Inoculation with Ericoid Mycorrhizal Associations Alleviates Drought Stress in Lowland and Upland Velvetleaf Blueberry (Vaccinium myrtilloides) Seedlings

Although velvetleaf blueberry (Vaccinium myrtilloides) is usually associated with sandy (upland) areas of the North American boreal forest, lowland populations can be also found in bogs, suggesting possible adaptations to different site conditions. In this study, we examined the role of ericoid mycorrhizal (ERM) fungi in conferring drought resistance to the upland and lowland velvetleaf blueberry seedlings. The seedlings were inoculated with four ERM fungi (Pezicula ericae, Pezoloma ericae, Meliniomyces variabilis, and Oidiodendron maius) isolated from the roots of ericaceous plants and grown under controlled environmental conditions in sterilized soil. The inoculated and non-inoculated (inoculation control) plants were subsequently subjected to three cycles of drought stress by withdrawing watering. Lowland plants appeared to benefit relatively more from mycorrhizal colonization, compared with the upland plants, in terms of plant growth and drought survival. After three weeks of treatments, the dry weights of non-inoculated well-watered upland plants were higher compared to the non-inoculated lowland plants. However, these differences were offset by the inoculation of plants with ERM fungi, some of which also significantly improved drought resistance characteristics of the upland and lowland plants. There were no major differences in the effects of different ERM fungal species on drought responses of upland and lowland plants. Of the examined ericoid mycorrhizal fungi, inoculation with Pezicula ericae was the most effective in conferring drought resistance characteristics to both upland and lowland seedlings and helped maintain higher shoot water potentials, net photosynthetic, and transpiration rates in plants subjected to drought stress.

Mycorrhizal Fungal Partners Remain Constant during a Root Lifecycle of Pleione bulbocodioides (Orchidaceae)

Mycorrhizal mutualisms are vital for orchids through germination to adulthood. Fungal species diversity and community composition vary across seasons and plant development stages and affect plant survival, adaptation, and community maintenance. Knowledge of the temporal turnover of mycorrhizal fungi (OMF) remains poorly understood in the eco-physiologically diverse orchids (especially in epiphytic orchids), although it is important to understand the function and adaptation of mycorrhizae. Some species of Pleione are epiphytic plants with annual roots and may recruit different fungal partners during their root lifecycle. Based on continuous samplings of Pleione bulbocodioides during a whole root lifecycle, we characterized the fungal temporal dynamics using Illumina sequencing of the ITS2 region. Our data showed that the plants of P. bulbocodioides were quickly colonized by OMF at root emergence and had a constant OMF composition throughout one root lifecycle, although the OMF richness declined with root aging after a peak occurrence during root elongation. In contrast, the richness of root-inhabiting fungal endophytes kept increasing with root aging and more drastic turnovers were found in their species compositions. Our findings of OMF temporal turnover contribute to further understanding of mycorrhizal associations and adaptation of Orchidaceae and will benefit orchid resource conservation and utilization.

Reciprocal interactions between aphids and arbuscular mycorrhizal fungi across plant generations

The conditions experienced by one plant generation can influence the growth of the offspring generation. These maternal effects can reduce performance of foliar-feeding insects, through accumulation of plant defences. Arbuscular mycorrhizal fungi (AMF) inhabit the roots of plants and are known to influence the performance of foliar-feeding insects. However, all published studies of the interactions between insects and AMF have taken place within one plant generation, but none across generations. Thus, in the present study, Senecio vulgaris plants were grown with or without aphids and AMF (termed ‘induction events’), and seeds from each treatment were used to grow plants experiencing that same treatment over four successive generations, all grown in identical environmental conditions. Naïve aphids were reared on Senecio plants whose parents had experienced 0, 1, 2 or 3 induction events. We found strong negative maternal effects of herbivory on aphid growth, which were not mitigated by the mycorrhiza. However, teneral weight and growth rate showed a gradual recovery; aphids reared on plants whose previous three generations suffered attack were similar in size to those at the beginning of the study. Herbivory had positive or negative effects on the mycorrhiza, dependent upon the number of previous generations suffering attack or having mycorrhizal associations. We conclude that the outcome of many insect plant fungal experiments is likely to have been influenced by and need to account for maternal effects of the parental plant’s growth conditions.

