References

Assessing Functions of Soil Microbes with Isotopic Measurements 391
Fig. 4. Movement and isotopic fractionation of carbon isotopes in different compounds
and components of plants, mycorrhizal fungi, and saprotrophic fungi. Separate pathways
are indicated for arbuscular mycorrhizal (AM) fungi and ectomycorrhizal fungi. Isotopic
fractionation along nonhorizontal arrows is 1–2‰
In Fig. 4, I propose a scheme of carbon isotope movement among plants,
mycorrhizal fungi, and saprotrophic fungi that accounts for current observationsfromfieldandculturestudies.Animportantcontributingfactorfor
many isotopic patterns is the enrichment in 13 Cofcarbohydratesrelativeto
other compound classes such as lignins and lipids. Because carbohydrate
polymers are abundant in plants and possess a regular structure that is
relatively amenable to enzymatic attack, they are the main carbon sources
for most saprotrophic fungi, ensuring that saprotrophic fungi will be enriched
in 13 C relative to their substrates. In addition, since both plants and
fungi transport carbon primarily as 13 C-enriched sugars, tissues such as
roots, wood, and fungal fruiting bodies apparently become increasingly
enriched in 13 C as the stream of labile carbohydrates becomes increasingly
metabolized. Many of the biochemical mechanisms causing isotopic fractionation
among different compounds are still unknown. Gleixner et al.
(1993) proposed that isotopic fractionation during triose interconversions
or by aldolase during cleavage of hexose to triose controlled the enrichment
in 13 C of fungal carbohydrates relative to cellulose. Because a “futile” cycle
of hexose to triose interconversions operates in plants (Hill et al. 1995), this
mechanism could conceivably contribute to increases in 13 Ccontentofcarbohydrates
with increasing distance from the “source” photosynthate. Since
lipids are important transport compounds in AM fungi (Pfeffer et al. 1999),
13 C enrichment along transport pathways of these fungi appears less likely
than in ectomycorrhizal fungi. Patterns of δ 13 C in AM or ectomycorrhizal
fungi relative to host-supplied sugars may accordingly reflect whether carbohydrates
or lipids are the primary storage compounds used for later
biosynthesis, as already shown for heterotrophically produced leaves derived
from lipid- versus starch-storing seeds (Luo and Sternberg 1994).