Links between boreal forest management, soil fungal communities and below‐ground carbon sequestration

Prospective Roles of Extremophilic Fungi in Climate Change Mitigation Strategies

Climate change and the resultant environmental deterioration signify one of the most challenging problems facing humankind in the 21st century. The origins of climate change are multifaceted and rooted in anthropogenic activities, resulting in increasing greenhouse gases in the environment and leading to global warming and weather drifts. Extremophilic fungi, characterized by their exceptional properties to survive extreme habitats, harbor great potential in mitigating climate change effects. This review provides insight into the potential applications of extremophilic fungi in climate change mitigation strategies. They are able to metabolize organic biomass and degrade carbon compounds, thereby safely sequestering carbon and extenuating its release into the environment as noxious greenhouse gases. Furthermore, they possess extremozymes, which break down recalcitrant organic species, including lignocellulosic biomass and hydrocarbons. Enzymatic machinery equips these extremophilic fungi to perform the bioremediation of polluted environments. Extremophilic fungi can also be exploited for various biological interventions, such as biofuels, bioplastics, and other bioprocessing applications. However, these fungi characterize a valued but underexplored resource in the arsenal of climate change mitigation strategies.

The biological controls of soil carbon accumulation following wildfire and harvest in boreal forests: A review

Boreal forests are frequently subjected to disturbances, including wildfire and clear-cutting. While these disturbances can cause soil carbon (C) losses, the long-term accumulation dynamics of soil C stocks during subsequent stand development is controlled by biological processes related to the balance of net primary production (NPP) and outputs via heterotrophic respiration and leaching, many of which remain poorly understood. We review the biological processes suggested to influence soil C accumulation in boreal forests. Our review indicates that median C accumulation rates following wildfire and clear-cutting are similar (0.15 and 0.20 Mg ha-1 year-1, respectively), however, variation between studies is extremely high. Further, while many individual studies show linear increases in soil C stocks through time after disturbance, there are indications that C stock recovery is fastest early to mid-succession (e.g. 15-80 years) and then slows as forests mature (e.g. >100 years). We indicate that the rapid build-up of soil C in younger stands appears not only driven by higher plant production, but also by a high rate of mycorrhizal hyphal production, and mycorrhizal suppression of saprotrophs. As stands mature, the balance between reductions in plant and mycorrhizal production, increasing plant litter recalcitrance, and ectomycorrhizal decomposers and saprotrophs have been highlighted as key controls on soil C accumulation rates. While some of these controls appear well understood (e.g. temporal patterns in NPP, changes in aboveground litter quality), many others remain research frontiers. Notably, very little data exists describing and comparing successional patterns of root production, mycorrhizal functional traits, mycorrhizal-saprotroph interactions, or C outputs via heterotrophic respiration and dissolved organic C following different disturbances. We argue that these less frequently described controls require attention, as they will be key not only for understanding ecosystem C balances, but also for representing these dynamics more accurately in soil organic C and Earth system models.

Ectomycorrhizal fungi are more sensitive to high soil nitrogen levels in forests exposed to nitrogen deposition

Ectomycorrhizal fungi are essential for nitrogen (N) cycling in many temperate forests and responsive to anthropogenic N addition, which generally decreases host carbon (C) allocation to the fungi. In the boreal region, however, ectomycorrhizal fungal biomass has been found to correlate positively with soil N availability. Still, responses to anthropogenic N input, for instance through atmospheric deposition, are commonly negative. To elucidate whether variation in N supply affects ectomycorrhizal fungi differently depending on geographical context, we investigated ectomycorrhizal fungal communities along fertility gradients located in two nemo-boreal forest regions with similar ranges in soil N : C ratios and inorganic N availability but contrasting rates of N deposition. Ectomycorrhizal biomass and community composition remained relatively stable across the N gradient with low atmospheric N deposition, but biomass decreased and the community changed more drastically with increasing N availability in the gradient subjected to higher rates of N deposition. Moreover, potential activities of enzymes involved in ectomycorrhizal mobilisation of organic N decreased as N availability increased. In forests with low external input, we propose that stabilising feedbacks in tree-fungal interactions maintain ectomycorrhizal fungal biomass and communities even in N-rich soils. By contrast, anthropogenic N input seems to impair ectomycorrhizal functions.

