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The hyphae of ectomycorrhizal and ericoid mycorrhizal fungi proliferate in nitrogen (N)-limited forests and tundra where the availability of inorganic N is low; under these conditions the most common fungal species are those capable of protein degradation that can supply their host plants with organic N. Although it is widely understood that these symbiotic fungi supply N to their host plants, the transfer is difficult to quantify in the field. A novel approach uses the natural 15N:14N ratios (expressed as δ15N values) in plants, soils, and mycorrhizal fungi to estimate the fraction of N in symbiotic trees and shrubs that enters through mycorrhizal fungi. This calculation is possible because mycorrhizal fungi discriminate against 15N when they create compounds for transfer to plants; host plants are depleted in 15N, whereas mycorrhizal fungi are enriched in 15N. The amount of carbon (C) supplied to these fungi can be stoichiometrically calculated from the fraction of plant N derived from the symbiosis, the N demand of the plants, the fungal C:N ratio, and the fraction of N retained in the fungi. Up to a third of C allocated belowground, or 20% of net primary production, is used to support ectomycorrhizal fungi. As anthropogenic N inputs increase, the C allocation to fungi decreases and plant δ15N increases. Careful analyses of δ15N patterns in systems dominated by ectomycorrhizal and ericoid mycorrhizal symbioses may reveal the ecosystem-scale effects of alterations in the plant–mycorrhizal symbioses caused by shifts in climate and N deposition. [ABSTRACT FROM AUTHOR]

The quantification of root turnover is necessary for a complete understanding of plant carbon (C) budgets, especially in terms of impacts of global climate change. To improve estimates of root turnover, we present a method to distinguish current- from prior-year allocation of carbon (C) to roots in global change experiments by using changes in 13C:12 C ratios (δ13C values) resulting from application of 13C-depleted, tank-derived CO2. In a 4-year study examining effects of elevated CO2 and temperature on reconstructed Douglas-fir (Pseudotsuga mensiezii) ecosystems, δ13C patterns of fine roots and foliage were measured yearly. Native soil of low nitrogen (N) content was used, so plant N supply relied on natural soil N processes. Under these N-limited conditions, elevated CO2 or temperature did not affect the proportion of fine root C derived from current-year photosynthate. Regression analyses showed that 75% of fine root C originated from current-year photosynthate. The method is useful as an independent measure of the contribution of current-year photosynthate to root C and could be used to improve estimates of root C budgets with concurrent measurements of root C pools. We calculated a 13C enrichment of root C relative to foliar C of 2‰. This enrichment agrees with prior measurements of the enrichment of heterotrophic versus autotrophic plant tissues and must be accounted for when using shifts in foliar δ13C to calculate inputs of plant C into the soil. This enrichment probably contributes to the progressive enrichment in 13C with increasing depth in soil profiles. [ABSTRACT FROM AUTHOR]

We compared the δ 13 C and δ 15 N of forest material with an extensive sporocarp collection to elucidate the role of litter, wood and soil as fungal carbon and nitrogen sources in Finnish boreal Picea abies-dominated forests. Ectomycorrhizal Hydnum and Cortinarius had higher δ 15 N than other ectomycorrhizal fungi, suggesting use of 15 N-enriched, deeper nitrogen. Russula had lower δ 15 N than other ectomycorrhizal fungi and resembled some litter decay genera, suggesting use of litter-derived nitrogen. There was little variation in δ 15 N among other genera of ectomycorrhizal fungi, indicating limited functional diversity in nitrogen use. Saprotrophic Leotia, Gymnopus, Hypholoma, Pholiota, Rhodocollybia and Calocera had δ 15 N values similar to ectomycorrhizal fungi, indicating overlap in use of older nitrogen from soil or roots or use of newly fixed nitrogen. Genera of litter and wood decay fungi varied up to 6‰ in δ 13 C and 10‰ in δ 15 N, suggesting large differences in carbon and nitrogen sources and processing. Similar δ 13 C between white and brown rot wood decay fungi also suggest that white rot fungi do not use lignin-derived carbon. Together, these δ 13 C and δ 15 N patterns of fungi from Finnish boreal forests enhance our knowledge of fungal functional diversity and indicate broad use of litter, wood and soil resources., (Copyright © 2019 The Author(s). Published by Elsevier Ltd.. All rights reserved.)