Atmospheric nitrogen enrichment changes nutrient stoichiometry and reduces fungal N supply to peatland ericoid mycorrhizal shrubs.

Peatlands store one third of global soil carbon (C) and up to 15% of global soil nitrogen (N) but often have low plant nutrient availability owing to slow organic matter decomposition under acidic and waterlogged conditions. In rainwater-fed ombrotrophic peatlands, elevated atmospheric N deposition has increased N availability with potential consequences to ecosystem nutrient cycling. Here, we studied how 14 years of continuous N addition with either nitrate or ammonium had affected ericoid mycorrhizal (ERM) shrubs at Whim Bog, Scotland. We examined whether enrichment has influenced foliar nutrient stoichiometry and assessed using N stable isotopes whether potential changes in plant nutrient constraints are linked with plant N uptake through ERM fungi versus direct plant uptake. High doses of ammonium alleviated N deficiency in Calluna vulgaris and Erica tetralix, whereas low doses of ammonium and nitrate improved plant phosphorus (P) nutrition, indicated by the lowered foliar N:P ratios. Root acid phosphatase activities correlated positively with foliar N:P ratios, suggesting enhanced P uptake as a result of improved N nutrition. Elevated foliar δ15N of fertilized shrubs suggested that ERM fungi were less important for N supply with N fertilization. Increases in N availability in peat porewater and in direct nonmycorrhizal N uptake likely have reduced plant nitrogen uptake via mycorrhizal pathways. As the mycorrhizal N uptake correlates with the reciprocal C supply from host plants to the soil, such reduction in ERM activity may affect peat microbial communities and even accelerate C loss via decreased ERM activity and enhanced saprotrophic activity. Our results thus introduce a previously unrecognized mechanism for how anthropogenic N pollution may affect nutrient and carbon cycling within peatland ecosystems.

Ericoid plant species and Pinus sylvestris shape fungal communities in their roots and surrounding soil.

Root-colonizing fungi can form mycorrhizal or endophytic associations with plant roots, the type of association depending on the host. We investigated the differences and similarities of the fungal communities of three boreal ericoid plants and one coniferous tree, and identified the community structure of fungi utilizing photosynthates from the plants studied. The fungal communities of roots and soils of Vaccinium myrtillus, Vaccinium vitis-idaea, Calluna vulgaris and Pinus sylvestris were studied in an 18-month-long experiment where the plants were grown individually in natural substrate. Photosynthates utilizing fungi were detected with DNA stable-isotope probing using 13 CO2 (13 C-DNA-SIP). The results indicated that the plants studied provide different ecological niches preferred by different fungal species. Those fungi which dominated the community in washed roots had also the highest 13 C-uptake. In addition, a common root endophyte without confirmed mycorrhizal status also obtained 13 C from all the plants, indicating close plant-association of this fungal species. We detect several fungal species inhabiting the roots of both ericoid mycorrhizal and ectomycorrhizal plants. Our results highlight that the ecological role of co-occurrence of fungi with different life styles (e.g. mycorrhizal or endophytic) in plant root systems should be further investigated.

Ericoid Roots and Mycospheres Govern Plant-Specific Bacterial Communities in Boreal Forest Humus.

In this study, the bacterial populations of roots and mycospheres of the boreal pine forest ericoid plants, heather (Calluna vulgaris), bilberry (Vaccinium myrtillus), and lingonberry (Vaccinium vitis-idaea), were studied by qPCR and next-generation sequencing (NGS). All bacterial communities of mycosphere soils differed from soils uncolonized by mycorrhizal mycelia. Colonization by mycorrhizal hyphae increased the total number of bacterial 16S ribosomal DNA (rDNA) gene copies in the humus but decreased the number of different bacterial operational taxonomic units (OTUs). Nevertheless, ericoid roots and mycospheres supported numerous OTUs not present in uncolonized humus. Bacterial communities in bilberry mycospheres were surprisingly similar to those in pine mycospheres but not to bacterial communities in heather and lingonberry mycospheres. In contrast, bacterial communities of ericoid roots were more similar to each other than to those of pine roots. In all sample types, the relative abundances of bacterial sequences belonging to Alphaproteobacteria and Acidobacteria were higher than the sequences belonging to other classes. Soil samples contained more Actinobacteria, Deltaproteobacteria, Opitutae, and Planctomycetia, whereas Armatimonadia, Betaproteobacteria, Gammaproteobacteria, and Sphingobacteriia were more common to roots. All mycosphere soils and roots harbored bacteria unique to that particular habitat. Our study suggests that the habitation by ericoid plants increases the overall bacterial diversity of boreal forest soils.