Metatranscriptomics sheds light on the links between the functional traits of fungal guilds and ecological processes in forest soil ecosystems

Soil fungi belonging to different functional guilds, such as saprotrophs, pathogens, and mycorrhizal symbionts, play key roles in forest ecosystems. To date, no study has compared the actual gene expression of these guilds in different forest soils. We used metatranscriptomics to study the competition for organic resources by these fungal groups in boreal, temperate, and Mediterranean forest soils. Using a dedicated mRNA annotation pipeline combined with the JGI MycoCosm database, we compared the transcripts of these three fungal guilds, targeting enzymes involved in C- and N mobilization from plant and microbial cell walls. Genes encoding enzymes involved in the degradation of plant cell walls were expressed at a higher level in saprotrophic fungi than in ectomycorrhizal and pathogenic fungi. However, ectomycorrhizal and saprotrophic fungi showed similarly high expression levels of genes encoding enzymes involved in fungal cell wall degradation. Transcripts for N-related transporters were more highly expressed in ectomycorrhizal fungi than in other groups. We showed that ectomycorrhizal and saprotrophic fungi compete for N in soil organic matter, suggesting that their interactions could decelerate C cycling. Metatranscriptomics provides a unique tool to test controversial ecological hypotheses and to better understand the underlying ecological processes involved in soil functioning and carbon stabilization.

Eastern Canadian boreal forest soil and foliar chemistry show evidence of resilience to long-term nitrogen addition

The boreal forest is one of the world's largest terrestrial biome and plays crucial roles in global biogeochemical cycles, such as carbon (C) sequestration in vegetation and soil. However, the impacts of decades of N deposition on N-limited ecosystems, like the eastern Canadian boreal forest, remain unclear. For 13 years, N deposition was simulated by periodically adding ammonium nitrate on soils of two boreal coniferous forests (i.e., balsam fir and black spruce) of eastern Canada, at low (LN) and high (HN) rates, corresponding to 3 and 10 times the ambient N deposition, respectively. We show that more than a decade of N addition had no strong effects on mineral soil C, N, P, and cation concentrations and on foliar total Ca, K, Mg, and Mn concentrations. In organic soil, C stock was not affected by N addition while N stock increased, and exchangeable Ca2+ and Mg2+ decreased at the balsam fir site under HN treatment. At both sites, LN treatment had nearly no impact on foliage and soil chemistry but foliar N and N:P significantly increased under HN treatment, potentially leading to foliar nutrient imbalance. Overall, our work indicates that, in the eastern Canadian boreal forest, soil and foliar nutrient concentrations and stocks are resilient to increasing N deposition potentially because, in the context of N limitation, extra N would be rapidly immobilized by soil micro-organisms and vegetation. These findings could improve modeling future boreal forest soil C stocks and biomass growth and could help in planning forest management strategies in eastern Canada.

Shifts in microbial community composition and metabolism correspond with rapid soil carbon accumulation in response to 20 years of simulated nitrogen deposition

