15. Biogenic Trace Gas Exchanges
Chanton and Whiting 1995 for a review). Chambers are the most simple and widely used method
for measuring gas emissions via plants. Although
different designs are used, they are typically made
of transparent film, Plexiglas, or polycarbonate, and
some can be sealed together to cover plants up to
3 m in height. The chamber is typically deployed
onto a collar that was previously placed in the sediment. Temperature control is usually necessary,
and CO 2 concentration is often monitored and controlled as well. While these chamber systems have
been principally used for estimation of CH 4 , the
fact that DMS is related to plant physiological processes suggests such chambers may be best for it
and other plant-derived gases as well. However, because many sulfur compounds are easily adsorbed
onto enclosure materials, care must be taken in
choice of materials for the enclosure (Kuster and
Goldan 1987). Chanton and Whiting (1995) discuss
the various approaches and review the published
research that has deployed the chambers.
Trace Gas Exchanges at Plant-Air Inteifaces
Plants playa number of different roles with respect
to biogenic trace gases, including providing a carbon or energy substrate for the microbial processes
that lead to gas production or consumption, acting
as conduits for gas transport, aerating the soil, and
otherwise influencing the diffusivity of the soil. For
some trace gases, plant tissues are the loci of production and emission. For example, a wide suite of
VOC compounds are produced and emitted by
plants. Isoprene and the monoterpenes are the most
widely studied of the biogenically emitted VOCs
(Fehsenfeld et al. 1992); however, many other
VOCs, including a variety of alkenes, alcohols, and
ketones, have been identified as plant emissions
(Nemecek-Marshall et al. 1995; Goldstein et al.
1996; Guenther et al. 1996; Winer et al. 1983).
Likewise, plants emit ammonia (NH3)' especially
during tissue senescence and under conditions of
high N availability (Schlesinger and Hartley 1992;
Freney and Simpson 1983; Schjoerring 1991). Numerous reduced sulfur compounds are also emitted
by plants (Renneberg 1991; Aneja and Cooper
1989; MacTaggart et al. 1987). These gases can
play critical roles in atmospheric chemistry (Logan
1983; Chameides et al. 1988; Sillman et al. 1990;
Brasseur and Chatfield 1991; Fehsenfeld et al.
1992).
241
A variety of techniques have been used to collect and analyze gases emitted from plants. Enclosures are often used for field measurements of flux.
For measurements of monoterpenes and isoprene,
an individual tree branch or several small plants
are typically enclosed in a Teflon bag, sometimes
fitted with a fan to mix the air (Zimmerman 1979;
Winer et al. 1983; Lamb et al. 1987; Juuti et al.
1990; Lerdau et al. 1994). The bag is sealed, evacuated, and refilled, or purged, with VOC-free air.
During enclosure, air flow is maintained through
the bag; the head-space is sampled at the end of
the enclosure period to determine gas concentration. Gas exchange chambers and leaf cuvettes
also have been used to estimate VOC emissions
from different species and under different environmental conditions (Isidorov et al. 1985; Tingey et
al. 1987; Monson and Fall 1989). These systems
function similarly to CO 2 gas exchange systems
(but with different, more sensitive analytical systems required). Because concentrations are typically very low, many analytical methods require
preconcentration of the gases via cryoconcentration or trapping on absorbents, followed by measurement with gas chromatography. For further description of these and other approaches, see Tingey
et al. (1991). Micrometeorological approaches that
estimate VOC exchange for whole ecosystems are
increasingly being used (Knoerr and Mawry 1981;
Lamb et al. 1985; Goldstein et al. 1996; Guenther
et al. 1996).
Enclosure methods are also frequently used to
estimate NH3 and reduced S gas emissions (e.g.,
Denmead 1983; MacTaggart et al. 1987; Fehsenfeld
1995). In the case of NH 3 , the gas is trapped on an
acid medium (e.g., dried filter paper initially saturated with oxalic acid) and then measured as ammonium with colorimetry or ion chromatography
(see Fehsenfeld 1995 for a recent review of trapping methods). Since NH3 is readily absorbed on
the materials used to make the chambers, the reliability of this approach under low flux conditions
is questionable. Gradient diffusion techniques (discussed below) have been widely used for NH3 in
agricultural systems (Denmead 1983) and appear to
be a better alternative under conditions suitable for
micrometeorological approaches (see below).
