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6
Terrestrial Trace Gas and
Nutrient Fluxes
OVERVIEW
The composition of the global atmosphere is influenced strongly by the
biosphere's activity. Although the importance of photosynthesis and respi-
ration in controlling carbon dioxide and oxygen has long been known, the
importance of biospheric processes controlling nitrogenous compounds such
as nitrous oxide, nitric oxide, and ammonia, sulfur compounds such as hy-
drogen sulfide, and various hydrocarbons has only recently been appreci-
ated. Moreover, human activities (industrial, agricultural, and others) now
affect these natural biospheric processes to such an extent that they may, in
many circumstances, overtake some of them in importance. For the first
time in the history of the earth, these natural and human-caused atmo-
spher~c-biospheric processes may alter the global climate, with potential
impacts on human welfare.
This chapter was prepared for the Committee on Global Change from the contribu-
tions of Paul Risser, University of New Mexico, Chair; Jim Brown, University of New
Mexico; Stuart Chapin, University of California, Berkeley; David Coleman, University
of Georgia; David Correll, Smithsonian Environmental Research Center; Mary Firestone,
University of California, Berkeley; Robert Howarth, Cornell University; Daniel Jacob,
Harvard University; Jerry Melillo, Marine Biological Laboratory; Robert Naiman,
University of Minnesota; William Parton, Jr., Colorado State University; William
Reiners, University of Wyoming; David Schimel, Colorado State University; Robert
Sievers, University of Colorado; Richard Sparks, Illinois Natural History Survey; Jack
Stanford, University of Montana; Peter Vitousek, Stanford University; and the National
Research Council's Committee on Atmospheric Chemistry. Daniel Albritton, NOAA,
participated as liaison representative from the Committee on Earth Sciences.
164
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TERRESTRIAL TRACE GAS AlID NUTRIENT FLUXES
165
There have always been global changes caused by natural processes such
as changes in solar activity, changes in the earth's orbit, volcanism, and
plate tectonics. The global changes under consideration today, however,
are affected by human activities and include a wide variety of causes and
effects, such as stratospheric ozone depletion, tropospheric ozone forma-
tion, global warming and sea level change, drought, deforestation, desertifi-
cation, and reduction in biological diversity. Climatic change has occurred
in the past on many occasions, but the projected rates now are much faster
owing to the combination of natural and human-caused processes. A major
challenge is to distinguish between these natural and human-influenced changes
and to predict their specific and cumulative impacts on the biosphere and its
inhabitants.
Reliably predicting changes on the global scale of some of these pro-
cesses requires an adequate understanding of the cycles of carbon, nitrogen,
oxygen, sulfur, and phosphorus. These required understandings involve
three important research components (CES, 1989~:
· biogeochemical processes occurring within oceans and on the land,
geophysical and biogeochemical processes that control the fluxes of
compounds between the atmosphere and the aquatic and terrestrial bio-
sphere, and
meteorological and chemical processes that control the distribution
and transformation of chemicals within the atmosphere.
Changes in patterns and rates of terrestrial biogeochemical cycling caused
by both natural and anthropogenic processes can cause changes in the glo-
bal atmosphere; for example, the increase in carbon dioxide and other trace
gases in the atmosphere can alter global temperature and rainfall patterns.
Conversely, global changes can influence biogeochemical cycling; for ex-
ample, global warming can cause an increase in the release of carbon diox-
ide and methane from boreal forest and tundra soils. Thus the connections
between the atmosphere and the terrestrial biosphere operate in both direc-
tions. The bidirectional relationships between the atmosphere and the bio-
sphere, and the complexity of these interactions, are the subject of this
chapter.
Problem Definition
Although these atmosphere-biosphere interactions are now recognized as
extremely important for environmental changes at the global scale, the physical
and biological processes that control the flux rates and magnitudes to and
from many ecosystem types are inadequately understood. This lack of un-
derstanding is caused by the complexity of these biological and physiochemical
systems, by the difficulty of measuring some of these flux exchanges in the
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166
RESEARCH STRATEGIES FOR THE USGCRP
field, and by the heretofore insufficient attention directed toward these cru-
cial studies. Analytical methods must be developed for measuring trace gas
and nutrient fluxes under ambient conditions, ecosystems need to be charac-
terized in terms of the connections between nutrient pathways and trace gas
sources and sinks, the physiological processes and biochemical controls of
these processes need to be understood, and these processes must be well
enough known to translate the results from local and regional scales to the
global scale and to predict their behavior under various conditions of glo-
bal change.
General Approach
The general approach for the research initiative proposed in this chapter
is designed to provide an adequate understanding of trace gas fluxes and
reservoirs and of the flows of nutrients. The following are addressed in the
chapter:
· Statement of the most crucial questions to be answered.
· Identification of the processes and variables that have the highest pri-
ority for attention.
Description of the data that will be required to build and test algo-
rithms for models describing and predicting these processes.
Designation of the most appropriate geographical areas and the envi-
ronmental conditions under which the studies should be conducted.
Description of the experiments that must be conducted and the data
that must be collected.
