Influence of soil nutrients on ectomycorrhizal communities in a chronosequence of mixed temperate forests

Unrevealing the potential of microbes in decomposition of organic matter and release of carbon in the ecosystem

Organic matter decomposition is a biochemical process with consequences affecting climate change and ecosystem productivity. Once decomposition begins, C is lost as CO2 or sequestered into more recalcitrant carbon difficult to further degradation. As microbial respiration releases carbon dioxide into the atmosphere, microbes act as gatekeepers in the whole process. Microbial activities were found to be the second largest CO2 emission source in the environment after human activities (industrialization), and research investigations suggest that this may have affected climate change over the past few decades. It is crucial to note that microbes are major contributors in the whole C cycle (decomposition, transformation, and stabilization). Therefore, imbalances in the C cycle might be causing changes in the entire carbon content of the ecosystem. The significance of microbes, especially soil bacteria in the terrestrial carbon cycle requires more attention. This review focuses on the factors that affect microorganism behavior during the breakdown of organic materials. The key factors affecting the microbial degradation processes are the quality of the input material, nitrogen, temperature, and moisture content. In this review, we suggest that to address global climate change and its effects on agricultural systems and vice versa, there is a need to double-up on efforts and conduct new research studies to further evaluate the potential of microbial communities to reduce their contribution to terrestrial carbon emission.

Communities of Mycorrhizal Fungi among Seedlings of Scots Pine (Pinus sylvestris L.) Growing on a Clearcut in Microsites Generated by Different Site-Preparation Methods

In European forests, the Scots pine (Pinus sylvestris L.) most often regenerates on clearcuts, following mechanical site preparation. Both of these silvicultural treatments (the removal of trees and preparation) have an impact on soil properties, and on the mycorrhizal fungi associated with the roots of seedlings. We therefore compared assemblages of mycorrhizal fungi associating with natural-regeneration pine seedlings growing on a clearcut, in relation to six types of microsite created using three mechanical site-preparation tools, i.e., a double-mouldboard forest plough (creating furrow and ridge), an active single-disc plough (establishing another type of furrow and ridge), and a forest mill—developing strips, as well as a non-mechanical site preparation control. A total of 46 taxa of mycorrhizal fungi were detected, with Wilcoxina mikolae being the most abundant species (relative abundance—79.8%), and the one occurring most frequently (96.8%). Other abundant mycorrhizal fungi were Thelephora terrestris (3.8%), Tylospora asterophora (3.2%), Hyaloscypha bicolor (2.2%), and Cenococcum geophilum (1.7%). The roots of seedlings growing in the non-mechanical site preparation control were characterised by a significantly greater presence of mycorrhizal root tips, compared with the roots of seedlings growing at other microsites. The highest percentage of non-mycorrhizal root tips was present on pines growing on the two types of ridge: the microsites which characterized the highest levels of mineral nutrients. Communities of mycorrhizal fungi differed between microsites. The five microsites: both types of furrow, forest plough ridge, forest mill strip, and non-mechanical site preparation control, were not found to differ from each other, but did differ from the active plough ridge treatment. The highest diversity of mycorrhizal fungi (Shannon–Wiener and Simpson indexes) was in the non-mechanical site preparation control. Any method of mechanical site preparation in the clearcut decreases the level of root mycorrhization and the biodiversity of mycorrhizal fungi. The least suitable method from the point of view of mycorrhizal fungal communities is the use of an active plough.

Forest Soil Phosphorus Resources and Fertilization Affect Ectomycorrhizal Community Composition, Beech P Uptake Efficiency, and Photosynthesis

Phosphorus (P) is an important nutrient, whose plant-available form phosphate is often low in natural forest ecosystems. Mycorrhizal fungi mine the soil for P and supply their host with this resource. It is unknown how ectomycorrhizal communities respond to changes in P availability. Here, we used young beech (Fagus sylvatica L.) trees in natural forest soil from a P-rich and P-poor site to investigate the impact of P amendment on soil microbes, mycorrhizas, beech P nutrition, and photosynthesis. We hypothesized that addition of P to forest soil increased P availability, thereby, leading to enhanced microbial biomass and mycorrhizal diversity in P-poor but not in P-rich soil. We expected that P amendment resulted in increased plant P uptake and enhanced photosynthesis in both soil types. Young beech trees with intact soil cores from a P-rich and a P-poor forest were kept in a common garden experiment and supplied once in fall with triple superphosphate. In the following summer, labile P in the organic layer, but not in the mineral top soil, was significantly increased in response to fertilizer treatment. P-rich soil contained higher microbial biomass than P-poor soil. P treatment had no effect on microbial biomass but influenced the mycorrhizal communities in P-poor soil and shifted their composition toward higher similarities to those in P-rich soil. Plant uptake efficiency was negatively correlated with the diversity of mycorrhizal communities and highest for trees in P-poor soil and lowest for fertilized trees. In both soil types, radioactive P tracing (H333PO4) revealed preferential aboveground allocation of new P in fertilized trees, resulting in increased bound P in xylem tissue and enhanced soluble P in bark, indicating increased storage and transport. Fertilized beeches from P-poor soil showed a strong increase in leaf P concentrations from deficient to luxurious conditions along with increased photosynthesis. Based on the divergent behavior of beech in P-poor and P-rich forest soil, we conclude that acclimation of beech to low P stocks involves dedicated mycorrhizal community structures, low P reserves in storage tissues and photosynthetic inhibition, while storage and aboveground allocation of additional P occurs regardless of the P nutritional status.

