ABSTRACT
The difficulty of experimental manipulations, observations, and measurements in the environments inhabited by fungi has long hindered studies of fungal functioning. One promising tool to explore fungal functioning, stable isotope measurements, is increasingly used to study links between fungi and biogeochemical processes. Such measurements rely on calculating ratios of heavy to light isotopes of key biological elements (e.g.,
C:
C,
N:
N,
O:
O,
H:
H, and
S:
S). Studies can generally be classified into two main types: those using natural abundance levels of isotopes in fungi or fungally produced compounds and those in which compounds or substances artificially enriched in one or more of the heavy isotopes are applied as tracers and the subsequent fate of the tracer is followed into different ecosystem components, including fungi. Natural abundance and tracer studies can be used at different levels of resolution, ranging from whole organisms to specific compound classes (e.g., lipids), specific compounds (e.g., N-acetyl glucosamine), or even DNA or RNA unique to a single species. Compound-specific analyses using gas chromatography of volatile compounds linked to isotope ratio mass spectrometry have been particularly widely applied (reviewed in Boschker and Middelburg
,
2002). The use of isotopic techniques has increased dramatically in the past 15 years as a result of several technological advances, including faster analyses with the widespread adoption of continuous-flow isotope ratio mass spectrometry, improvements in our ability to measure hydrogen and oxygen isotope ratios, and steady increases in the quality and variety of compound-specific measurements possible. In the following chapter, I focus primarily on using natural abundance measurements of carbon and nitrogen isotopes to assess fungal
functioning in ecosystems, with some discussion of tracer experiments. Because interpreting natural abundance measurements requires an understanding of how isotopic differences are created, I also discuss the mechanisms creating nitrogen and carbon isotope patterns. Recent useful reviews on natural abundance measurements include Adams and Grierson (2001), Evans (2001), Werner and Schmidt (2002), and Hayes (2002).