Anthropogenic nitrogen (N) deposition and fertilization in boreal forests frequently reduces decomposition and soil respiration and enhances C storage in the topsoil. This enhancement of the C sink can be as strong as the aboveground biomass response to N additions and has implications for the global C cycle, but the mechanisms remain elusive. We hypothesized that this effect would be associated with a shift in the microbial community and its activity, and particularly by fungal taxa reported to be capable of lignin degradation and organic N acquisition. We sampled the organic layer below the intact litter of a Norway spruce (Picea abies (L.) Karst) forest in northern Sweden after 20 years of annual N additions at low (12.5 kg N ha-1 yr-1) and high (50 kg N ha-1 yr-1) rates. We measured microbial biomass using phospholipid fatty-acid analysis (PLFA) and ergosterol measurements and used ITS metagenomics to profile the fungal community of soil and fine-roots. We probed the metabolic activity of the soil community by measuring the activity of extracellular enzymes and evaluated its relationships with the most N responsive soil fungal species. Nitrogen addition decreased the abundance of fungal PLFA markers and changed the fungal community in humus and fine-roots. Specifically, the humus community changed in part due to a shift from Oidiodendron pilicola, Cenococcum geophilum, and Cortinarius caperatus to Tylospora fibrillosa and Russula griseascens. These microbial community changes were associated with decreased activity of Mn-peroxidase and peptidase, and an increase in the activity of C acquiring enzymes. Our results show that the rapid accumulation of C in the humus layer frequently observed in areas with high N deposition is consistent with a shift in microbial metabolism, where decomposition associated with organic N acquisition is downregulated when inorganic N forms are readily available.

Fungal community composition predicts forest carbon storage at a continental scale

Forest soils harbor hyper-diverse microbial communities which fundamentally regulate carbon and nutrient cycling across the globe. Directly testing hypotheses on how microbiome diversity is linked to forest carbon storage has been difficult, due to a lack of paired data on microbiome diversity and in situ observations of forest carbon accumulation and storage. Here, we investigated the relationship between soil microbiomes and forest carbon across 238 forest inventory plots spanning 15 European countries. We show that the composition and diversity of fungal, but not bacterial, species is tightly coupled to both forest biotic conditions and a seven-fold variation in tree growth rates and biomass carbon stocks when controlling for the effects of dominant tree type, climate, and other environmental factors. This linkage is particularly strong for symbiotic endophytic and ectomycorrhizal fungi known to directly facilitate tree growth. Since tree growth rates in this system are closely and positively correlated with belowground soil carbon stocks, we conclude that fungal composition is a strong predictor of overall forest carbon storage across the European continent.

Effects of phase change material inclusion on reducing greenhouse gas emissions from soil in cold region

Increased gas emissions from soil into the atmosphere are one form of ecosystem feedback in response to climate change. Soil temperature plays a critical role in the soil emission of carbon dioxide (CO2) and nitrous oxide (N2O) suggesting that the release of gases can be reduced by regulating soil temperature. This study proposes a green microencapsulated phase-change material (mPCM) as a soil temperature regulator due to its ability to absorb and release heat during temperature phase transition. The objective is to test how mPCM in soil mixtures influences CO2 and N2O fluxes under laboratory-controlled conditions. For this purpose, a series of soil incubations were carried out with different temperature regimes and soil moisture. The test results revealed that at 20% soil moisture mPCM reduced cumulative CO2 emissions from the soil by 16.4% during the thawing stage and by 20.5% during the freezing stage. At 25% soil moisture, mPCM showed a greater effect reducing cumulative CO2 emissions by 23.9% during the thawing stage and by 24.2% during the freezing stage. At below-zero temperatures, mPCM reduced the total N2O flux by 11.6% at 20% soil moisture and by 26.0% at 25% soil moisture, compared to soil without mPCM. As soil moisture increased, the effects of mPCM on CO2 and N2O fluxes became more pronounced. Cyclic freezing and thawing of soil led to an increase in gas flux. This variation was reduced by the mPCM due to its ability to mitigate the change of soil temperature. Inhibition of the rise in soil temperature due to the inclusion of mPCM reduced the rate of activation of soil mineralization, which reduced gas fluxes. This study demonstrates the potential of mPCM application to reduce greenhouse gas emissions from soil through thermoregulation.