Khasa D. Edaphic Selection Pressures as Drivers of Contrasting White Spruce Ectomycorrhizal Fungal Community Structure and Diversity in the Canadian Boreal Forest of Abitibi-Témiscamingue Region

Little is known about edaphic selection pressures as drivers of contrasting white spruce ectomycorrhizal fungal community structure and diversity in the Canadian boreal forest. We hypothesized that community composition differs among the four sites sampled-nursery, mining site, forest edge, and natural forest. Ectomycorrhizal (ECM) fungal community structure and diversity was studied at the four locations with soil fertility gradient through morpho-molecular and phylogenetic analyses in relationships with rhizospheric soil chemical properties. 41 different species were identified. Mining site had a significantly different species composition than the surrounding environments. Soil pH and percentage of roots colonized by ECM fungi increased while soil P, N, Fe, C, K, Mg, Al, Ca, and Na contents declined across the soil fertility gradient: nursery → natural forest → forest edge → mining site. Contrary to the preference of acid soils by ECM fungi, a few ecologically adapted to high pH, poor soil chemical fertility, and low organic matter content colonize white spruce roots on the non-acidogenic mining site, allowing natural regeneration of white spruce seedlings. Other ECM fungi are adapted to high fertigation level of commercial nursery. This study clearly shows the contrasting difference in white spruce ectomycorrhizal fungal community structure and diversity driven by edaphic selection pressures.

Changes in ectomycorrhizal fungal community composition and declining diversity along a 2-million-year soil chronosequence

Ectomycorrhizal (ECM) fungal communities covary with host plant communities along soil fertility gradients, yet it is unclear whether this reflects changes in host composition, fungal edaphic specialization or priority effects during fungal community establishment. We grew two co-occurring ECM plant species (to control for host identity) in soils collected along a 2-million-year chronosequence representing a strong soil fertility gradient and used soil manipulations to disentangle the effects of edaphic properties from those due to fungal inoculum. Ectomycorrhizal fungal community composition changed and richness declined with increasing soil age; these changes were linked to pedogenesis-driven shifts in edaphic properties, particularly pH and resin-exchangeable and organic phosphorus. However, when differences in inoculum potential or soil abiotic properties among soil ages were removed while host identity was held constant, differences in ECM fungal communities and richness among chronosequence stages disappeared. Our results show that ECM fungal communities strongly vary during long-term ecosystem development, even within the same hosts. However, these changes could not be attributed to short-term fungal edaphic specialization or differences in fungal inoculum (i.e. density and composition) alone. Rather, they must reflect longer-term ecosystem-level feedback between soil, vegetation and ECM fungi during pedogenesis.

Summer temperature increase has distinct effects on the ectomycorrhizal fungal communities of moist tussock and dry tundra in Arctic Alaska

Arctic regions are experiencing the greatest rates of climate warming on the planet and marked changes have already been observed in terrestrial arctic ecosystems. While most studies have focused on the effects of warming on arctic vegetation and nutrient cycling, little is known about how belowground communities, such as fungi root-associated, respond to warming. Here, we investigate how long-term summer warming affects ectomycorrhizal (ECM) fungal communities. We used Ion Torrent sequencing of the rDNA internal transcribed spacer 2 (ITS2) region to compare ECM fungal communities in plots with and without long-term experimental warming in both dry and moist tussock tundra. Cortinarius was the most OTU-rich genus in the moist tundra, while the most diverse genus in the dry tundra was Tomentella. On the diversity level, in the moist tundra we found significant differences in community composition, and a sharp decrease in the richness of ECM fungi due to warming. On the functional level, our results indicate that warming induces shifts in the extramatrical properties of the communities, where the species with medium-distance exploration type seem to be favored with potential implications for the mobilization of different nutrient pools in the soil. In the dry tundra, neither community richness nor community composition was significantly altered by warming, similar to what had been observed in ECM host plants. There was, however, a marginally significant increase in OTUs identified as ECM fungi with the medium-distance exploration type in the warmed plots. Linking our findings of decreasing richness with previous results of increasing ECM fungal biomass suggests that certain ECM species are favored by warming and may become more abundant, while many other species may go locally extinct due to direct or indirect effects of warming. Such compositional shifts in the community might affect nutrient cycling and soil organic C storage.

