Insights into nitrogen and carbon dynamics of ectomycorrhizal and saprotrophic fungi from isotopic evidence

Abstract

The successful use of natural abundances of carbon (C) and nitrogen (N) isotopes in the study of ecosystem dynamics suggests that isotopic measurements could yield new insights into the role of fungi in nitrogen and carbon cycling. Sporocarps of mycorrhizal and saprotrophic fungi, vegetation, and soils were collected in young, deciduous-dominated sites and older, coniferous-dominated sites along a successional sequence at Glacier Bay National Park, Alaska. Mycorrhizal fungi had consistently higher δN and lower δC values than saprotrophic fungi. Foliar δC values were always isotopically depleted relative to both fungal types. Foliar δN values were usually, but not always, more depleted than those in saprotrophic fungi, and were consistently more depleted than in mycorrhizal fungi. We hypothesize that an apparent isotopic fractionation by mycorrhizal fungi during the transfer of nitrogen to plants may be attributed to enzymatic reactions within the fungi producing isotopically depleted amino acids, which are subsequently passed on to plant symbionts. An increasing difference between soil mineral nitrogen δN and foliar δN in later succession might therefore be a consequence of greater reliance on mycorrhizal symbionts for nitrogen supply under nitrogen-limited conditions. Carbon signatures of mycorrhizal fungi may be more enriched than those of foliage because the fungi use isotopically enriched photosynthate such as simple sugars, in contrast to the mixture of compounds present in leaves. In addition, some C fractionation may occur during transport processes from leaves to roots, and during fungal chitin biosynthesis. Stable isotopes have the potential to help clarify the role of fungi in ecosystem processes.

Stratégie saprophyte ou symbiotique durant le développement d’ascocarpes de truffes dans une truffière à chêne vert. Une réponse basée sur l’abondance naturelle du 13C et du 15N

“…Fungal sheaths were 2.4 to 6.4% enriched compared to the root core. Other studies have confirmed that sporophores of ectomycorrhizal fungi are often more enriched in 15 N than sporophores of saprotrophic fungi, whereas sporophores of saprotrophic fungi are almost always enriched in 13 C compared to sporophores of ectomycorrhizal fungi (Hobbie et al, 1999;2002;Kohzu et al, 1999;2000;Taylor et al, 2003;Trudell et al, 2004). Lilleskov et al (2002) demonstrated that isotope signatures reflect ecophysiological functions.…”

“…They concluded that 15 N uptake was coupled with an internally fractionating process. However, Hobbie et al (1999;2001;2002), Hobbie and Colpaert (2003;, 2005) considered that in natural conditions with low nitrogen availability there is no fractionation upon uptake.…”

Ectomycorrhizal fungal taxa differing in response to nitrogen deposition also differ in pure culture organic nitrogen use and natural abundance of nitrogen isotopes

“…chitin, proteins or transfer amino Table 3 Per cent root-level abundance of ectomycorrhizal fungal taxa at five sites across the N deposition gradient (Lilleskov et al, 2002), their protein use status (from Table 2) and the regression of abundance of each group vs organic horizon mineral N. Sites are presented from high to low soil mineral N (from site IDs 111 to 13, respectively) acids), the proportion of fungal N uptake transferred to hosts or the allocation of N to different tissues. We know that sporocarps are isotopically enriched compared with their host plants and saprobic fungi, suggesting that fractionation during transfer of N from fungus to host is likely (Handley et al, 1996;Högberg et al, 1996Högberg et al, , 1999Hobbie et al, 1999;Kohzu et al, 2000). Variation in the specific physiology and biochemistry of N transfer, or in the proportion of N transferred to hosts, could lead to distinctive signatures among EMF.…”

“…Russula ). EMF taxa differ in their N isotopic signatures (Taylor et al , 1997;Gebauer & Taylor, 1999;Hobbie et al , 1999;Kohzu et al , 1999). Nitrogen isotopes may provide an independent line of evidence regarding N-use patterns if the combination of soil N pool signatures and physiological mechanisms of isotopic alteration during uptake, transformations, transfers and losses lead to a consistent isotopic distinction among physiological classes of fungi.…”

“…transamination), and transfers of depleted N (to soil, host plants) all have the potential to alter fungal and plant isotopic signatures. Fungal tissues are typically enriched relative to their host plants (Högberg et al , 1996(Högberg et al , , 1999Handley et al , 1996;Hobbie et al , 1999;Kohzu et al , 2000), so any effect of sources on isotopic signatures would be in addition to effects of enrichment relative to hosts. Given the number of processes that could influence isotopic signatures, isotopic differences among EMF taxa resulting from different pool use could be masked by other processes.…”