, intensity of colonization, types of mycorrhizal structures formed, and number of spores produced. Fungal species also differed in effects on plant growth. At harvest, five treatments were taller than the non‐mycorrhizal control trees, and two were not. Stem diameter and total dry weight were increased by only three treatments. At high P, no treatment resulted in greater than 5 % of root length colonized, and endogonaceous fungi had little influence on plant growth.
The mechanism responsible for inhibition of the establishment of mycorrhizal associations in Sorghum vulgare Pers. (herbaceous monocot) and Citrus aurantium L. (woody dicot) under high levels of soil phosphorus (P) was studied. Plants were grown on low fertility loamy sand (4.5 ppm P), receiving superphosphate [Ca (H2PO4)2H2O] at 0, 6, 28, 56, 228 and 556 ppm P along with all the other necessary nutrients. The percentage P content of root tissue was correlated with the amount of P added to the soil. Root exudation, measured in terms of the net leakage of soluble amino acids and reducing sugars from the roots within a 17–h period, was significantly higher under low P levels (0, 6 and 28 ppm P) than under high P levels (56, 228 and 556 ppm P). The amount of exudation was correlated with a P-induced decrease in phospholipid levels and associated changes in permeability properties of root membranes, rather than with changes in the root content of sugars and amino nitrogen. The hypothesis is proposed that phosphorus inhibition of mycorrhizal symbiosis is associated with a membrane-mediated decrease in root exudation.
summaryAlthough the overwhelming majority of non-aquatic vascular plants form vesicular-arbuscular (VA) mycorrhizal associations, the extent of colonization of the host root by any given fungal symbiont varies considerably depending on host and environmental factors. Because VA mycorrhizal fungi are obligate biotrophs, transfer of photosynthate from host to fungus may be an important factor in regulating the extent of VA mycorrhizal formation. Host metabolites must cross the plasma membrane before becoming available to the fungus. Several studies on rates of root exudation under various environmental conditions show a strong correlation between rates of root exudation and percent of root length colonized by VA mycorrhizal fungi. However, passive leakage of simple metabolites from roots as the sole means of regulating fungal colonization seems improbable for an obligate biotroph which has not yet been successfully cultured on any artificial medium. So far there has been insufficient investigation of hormone interactions between symbionts, and of the interference by the fungus in host cell wall synthesis, to evaluate the possible role of these factors in controlling growth of VA mycorrhizal fungi. Cytochemical studies of the host-fungus interface suggest modification of host plasma membrane ATPase activity as arbuscules develop, but the function of this altered activity remains unresolved. The presence of a linked P1-photosynthate exchange mechanism on the host plasma membrane analogous to the P1-photosynthate translocator known to exist in the outer membrane of chloroplasts remains an uninvestigated possible mechanism for balancing photosynthate demand by the fungus with enhanced P uptake.
At several times during plant growth, we compared the effects of two irradiances (plants were either not shaded or were shaded to reduce 60–70% of the photon flux in the glasshouse) and two concentrations of soil phosphorus (P) (35 and 105 μg−1 P g−1 soil) on the formation of vesicular-arbuscular (VA) mycorrhizas on Trifolium subterraneum L. by Gigaspora calospora (Nicol. & Gerd.) Gerd. and by Glomus fasciculatum (Thaxter sensu Gerd.) Gerd. & Trappe.
By the fifth week from sowing, the proportion of root length infected by both G. calospora and G. fasciculatum was reduced in plants grown at the greater concentration of P and in plants grown without shade. These effects were associated with decreased concentrations of soluble carbohydrates in roots at earlier harvests. Decreased concentrations of soluble carbohydrates in the roots of unshaded plants were not associated with increased concentrations of P in the plant.
The percentage of root length infected by G, calospora was decreased to a greater extent than that of G. fasciculatum with similar reductions in the concentrations of soluble carbohydrates in roots. This effect may partly explain why the percentage of root length infected by G. calospora is more sensitive than that of G. fasciculatum to increasing P supply. G. calospora and G. fasciculatum may also differ in the extent to which P may directly affect their ability to form mycorrhizas.
The effects of P supply and shading on the percentage of root length infected by G. calospora and by G. fasciculatum were not apparent before the fourth week from sowing. These factors may have affected the ability of the mycorrhizal fungi to spread by secondary infections.