12
Assessing Ecosystem-Level Water
Relations Through Stable Isotope
Ratio Analyses
James R. Ehleringer, John Roden, and Todd E. Dawson
Introduction
Virtually all elements of biological interest have
multiple stable isotopic forms and the fractionation
events associated with biological and physical processes help to create spatial and temporal variations in isotopic abundance that can be used to
understand the dynamics of ecological systems.
Stable isotope ratio analyses at natural abundance
levels can provide integrated information on ecosystem functioning, such as variations in water-use
activities by different elements within an ecosystem (Ehleringer et al. 1993; Dawson and Ehleringer 1998). Stable isotope ratio analyses do not provide information on water flux rates through the
ecosystem, but instead they help constrain the
analysis of flux data, such as through identifying
those specific soil layers that are the source of current moisture use by the vegetation or the ratio of
carbon dioxide-to-water (C02-to-H20) flux.
Stable Isotopes: Natural
Abundances and 8 Notation
Most elements are present in multiple stable forms,
although one isotope is usually far more abundant
than the others. Rather than consider the abundances of different stable isotopes in absolute
terms, it is much more common and convenient to
describe the abundance of stable isotopes as the
molar ratios of the heavy-to-light forms (R) in a
sample (Rsample) relative to that of an international
standard (Rstandard)' This is the so called delta notation (0) where
o = (RsamPle - 1) . 1000%0 (12.1)
Rstandard
and the 0 value has units of parts per thousand, or
per mil (%0).
With regard to water relations functions within
ecosystems, carbon (C), hydrogen (H), and oxygen
(0) isotopes are most relevant. Table 12.1 presents
the typical background value for different isotopic
forms as well as the range of 0 values that one
might encounter when investigating different ecosystem components on a global basis.
Isotope Ratio Mass Spectrometry
In isotope ratio mass spectrometry, light gases are
introduced into a chamber maintained at a very
low pressure «10- 6 torr), in which there is an
electron source. The gases (in our case H2 or CO 2 )
are bombarded by this electron source, resulting in
ion formation in approximately lout of every
1000-4000 molecules. The most likely molecular
possibilities that are ionized include lHIH and
2HIH for H2 and 12CI60160, 13CI60160, and
12C180160 for CO 2 , although other rarer combinations most certainly occur. These charged ions
are then accelerated and pass along a flight tube
with an even lower pressure (10- 8 torr). During
this flight, their trajectory is deflected by a magnet
located at the center point. The extent of an ion's
magnetic deflection depends on its mJe (i.e., mass
to charge) ratio. Thus, 'H1H+ is deflected more
than 2H'H+ and likewise 12C'60'60+ more than
12CI80'60+. At the end of the flight tube are a
181
Assessing Ecosystem-Level Water
Relations Through Stable Isotope
Ratio Analyses
James R. Ehleringer, John Roden, and Todd E. Dawson
Introduction
Virtually all elements of biological interest have
multiple stable isotopic forms and the fractionation
events associated with biological and physical processes help to create spatial and temporal variations in isotopic abundance that can be used to
understand the dynamics of ecological systems.
Stable isotope ratio analyses at natural abundance
levels can provide integrated information on ecosystem functioning, such as variations in water-use
activities by different elements within an ecosystem (Ehleringer et al. 1993; Dawson and Ehleringer 1998). Stable isotope ratio analyses do not provide information on water flux rates through the
ecosystem, but instead they help constrain the
analysis of flux data, such as through identifying
those specific soil layers that are the source of current moisture use by the vegetation or the ratio of
carbon dioxide-to-water (C02-to-H20) flux.
Stable Isotopes: Natural
Abundances and 8 Notation
Most elements are present in multiple stable forms,
although one isotope is usually far more abundant
than the others. Rather than consider the abundances of different stable isotopes in absolute
terms, it is much more common and convenient to
describe the abundance of stable isotopes as the
molar ratios of the heavy-to-light forms (R) in a
sample (Rsample) relative to that of an international
standard (Rstandard)' This is the so called delta notation (0) where
o = (RsamPle - 1) . 1000%0 (12.1)
Rstandard
and the 0 value has units of parts per thousand, or
per mil (%0).
With regard to water relations functions within
ecosystems, carbon (C), hydrogen (H), and oxygen
(0) isotopes are most relevant. Table 12.1 presents
the typical background value for different isotopic
forms as well as the range of 0 values that one
might encounter when investigating different ecosystem components on a global basis.
Isotope Ratio Mass Spectrometry
In isotope ratio mass spectrometry, light gases are
introduced into a chamber maintained at a very
low pressure «10- 6 torr), in which there is an
electron source. The gases (in our case H2 or CO 2 )
are bombarded by this electron source, resulting in
ion formation in approximately lout of every
1000-4000 molecules. The most likely molecular
possibilities that are ionized include lHIH and
2HIH for H2 and 12CI60160, 13CI60160, and
12C180160 for CO 2 , although other rarer combinations most certainly occur. These charged ions
are then accelerated and pass along a flight tube
with an even lower pressure (10- 8 torr). During
this flight, their trajectory is deflected by a magnet
located at the center point. The extent of an ion's
magnetic deflection depends on its mJe (i.e., mass
to charge) ratio. Thus, 'H1H+ is deflected more
than 2H'H+ and likewise 12C'60'60+ more than
12CI80'60+. At the end of the flight tube are a
181