15
Biogenic Trace Gas Exchanges
Pamela Matson and Allen Goldstein
Biogenic trace gases are defined as those gases of
biological origin whose atmospheric mixing ratios
(concentrations by volume) are less than a few parts
per million by volume (ppmv). In the past several
decades, knowledge of trace gas exchanges between terrestrial or aquatic systems and the atmosphere, and of the biological, physical, and chemical processes that control them, has increased
dramatically. The increased attention and research
effort in this area stems primarily from the fact that
the atmospheric concentrations of a number of trace
gases are increasing. The increases of some of the
less reactive gases such as nitrous oxide (N 2 0) and
methane (CH4) are documented in the 20-year or
longer record of atmospheric measurements at sampling stations throughout the world (e.g., Prinn et
al. 1990, Steele et al. 1992) and by the long-term
record provided through the analysis of gases
trapped in glacial ice (e.g., Raynaud et al. 1988);
increases in some reactive trace gases such as oxides of nitrogen (NOJ and ozone have been determined via ground-based ambient concentration
measurements, ground- or aircraft-based flux measurements and atmospheric models. The consequences of these atmospheric changes are felt at a
variety of scales; some contribute to the radiative
balance of Earth and hence climate change, and
others have critical roles in regional and global atmospheric chemistry.
The understanding of the sources and sinks of
the fluxes has depended on the contributions of a
number of disciplines, including atmospheric
chemistry, ecology, biogeochemistry, microbiology, soil physics and chemistry, meteorology, and
hydrology. Exchanges of biogenic trace gases between surfaces and the atmosphere are controlled
by the production and consumption of gases by
plants and microbial processes, the physical transport through soils, sediments, or water, and the flux
across the air-surface boundary. These biological
and physical processes in tum depend on other biotic and abiotic properties and processes within
ecosystems. If we want to use our understanding of
trace gas fluxes to develop estimates at regional or
global scales (see Chapter 18), and to develop predictive models of fluxes with future environmental
and anthropogenic change (see Chapters 13 and
25), studies of trace gas exchange must be carried
out in the context of ecosystem processes and properties (Matson and Harriss 1995). Clearly, the development of an understanding of trace gas fluxes
requires far more than measurements of flux, and
thus, ultimately, requires the use of many of the
other methods described in this book.
Approaches for Estimation
of Fluxes
There are a number of different approaches to measuring trace gas fluxes, each with its own advantages and limitations, each with a set of conditions
or range of questions for which it is most appropriate. No single gas measurement technique works
for all gases under all situations. Choice of the approach or combination of approaches depends on
the scientific questions being addressed, the biophysical characteristics of the study site, the analytical capabilities for the gases of interest, and the
235
Biogenic Trace Gas Exchanges
Pamela Matson and Allen Goldstein
Biogenic trace gases are defined as those gases of
biological origin whose atmospheric mixing ratios
(concentrations by volume) are less than a few parts
per million by volume (ppmv). In the past several
decades, knowledge of trace gas exchanges between terrestrial or aquatic systems and the atmosphere, and of the biological, physical, and chemical processes that control them, has increased
dramatically. The increased attention and research
effort in this area stems primarily from the fact that
the atmospheric concentrations of a number of trace
gases are increasing. The increases of some of the
less reactive gases such as nitrous oxide (N 2 0) and
methane (CH4) are documented in the 20-year or
longer record of atmospheric measurements at sampling stations throughout the world (e.g., Prinn et
al. 1990, Steele et al. 1992) and by the long-term
record provided through the analysis of gases
trapped in glacial ice (e.g., Raynaud et al. 1988);
increases in some reactive trace gases such as oxides of nitrogen (NOJ and ozone have been determined via ground-based ambient concentration
measurements, ground- or aircraft-based flux measurements and atmospheric models. The consequences of these atmospheric changes are felt at a
variety of scales; some contribute to the radiative
balance of Earth and hence climate change, and
others have critical roles in regional and global atmospheric chemistry.
The understanding of the sources and sinks of
the fluxes has depended on the contributions of a
number of disciplines, including atmospheric
chemistry, ecology, biogeochemistry, microbiology, soil physics and chemistry, meteorology, and
hydrology. Exchanges of biogenic trace gases between surfaces and the atmosphere are controlled
by the production and consumption of gases by
plants and microbial processes, the physical transport through soils, sediments, or water, and the flux
across the air-surface boundary. These biological
and physical processes in tum depend on other biotic and abiotic properties and processes within
ecosystems. If we want to use our understanding of
trace gas fluxes to develop estimates at regional or
global scales (see Chapter 18), and to develop predictive models of fluxes with future environmental
and anthropogenic change (see Chapters 13 and
25), studies of trace gas exchange must be carried
out in the context of ecosystem processes and properties (Matson and Harriss 1995). Clearly, the development of an understanding of trace gas fluxes
requires far more than measurements of flux, and
thus, ultimately, requires the use of many of the
other methods described in this book.
Approaches for Estimation
of Fluxes
There are a number of different approaches to measuring trace gas fluxes, each with its own advantages and limitations, each with a set of conditions
or range of questions for which it is most appropriate. No single gas measurement technique works
for all gases under all situations. Choice of the approach or combination of approaches depends on
the scientific questions being addressed, the biophysical characteristics of the study site, the analytical capabilities for the gases of interest, and the
235