Abiotic and Biotic Drivers of Soil Fungal Communities in Response to Dairy Manure Amendment

Modern agriculture often relies on large inputs of synthetic fertilizers to maximize crop yield potential, yet their intensive use has led to nutrient losses and impaired soil health. Alternatively, manure amendments provide plant available nutrients, build organic carbon, and enhance soil health. However, we lack a clear understanding of how consistently manure impacts fungal communities, the mechanisms via which manure impacts soil fungi, and the fate of manure-borne fungi in soils. We assembled soil microcosms using five soils to investigate how manure amendments impact fungal communities over a 60-day incubation. Further, we used autoclaving treatments of soils and manure to determine if observed changes in soil fungal communities were due to abiotic or biotic properties, and if indigenous soil communities constrained colonization of manure-borne fungi. We found that manure amended soil fungal communities diverged from nonamended communities over time, often in concert with a reduction in diversity. Fungal communities responded to live and autoclaved manure in a similar manner, suggesting that abiotic forces are primarily responsible for the observed dynamics. Finally, manure-borne fungi declined quickly in both live and autoclaved soil, indicating that the soil environment is unsuitable for their survival. IMPORTANCE Manure amendments in agricultural systems can impact soil microbial communities via supplying growth substrates for indigenous microbes or by introducing manure-borne taxa. This study explores the consistency of these impacts on soil fungal communities and the relative importance of abiotic and biotic drivers across distinct soils. Different fungal taxa responded to manure among distinct soils, and shifts in soil fungal communities were driven largely by abiotic factors, rather than introduced microbes. This work demonstrates that manure may have inconsistent impacts on indigenous soil fungi, and that abiotic properties of soils render them largely resistant to invasion by manure-borne fungi.

Forest microbiome and global change

Forests influence climate and mitigate global change through the storage of carbon in soils. In turn, these complex ecosystems face important challenges, including increases in carbon dioxide, warming, drought and fire, pest outbreaks and nitrogen deposition. The response of forests to these changes is largely mediated by microorganisms, especially fungi and bacteria. The effects of global change differ among boreal, temperate and tropical forests. The future of forests depends mostly on the performance and balance of fungal symbiotic guilds, saprotrophic fungi and bacteria, and fungal plant pathogens. Drought severely weakens forest resilience, as it triggers adverse processes such as pathogen outbreaks and fires that impact the microbial and forest performance for carbon storage and nutrient turnover. Nitrogen deposition also substantially affects forest microbial processes, with a pronounced effect in the temperate zone. Considering plant-microorganism interactions would help predict the future of forests and identify management strategies to increase ecosystem stability and alleviate climate change effects. In this Review, we describe the impact of global change on the forest ecosystem and its microbiome across different climatic zones. We propose potential approaches to control the adverse effects of global change on forest stability, and present future research directions to understand the changes ahead.

Consolidation of Chloridium: new classification into eight sections with 37 species and reinstatement of the genera Gongromeriza and Psilobotrys