Host plant genus-level diversity is the best predictor of ectomycorrhizal fungal diversity in a Chinese subtropical forest

Microbial diversity is generally far higher than plant diversity, but the relationship between microbial diversity and plant diversity remains enigmatic. To shed light on this problem, we examined the diversity of a key guild of root-associated microbes,that is, ectomycorrhizal (EM) fungi along a plant diversity gradient in a Chinese subtropical forest. The results indicated that EM fungal diversity was positively correlated with host plant diversity. Furthermore, this relationship was best predicted by host genus-level diversity, rather than species-level diversity or family-level diversity. The generality of this finding was extended beyond our study system through the analyses of 100 additional studies of EM fungal communities from tropical and temperate forests.Here as well, EM fungal lineage composition was significantly affected by EM plant diversity levels, and some EM fungal lineages were co-associated with some host plant genera. These results suggest a general diversity maintenance mechanism for host-specific microbes based on higher order host plant phylogenetic diversity.

Structure and phylogenetic diversity of post-fire ectomycorrhizal communities of maritime pine

Environmental disturbances define the diversity and assemblage of species, affecting the functioning of ecosystems. Fire is a major disturbance of Mediterranean pine forests. Pines are highly dependent on the ectomycorrhizal (EM) fungal symbiosis, which is critical for tree recruitment under primary succession. To determine the effects of time since fire on the structure and recovery of EM fungal communities, we surveyed the young Pinus pinaster regenerate in three sites differing in the elapsed time after the last fire event. Pine roots were collected, and EM fungi characterized by sequencing the internal transcribed spacer (ITS) and the large subunit (LSU) regions of the nuclear ribosomal (nr)-DNA. The effects of the elapsed time after fire on the EM community structure (richness, presence/absence of fungi, phylogenetic diversity) and on soil properties were analysed.Fungal richness decreased with the elapsed time since the fire; although, the phylogenetic diversity of the EM community increased. Soil properties were different depending on the elapsed time after fire and particularly, the organic matter, carbon-to-nitrogen (C/N) ratio, nitrogen and iron significantly correlated with the assemblage of fungal species. Ascomycetes, particularly Tuberaceae and Pezizales, were significantly over-represented on saplings in the burned site. On seedlings, a significant over-representation of Rhizopogonaceae and Atheliaceae was observed in the most recently burned site, while other fungi (i.e. Cortinariaceae) were significantly under-represented. Our results are consistent with the hypothesis that fire can act as a selective agent by printing a phylogenetic signal on the EM fungal communities associated with naturally regenerated pines, pointing out to some groups as potential fire-adapted fungi.

Substantial compositional turnover of fungal communities in an alpine ridge-to-snowbed gradient

The main gradient in vascular plant, bryophyte and lichen species composition in alpine areas, structured by the topographic gradient from wind-exposed ridges to snowbeds, has been extensively studied. Tolerance to environmental stress, resulting from wind abrasion and desiccation towards windswept ridges or reduced growing season due to prolonged snow cover towards snowbeds, is an important ecological mechanism in this gradient. The extent to which belowground fungal communities are structured by the same topographic gradient and the eventual mechanisms involved are less well known. In this study, we analysed variation in fungal diversity and community composition associated with roots of the ectomycorrhizal plant Bistorta vivipara along the ridge-to-snowbed gradient. We collected root samples from fifty B. vivipara plants in ten plots in an alpine area in central Norway. The fungal communities were analysed using 454 pyrosequencing analyses of tag-encoded ITS1 amplicons. A distinct gradient in the fungal community composition was found that coincided with variation from ridge to snowbeds. This gradient was paralleled by change in soil content of carbon, nitrogen and phosphorus. A large proportion (66%) of the detected 801 nonsingleton operational taxonomic units (OTUs) were ascomycetes, while basidiomycetes dominated quantitatively (i.e. with respect to number of reads). Numerous fungal OTUs, many with taxonomic affinity to Sebacinales, Cortinarius and Meliniomyces, showed distinct affinities either to ridge or to snowbed plots, indicating habitat specialization. The compositional turnover of fungal communities along the gradient was not paralleled by a gradient in species richness.