Enriched CO2 and Root-Associated Fungi (Mycorrhizae) Yield Inverse Effects on Plant Mass and Root Morphology in Six Asclepias Species

While milkweeds (Asclepias spp.) are important for sustaining biodiversity in marginal ecosystems, CO2 flux may afflict Asclepias species and cause detriment to native communities. Negative CO2-induced effects may be mitigated through mycorrhizal associations. In this study, we sought to determine how mycorrhizae interacts with CO2 to influence Asclepias biomass and root morphology. A broad range of Asclepias species (n = 6) were chosen for this study, including four tap-root species (A. sullivantii, A. syriaca, A. tuberosa, and A. viridis) and two fibrous root species (A. incarnata and A. verticillata). Collectively, the six Asclepias species were manipulated under a 2 × 2 full-factorial design that featured two mycorrhizal levels (−/+ mycorrhizae) and two CO2 levels (ambient and enriched (i.e., 3.5× ambient)). After a duration of 10 months, Asclepias responses were assessed as whole dry weight (i.e., biomass) and relative transportive root. Relative transportive root is the percent difference in the diameter of highest order root (transportive root) versus that of first-order absorptive roots. Results revealed an asymmetrical response, as mycorrhizae increased Asclepias biomass by ~12-fold, while enriched CO2 decreased biomass by about 25%. CO2 did not impact relative transportive roots, but mycorrhizae increased root organ’s response by more than 20%. Interactions with CO2 and mycorrhizae were observed for both biomass and root morphology (i.e., relative transportive root). A gene associated with CO2 fixation (rbcL) revealed that the two fibrous root species formed a phylogenetic clade that was distant from the four tap-root species. The effect of mycorrhizae was most profound in tap-root systems, as mycorrhizae modified the highest order root into tuber-like structures. A strong positive correlation was observed with biomass and relative transportive root. This study elucidates the interplay with roots, mycorrhizae, and CO2, while providing a potential pathway for mycorrhizae to ameliorate CO2 induced effects.

Implementation of mycorrhizal mechanisms into soil carbon model improves the prediction of long-term processes of plant litter decomposition

Ecosystems dominated by plants featuring ectomycorrhizae (EM) and arbuscular mycorrhizae (AM) promote distinct soil carbon dynamics. AM and EM soil environments can thus have different impacts on litter decomposition. However, current soil carbon models treat mycorrhizal impacts on the processes of soil carbon transformation as a black box. We re-formulated the soil carbon model Yasso15, and incorporated impacts of mycorrhizal vegetation on soil carbon pools of different recalcitrance. We examined alternative conceptualizations of mycorrhizal impacts on transformations of labile and stable carbon, and quantitatively assessed the performance of the selected optimal model in terms of the long-term fate of plant litter. We found that mycorrhizal impacts on pools of labile carbon in the litter are distinct from that on recalcitrant pools. Plant litter of the same chemical composition decomposes slower when exposed to EM-dominated ecosystems compared to AM-dominated ones, and across time, EM-dominated ecosystems accumulate more recalcitrant residues of non-decomposed litter. Overall, adding our mycorrhizal module into the Yasso model improved the accuracy of the temporal dynamics of carbon sequestration predictions. Our results suggest that mycorrhizal impacts on litter decomposition are underpinned by distinct decomposition pathways in AM- and EM-dominated ecosystems. Ignoring mycorrhiza-induced mechanisms will thus lead to an overestimation of climate impacts on decomposition dynamics. Our new model provides a benchmark for mechanistic and quantitative modelling of microbial impact on soil carbon. It helps to determine the relative importance of mycorrhizal associations and climate on organic matter decomposition rate and reduces the uncertainties in estimating soil carbon sequestration.