Chloridium is a little-studied group of soil- and wood-inhabiting dematiaceous hyphomycetes that share a rare mode of phialidic conidiogenesis on multiple loci. The genus has historically been divided into three morphological sections, i.e. Chloridium, Gongromeriza, and Psilobotrys. Sexual morphs have been placed in the widely perceived genus Chaetosphaeria, but unlike their asexual counterparts, they show little or no morphological variation. Recent molecular studies have expanded the generic concept to include species defined by a new set of morphological characters, such as the collar-like hyphae, setae, discrete phialides, and penicillately branched conidiophores. The study is based on the consilience of molecular species delimitation methods, phylogenetic analyses, ancestral state reconstruction, morphological hypotheses, and global biogeographic analyses. The multilocus phylogeny demonstrated that the classic concept of Chloridium is polyphyletic, and the original sections are not congeneric. Therefore, we abolish the existing classification and propose to restore the generic status of Gongromeriza and Psilobotrys. We present a new generic concept and define Chloridium as a monophyletic, polythetic genus comprising 37 species distributed in eight sections. In addition, of the taxa earlier referred to Gongromeriza, two have been redisposed to the new genus Gongromerizella. Analysis of published metabarcoding data showed that Chloridium is a common soil fungus representing a significant (0.3 %) proportion of sequence reads in environmental samples deposited in the GlobalFungi database. The analysis also showed that they are typically associated with forest habitats, and their distribution is strongly influenced by climate, which is confirmed by our data on their ability to grow at different temperatures. We demonstrated that Chloridium forms species-specific ranges of distribution, which is rarely documented for microscopic soil fungi. Our study shows the feasibility of using the GlobalFungi database to study the biogeography and ecology of fungi. Taxonomic novelties: New genus: Gongromerizella Réblová; New sections: Chloridium section Cryptogonytrichum Réblová, Hern.-Restr., M. Kolařík & F. Sklenar, Chloridium section Gonytrichopsis Réblová, Hern.-Restr., M. Kolařík & F. Sklenar, Chloridium section Metachloridium Réblová, Hern.-Restr., M. Kolařík & F. Sklenar, Chloridium section Volubilia Réblová, Hern.-Restr., M. Kolařík & F. Sklenar; New species: Chloridium bellum Réblová & Hern.-Restr., Chloridium biforme Réblová & Hern.-Restr., Chloridium detriticola Réblová & Hern.-Restr., Chloridium gamsii Réblová & Hern.-Restr., Chloridium guttiferum Réblová & Hern.-Restr., Chloridium moratum Réblová & Hern.-Restr., Chloridium peruense Réblová & Hern.-Restr., Chloridium novae-zelandiae Réblová & Hern.-Restr., Chloridium elongatum Réblová & Hern.-Restr., Chloridium volubile Réblová & Hern.-Restr.; New varieties: Chloridium bellum var. luteum Réblová & Hern.-Restr., Chloridium detriticola var. effusum Réblová & Hern.-Restr., Chloridium chloridioides var. convolutum Réblová & Hern.-Restr.; New combinations: Chloridium section Gonytrichum (Nees & T. Nees) Réblová, Hern.-Restr., M. Kolařík & F. Sklenar, Chloridium section Mesobotrys (Sacc.) Réblová, Hern.-Restr., M. Kolařík & F. Sklenar, Chloridium section Pseudophialocephala (M.S. Calabon et al.) Réblová, Hern.-Restr., M. Kolařík & F. Sklenar, Chloridium simile (W. Gams & Hol.-Jech.) Réblová & Hern.-Restr., Chloridium chloridioides (W. Gams & Hol.-Jech.) Réblová & Hern.-Restr., Chloridium subglobosum (W. Gams & Hol.-Jech.) Réblová & Hern.-Restr., Chloridium fuscum (Corda) Réblová & Hern.-Restr., Chloridium ypsilosporum (Hol.-Jech.) Réblová & Hern.-Restr., Chloridium costaricense (G. Weber et al.) Réblová & Hern.-Restr., Chloridium cuneatum (N.G. Liu et al.) Réblová & Hern.-Restr., Fusichloridium cylindrosporum (W. Gams & Hol.-Jech.) Réblová, Gongromeriza myriocarpa (Fr.) Réblová, Gongromeriza pygmaea (P. Karst.) Réblová, Gongromerizella lignicola (F. Mangenot) Réblová, Gongromerizella pachytrachela (W. Gams & Hol.-Jech) Réblová, Gongromerizella pini (Crous & Akulov) Réblová; New name: Chloridium pellucidum Réblová & Hern.-Restr.; Epitypifications (basionyms): Chaetopsis fusca Corda, Gonytrichum caesium var. subglobosum W. Gams & Hol.-Jech.; Lectotypification (basionym): Gonytrichum caesium Nees & T. Nees. Citation: Réblová M, Hernández-Restrepo M, Sklenář F, Nekvindová J, Réblová K, Kolařík M (2022). Consolidation of Chloridium: new classification into eight sections with 37 species and reinstatement of the genera Gongromeriza and Psilobotrys. Studies in Mycology 103: 87-212. doi: 10.3114/sim.2022.103.04.