References

Assessing Functions of Soil Microbes with Isotopic Measurements 385
atmospheric N2 ( 15 N/ 14 N = 0. 0036765), for sulfur, Canyon Diablo troilite
( 34 S/ 32 S = 0. 0450045), and for hydrogen and oxygen, Vienna Standard
Mean Ocean Water (V-SMOW, D/H = 0. 00015576, 18 O/ 16 O = 0. 00200520)
(Hoefs 1997). Isotopic values for carbon generally range from 0‰ (carbonates)
to −50‰ (methane in some systems), with mostδ 13 Cvaluesinsystems
dominated by plants of the C3 photosynthetic pathway ranging from −20 to
−35‰. In systems dominated by plants of the C4 photosynthetic pathway,
values range from −10 to −20‰.Forδ 15 N, pools in terrestrial ecosystems
are usually between 20 and −10‰. Incomparisonsamongsamples,samples
with more of the heavy isotope are commonly referred to as isotopically
enriched, or heavy, and samples with less of the heavy isotope are referred
to as isotopically depleted, or light. Isotopic fractionation (∆)forreactions
in open systems can be calculated based on the isotopic signature of the
source and product according to (18.2).
∆(δ n Xsource − δ n Xproduct)/(1 + δ n Xproduct) (18.2)
2.1
Fungi
Fungi can be divided into two main life history strategies depending on
their carbon source. Saprotrophic fungi obtain their carbon from the decay
of dead organic matter, whereas mycorrhizal fungi form symbiotic
associations with plants in which plant-supplied sugars are exchanged
for nutrients obtained by fungi from the soil. Mycorrhizal fungi can be
further divided into several types, of which the two most important are
arbuscular mycorrhizal (AM) fungi (belonging to the Glomerales), symbiotic
on most herbaceous plants and some trees, and ectomycorrhizal
fungi, associated with many of the dominant tree families of temperate
and boreal regions (e.g., Pinaceae, Betulaceae, Salicaceae, and Fagaceae),
but also some families distributed in the tropics (e.g., Dipterocarpaceae
and Myrtaceae). Higher fungi in the phyla Basidiomycota and Ascomycota
have been tempting targets for isotopic measurements in recent years because
many produce large, conspicuous fruiting bodies that often can be
identified to species. The production of large fruiting bodies has allowed
researchers to analyze isotopes at the species level in situ, without resorting
to techniques such as DNA- or RNA-specific analyses. Such isotopic analyses
are providing interesting insights into patterns of carbon and nitrogen
cycling by both ectomycorrhizal and saprotrophic fungi in these phyla.