Fluxes of sulfur gases are measured almost exclusively with enclosures (MacTaggart et al. 1987;
Fehsenfeld 1995), in which a purge gas flows
Chanton and Whiting 1995 for a review). Chambers are the most simple and widely used method
for measuring gas emissions via plants. Although
different designs are used, they are typically made
of transparent film, Plexiglas, or polycarbonate, and
some can be sealed together to cover plants up to
3 m in height. The chamber is typically deployed
onto a collar that was previously placed in the sediment. Temperature control is usually necessary,
and CO 2 concentration is often monitored and controlled as well. While these chamber systems have
been principally used for estimation of CH 4 , the
fact that DMS is related to plant physiological processes suggests such chambers may be best for it
and other plant-derived gases as well. However, because many sulfur compounds are easily adsorbed
onto enclosure materials, care must be taken in
choice of materials for the enclosure (Kuster and
Goldan 1987). Chanton and Whiting (1995) discuss
the various approaches and review the published
research that has deployed the chambers.
Trace Gas Exchanges at Plant-Air Inteifaces
Plants playa number of different roles with respect
to biogenic trace gases, including providing a carbon or energy substrate for the microbial processes
that lead to gas production or consumption, acting
as conduits for gas transport, aerating the soil, and
otherwise influencing the diffusivity of the soil. For
some trace gases, plant tissues are the loci of production and emission. For example, a wide suite of
VOC compounds are produced and emitted by
plants. Isoprene and the monoterpenes are the most
widely studied of the biogenically emitted VOCs
(Fehsenfeld et al. 1992); however, many other
VOCs, including a variety of alkenes, alcohols, and
ketones, have been identified as plant emissions
(Nemecek-Marshall et al. 1995; Goldstein et al.
1996; Guenther et al. 1996; Winer et al. 1983).
Likewise, plants emit ammonia (NH3)' especially
during tissue senescence and under conditions of
high N availability (Schlesinger and Hartley 1992;
Freney and Simpson 1983; Schjoerring 1991). Numerous reduced sulfur compounds are also emitted
by plants (Renneberg 1991; Aneja and Cooper
1989; MacTaggart et al. 1987). These gases can
play critical roles in atmospheric chemistry (Logan
1983; Chameides et al. 1988; Sillman et al. 1990;
Brasseur and Chatfield 1991; Fehsenfeld et al.
1992).
241
A variety of techniques have been used to collect and analyze gases emitted from plants. Enclosures are often used for field measurements of flux.
For measurements of monoterpenes and isoprene,
an individual tree branch or several small plants
are typically enclosed in a Teflon bag, sometimes
fitted with a fan to mix the air (Zimmerman 1979;
Winer et al. 1983; Lamb et al. 1987; Juuti et al.
1990; Lerdau et al. 1994). The bag is sealed, evacuated, and refilled, or purged, with VOC-free air.
During enclosure, air flow is maintained through
the bag; the head-space is sampled at the end of
the enclosure period to determine gas concentration. Gas exchange chambers and leaf cuvettes
also have been used to estimate VOC emissions
from different species and under different environmental conditions (Isidorov et al. 1985; Tingey et
al. 1987; Monson and Fall 1989). These systems
function similarly to CO 2 gas exchange systems
(but with different, more sensitive analytical systems required). Because concentrations are typically very low, many analytical methods require
preconcentration of the gases via cryoconcentration or trapping on absorbents, followed by measurement with gas chromatography. For further description of these and other approaches, see Tingey
et al. (1991). Micrometeorological approaches that
estimate VOC exchange for whole ecosystems are
increasingly being used (Knoerr and Mawry 1981;
Lamb et al. 1985; Goldstein et al. 1996; Guenther
et al. 1996).
Enclosure methods are also frequently used to
estimate NH3 and reduced S gas emissions (e.g.,
Denmead 1983; MacTaggart et al. 1987; Fehsenfeld
1995). In the case of NH 3 , the gas is trapped on an
acid medium (e.g., dried filter paper initially saturated with oxalic acid) and then measured as ammonium with colorimetry or ion chromatography
(see Fehsenfeld 1995 for a recent review of trapping methods). Since NH3 is readily absorbed on
the materials used to make the chambers, the reliability of this approach under low flux conditions
is questionable. Gradient diffusion techniques (discussed below) have been widely used for NH3 in
agricultural systems (Denmead 1983) and appear to
be a better alternative under conditions suitable for
micrometeorological approaches (see below).
Fluxes of sulfur gases are measured almost exclusively with enclosures (MacTaggart et al. 1987;
Fehsenfeld 1995), in which a purge gas flows