Carbon-nitrogen relations of ectomycorrhizal mycelium across a natural nitrogen supply gradient in boreal forest

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.

Modified source-sink dynamics govern resource exchange in ectomycorrhizal symbiosis

Ectomycorrhizal symbiosis between roots and fungi is founded on the movement of carbon from plants to fungi, and of soil resources from fungi to plants. Framing this movement as a trade can facilitate an understanding of how this mutualism has developed over evolutionary time, but fails to explain experimental observations of carbon and nutrient movement. Here, I propose that source-sink dynamics are an essential basic model to explain the movement of plant and fungal resources, which may be modified by plant immune response, variability in fungal molecular repertoires, and competition in the soil. Source-sink dynamics provide testable hypotheses to illuminate mechanisms of ectomycorrhizal resource movement and its consequences for mutualism stability and forest function under climate change.

Untargeted metabolomics reveals sour jujube kernel benefiting the nutritional value and flavor of Morchella esculenta

Nucleosides, organic acids, and amino acids separated from Morchella esculenta are well known for their nutritional value and flavor. However, how to increase their content in a better way has been a challenge. In this study, the effect of adding jujube kernel on the active components of M. esculenta was investigated by untargeted metabolomics using UPLC-MS/MS. A total of 1,243 metabolites were identified, of which 262 metabolites (21.078%) were organic acids and derivatives, 245 metabolites (19.71%) were lipids and lipid-like molecules, and 26 metabolites (2.092%) were nucleosides, nucleotides, and analogues. Subsequently, differential metabolites between groups were screened by the orthogonal partial least squares-discriminant analysis model, which showed that 256 metabolites were identified as significantly different for the positive ion model and 149 for the negative ion model. Moreover, significant differential metabolites (VIP > 1, P < 0.05) in annotation of kyoto encyclopedia of genes and genomes pathway were investigated, which showed that ABC transporters were the most commonly observed transporters, followed by pyrimidine metabolism and purine metabolism. The results indicated that the main components of jujube kernel might be conducive to the accumulation of nucleoside organic acids and amino acid metabolites in M. esculenta. These results provide important information for the understanding of more suitable way for cultivation of M. esculenta.

Mixed conifer-broadleaf trees on arbuscular mycorrhizal and ectomycorrhizal communities in rhizosphere soil of different plantation stands in the temperate zone, Northeast China

In comparison with ectomycorrhizal (EM) tree species, arbuscular mycorrhizal (AM) trees have different litter quality and nitrogen cycle modes, which may affect mycorrhizal colonization and the community composition and diversity. However, available studies addressing the mycorrhizal fungal colonization rate, diversity and community composition in mixed forest stands composed of AM and EM trees are rare. In the present study, we assessed litter quality, soil physicochemical properties and correlated them with mycorrhizal community characteristics in rhizosphere soils of monoculture and mixture plantation stands of AM tree species (Fraxinus mandschurica Rupr.) and EM tree species (Larix gmelinii Rupr., Picea koraiensis Nakai) in Northeast China. We hypothesized that (1) the effect of mixture pattern on mycorrhizal colonization rate and diversity would change with tree species, (2) the effect of mixture pattern on mycorrhizal community composition would be less pronounced in comparison with that of tree species. We found that mixture did not change AMF colonization rate regardless of mixture identity, whereas mixture and tree species exerted significant effects on EMF colonization rate. For AMF community, both M-AS (Fraxinus mandschurica Rupr. and Picea koraiensis Nakai) and M-AL (Fraxinus mandschurica Rupr. and Larix gmelinii Rupr.) mixtures significantly increased Pielou index and Simpson index, whereas only M-AS significantly increased Sobs. For EMF community, mixture significantly affected examined diversity indices except for Chao1. Mixture significantly shifted AMF and EMF community, and the magnitude was tree species dependent. The dominant genera in AMF and EMF communities in plantation stands were Glomus and Tomentella, respectively. The EnvFit analysis showed that the determinant factors of EMF community are soil moisture, pH, nitrate nitrogen content, dissolved organic nitrogen content, soil organic matter content, soil organic carbon/total nitrogen and litter carbon/total nitrogen. In conclusion, mixed conifer-broadleaf trees significantly changed soil physicochemical properties, litter quality as well as mycorrhizal fungi community diversity and composition.

Metatranscriptomics captures dynamic shifts in mycorrhizal coordination in boreal forests

Advances in DNA sequencing have provided an unprecedented view of the complex microbial communities that populate global ecosystems. We present a metatranscriptomic analysis of samples from the boreal forest—the largest terrestrial carbon store—capturing the seasonally resolved transcriptomes of Norway spruce roots and more than 350 root-associated fungal species. Our findings link the functional response of host-trees to increased nutrient availability, with profound perturbations in the fungal community. Notably, we observed an exchange in prevalence and host-coordination of specialist ectomycorrhizal species critical for enzymatic cycling of recalcitrant carbon, to metabolically versatile species with resilient melanized cell walls. This research unites kingdom-spanning taxonomic and functional details of the boreal root microbiome, contributing a missing perspective toward modeling global carbon cycling.

Carbon storage and cycling in boreal forests—the largest terrestrial carbon store—is moderated by complex interactions between trees and soil microorganisms. However, existing methods limit our ability to predict how changes in environmental conditions will alter these associations and the essential ecosystem services they provide. To address this, we developed a metatranscriptomic approach to analyze the impact of nutrient enrichment on Norway spruce fine roots and the community structure, function, and tree–microbe coordination of over 350 root-associated fungal species. In response to altered nutrient status, host trees redefined their relationship with the fungal community by reducing sugar efflux carriers and enhancing defense processes. This resulted in a profound restructuring of the fungal community and a collapse in functional coordination between the tree and the dominant Basidiomycete species, and an increase in functional coordination with versatile Ascomycete species. As such, there was a functional shift in community dominance from Basidiomycetes species, with important roles in enzymatically cycling recalcitrant carbon, to Ascomycete species that have melanized cell walls that are highly resistant to degradation. These changes were accompanied by prominent shifts in transcriptional coordination between over 60 predicted fungal effectors, with more than 5,000 Norway spruce transcripts, providing mechanistic insight into the complex molecular dialogue coordinating host trees and their fungal partners. The host–microbe dynamics captured by this study functionally inform how these complex and sensitive biological relationships may mediate the carbon storage potential of boreal soils under changing nutrient conditions.