Differences in C, N, δ13C and δ15N among plant functional types after a wildfire in a black spruce forest, interior Alaska

We measured differences in %C, %N,  13C and  15N of plant functional types 17 (PFTs) between burned and unburned ground surfaces soon after a wildfire on a north-18 facing slope in interior Alaska. The C and N were measured for 16 species and 19 Sphagnum litter.  13C differed among the PFTs and was low for trees and shrubs, 20 suggesting that woody stems slowed C dynamics or showed low water use efficiency. 21  15N concentrations suggested that the herbaceous plants depended less on the 22 mycorrhizal associations that became weak on the burned surfaces. The shrub leaves 23 showed the lowest  15N of PFTs and showed higher  15N on the burned surface, showing 24 that N transfer from the soils to the leaves in the shrubs was slowed by the wildfire. 25 Mosses showed the highest C/N ratio. Sphagnum litter decomposed faster on the burned 26 surface, and %N and  15N in the litter increased from the second to third year on both 27 burned and unburned surfaces, while %C changed little. In conclusion, the responses to 28 the wildfire differed among the PFTs as characterized by their C and N dynamics. 29 30 Key words: Burned and unburned ground surface, carbon (C) and nitrogen (N), Alaskan 31 taiga, plant functional type, stable isotope

Phosphorus uptake and toxicity is delimited by mycorrhizal symbiosis in P-sensitive Eucalyptus marginata but not in P-tolerant Acacia celastrifolia

Many plant species from regions with ancient, highly-weathered nutrient-depleted soils have specialised adaptations for acquiring P and are sensitive to excess P-supply. Mycorrhizal associations may regulate P-uptake at high external P-concentrations, potentially reducing P-toxicity. We predicted that excess P-application will negatively impact species from the nutrient-depleted jarrah forest of Western Australia and that mycorrhizal inoculation will reduce P-toxicity by regulating P-uptake. For seedlings of the N2-fixing legume Acacia celastrifolia and the tree species Eucalyptus marginata, we measured growth at P-concentrations of 0 to 90 mg kg-1 soil and in relation to inoculation with the arbuscular mycorrhizal fungus (AMF) Rhizophagus irregularis. Non-inoculated A. celastrifolia maintained leaf P-concentrations at <2 mg g-1 dry mass (DM) across the range of external P-concentrations. However, for non-inoculated E. marginata, as external P-concentrations increased leaf P also increased, reaching >9 mg g-1 DM at 30 mg P kg-1 soil. A. celastrifolia DM increased with increasing external P-concentrations, while E. marginata DM was maximal at 15 mg P kg-1 soil, declining at higher external P concentrations. Neither DM nor leaf P of A. celastrifolia were affected by inoculation with AMF. For E. marginata, even at 90 mg P kg-1 soil, inoculation with AMF resulted in leaf P remaining <1 mg g-1 DM, and DM being maintained. These data strengthen the evidence base that AMF may not only facilitate P-uptake at low external P-concentrations, but are also important for moderating P-uptake at elevated external P-concentrations and maintaining plant P concentrations within a relatively narrow concentration range.

Dominant Tree Mycorrhizal Associations Affect Soil Nitrogen Transformation Rates Through Mediating Microbial Abundances in a Temperate Forest

Tree-fungal symbioses are increasingly recognized to affect soil nitrogen (N) transformations, yet the role of soil microbes in the process is largely unclear. Soil microbes directly interact with trees and are a primary driver of many N transformation processes. Here, we explored the linkage among tree mycorrhizal associations, soil microbes and N transformation rates in a temperate forest of Northeast China. Across a gradient of increasing ectomycorrhizal (ECM) tree dominance, we measured soil acid-base chemistry, bacterial and fungal abundances, N-hydrolyzing enzyme activities, abundances and community composition of ammonia-oxidizing archaea (AOA) and bacteria, and net N mineralization and net nitrification rates. Results showed that soil pH, exchangeable base cations, inorganic N concentrations and N transformation rates decreased with increasing ECM tree dominance. The ECM tree dominance was negatively related to soil bacterial and AOA amoA gene abundances, and positively to soil fungal abundances and β-N-acetylglucosaminidase activities. These shifts in soil microbial abundances and enzyme activities along the mycorrhizal gradient were linked with the increase in soil acidity with increasing ECM tree dominance. Structural equation models revealed that ECM tree dominance was not directly related to N transformation rates, but indirectly to net N mineralization rates via affecting bacterial and fungal abundances, and indirectly to net nitrification rates via influencing AOA amoA gene abundances. Collectively, our results indicate that soil microbes provide a mechanistic link between mycorrhizal associations and soil N transformations, and suggest that shifts in forest mycorrhizal associations under global change could have profound consequences for biogeochemical cycling of temperate forests.