Ectomycorrhizal fungal succession coincides with shifts in organic nitrogen availability and canopy closure in post-wildfire jack pine forests

Successional changes in belowground ectomycorrhizal fungal (EMF) communities have been observed with increasing forest stand age; however, mechanisms behind this change remain unclear. It has been hypothesized that declines of inorganic nitrogen (N) and increases of organic N influence changes in EMF taxa over forest development. In a post-wildfire chronosequence of six jack pine (Pinus banksiana) stands ranging in age from 5 to 56 years, we investigated EMF community composition and compared shifts in taxa with detailed soluble inorganic and organic N data. Taxa were identified by internal transcribed spacer rDNA sequencing, and changes in community composition evaluated with non-metric multi-dimensional scaling (NMDS). Dissimilarities in the community data were tested for correlations with N variables. We observed a successional shift along NMDS axis 1 from such taxa as Suillus brevipes and Thelephora terrestris in sites age 5 and 11 to species of Cortinarius and Russula, among others, in the four older sites. This change was positively correlated with soluble organic N (SON) (r(2) = 0.902, P = 0.033) and free amino-acid N (r(2) = 0.945, P = 0.021), but not inorganic N. Overall, our results show a successional shift of EMF communities occurring between stand initiation and canopy closure without a change in species of the dominant plant-host, and associated with SON and free amino-acid N in soil. It is uncertain whether EMF taxa are responding to these organic N forms directly, affecting their availability, or are ultimately responding to changes in other site variables, such as belowground productivity.

Mycorrhizal networks affect ectomycorrhizal fungal community similarity between conspecific trees and seedlings

Ectomycorrhizal (EM) networks (MN) are thought to be an important mode of EM fungal colonization of coniferous seedlings. How MNs affect EM communities on seedlings, and how this varies with biotic and abiotic factors, is integral to understanding their importance in seedling establishment. We examined EM fungal community similarity between mature trees and conspecific interior Douglas-fir (Pseudotsuga menziesii var. glauca) seedlings in two experiments where seed and nursery-grown seedlings originating from different locations were planted at various distances from trees along a climatic gradient. At harvest, trees shared 60% of their fungal taxa in common with outplanted seedlings and 77% with germinants, indicating potential for seedlings to join the network of residual trees. In both experiments, community similarity between trees and seedlings increased with drought. However, community similarity was lower among nursery seedlings growing at 2.5 m from trees when they were able to form an MN, suggesting MNs reduced seedling EM fungal richness. For field germinants, MNs resulted in lower community similarity in the driest climates. Distance from trees affected community similarity of nursery seedlings to trees, but there was no interaction of provenance with MNs in their effect on similarity in either nursery seedlings or field germinants as hypothesized. We conclude that MNs of trees influence EM colonization patterns of seedlings, and the strength of these effects increases with climatic drought.

Effects of growth medium, nutrients, water, and aeration on mycorrhization and biomass allocation of greenhouse-grown interior Douglas-fir seedlings

Commercial nursery practices usually fail to promote mycorrhization of interior Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco var. glauca (Beissn.) Franco] seedlings in British Columbia, which may account for their poor performance following planting in the field. We tested the effects of four nursery cultivation factors (nitrogen fertilization, phosphorus fertilization, watering, and soil aeration) and field soil addition on mycorrhization, survival, growth, and biomass allocation of interior Douglas-fir seedlings in a series of greenhouse experiments. Where field soil was added to the growing medium, mycorrhization and root/shoot ratios were maximized at lower levels of mineral nutrient application and aeration. Where field soil was not added, mycorrhization was negligible across all fertilization and aeration treatments, but root/shoot ratio was maximized at lower levels of mineral nutrients and the highest level of aeration. Regardless of whether field soil was added, intermediate levels of soil water resulted in the best mycorrhizal colonization and root/shoot ratios. However, field soil addition reduced seedling mortality at the two lowest water levels. A cluster analysis placed ectomycorrhizal morphotypes into three groups (Mycelium radicis-atrovirens Melin, Wilcoxina, and mixed) based on their treatment response, with all but two morphotypes in the mixed group whose abundance was maximized under conditions common to advanced seedling establishment. For maximal mycorrhization and root development of interior Douglas-fir seedlings, nurseries should minimize addition of nitrogen and phosphorus nutrients, maximize aeration, provide water at moderate rates, and, where possible, add small amounts of field soil to the growing medium.