· Method for organizing the resulting data and information into coher-
ent data sets and models for describing the processes and for predicting
their behavior under alternative conditions of global change.
To proceed with this general approach, data and information must be
provided from efforts discussed in other chapters of this report. Some of
these interactions are shown in Figure 6.1. Arrows A and B refer, respec-
tively, to the trace gases and nutrient fluxes that are essential components
of the research programs proposed in this report. Conducting these studies
will depend on (1) models and measurements describing chemical composition
of and reactions in the atmosphere, (2) predictions of changing climate, (3)
measurements of changes in land use, and (4) assessing the influence of
other anthropogenic activities. Data and information about these input vari-
ables will be generated from other coordinated studies in the U.S. Global
Change Research Program (USGCRP).
The key variables regulating the fluxes of trace gases to and from terres-
trial ecosystems vary from gas to gas. Thus, measuring one set of variables
for one gas may not be appropriate to the understanding of another gas. On
.
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TERRESTRIAL TRACE GAS AND NUTRIENT FLUXES
ll
1 Atmospheric characteristics
a-----------_____
, 2 Climate
______ ___ ________
, 3 Land use
Ecosystem
Function
1
ll
-
167
Ecosystem
Function
J
1
1
Rae
Ecosystem ~Ecosystem I I
Structure ' Structure ,,
1 1
1 1
1 1
1 1
ll
FIGURE 6.1 Coordination of the Mace gas (A) arid nutrient flux (B) studies with
Rose described in other parts of die USGCRP (1, 2, and 3~.
the basis of our current knowledge, it is possible to predict the necessary
variables for modeling each gas. Table 6.1 is a first approximation of a
summary of the variables needed to predict the exchanges of the major trace
gases discussed in this report. The "influence" characteristics are expressed
in general terms only. These variables may affect the fluxes through their
influences on biomass loading, leaf resistances (e.g., stomata! opening),
plant biological activity, soil chemical activity, microbial activity, or surface
layer turbulence. Many of these variables can be mapped from satellite
observations, others from land-based surveys.
This chapter consists of two related topics, namely, the exchange of
radiatively, chemically, and biologically active trace gas species between
the atmosphere and terrestrial ecosystems and the fluxes of nutrients within
and among landscape units. Trace gas emissions lead directly to local
effects but also may move laterally and affect adjacent or more distant
landscape units. Materials (e.g., nutrients and pollutants) move within eco-
systems but also move laterally in the hydrological cycle when they constitute
or are attached to airborne particles. Moreover, lateral flows of nutrients,
especially nitrogen and phosphorus, affect the sources and sinks of trace
gases. The interactions between Face gas and nutrient fluxes are included
in the described research programs.
The major global changes affecting the fluxes of water, sediment, nutri
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168
RESEARCH STRATEGIES FOR THE USGCRP
TABLE 6.1 Environmental Variables Regulating the Fluxes of Trace
Gases from Terrestrial Ecosystems
Variable
Influence
Mapping
Strategy
Surface temperature
Solar radiation (PAR)
Leaf area index
Greenness index (chlorophyll)
Vegetation type
Plant stress
Surface roughness
Sensible heat flux
Surface wind
Soil moisture
Soil type
Soil chemistry
Leaf resistance
Plant biological activity
Soil chemical activity
Microbial activity
Leaf resistance
Plant biological activity
Biomass loading
Leaf resistance
Plant biological activity
Leaf resistance
Plant biological activity
Leaf resistance
Plant biological activity
Turbulence Land-based
Turbulence Satellite
Turbulence Land-based
Soil chemical activity Land-based
Microbial activity
Soil chemical activity
Microbial activity
Soil chemical activity
Microbial activity
Satellite
Land-based
Satellite
Land-based
Satellite
Satellite
Satellite
Land-based
Land-based
Lard-based
Land-based
ents, and pollutants are land use and climate. Of these, altered land use will
cause larger changes in these fluxes in the next years and few decades than
will climatic change. However, climatic change will also affect lateral
water flows and nutrient cycling, which, in turn, will affect trace gas flux
and indirectly the climate. Thus there are specific links between climatic
change, water and nutrient fluxes, and feedbacks to trace gas flux.
Many nutrient cycling studies to date have been conducted in relatively
homogeneous areas (Likens et al., 1985~. Much less is known about the
transfer of nutrients across boundaries between ecosystems, but such trans-
fers may greatly affect trace gas fluxes (Schimel et al., 1989~. Therefore
more careful attention should be given to these boundaries in terms of the
fluxes that occur across boundaries and their controls.
At the global scale, the most significant issue concerning biogeochemis-
try is the effect of land use (e.g., cultivation and deforestations on the flows
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TERRESTRIAL TRACE GAS AND NUTRIENT FLUXES
169
of carbon, nitrogen, phosphorus, and sulfur from specific ecosystems and
across the landscape. These losses from terrestrial systems occur by several
atmospheric and soil surface and subsurface pathways and involve various
interlocking biogeochemical cycles. Moreover, these exchanges ultimately
affect the productivity and behavior of terrestrial, freshwater, and marine
ecosystems.
The purpose of this chapter is to describe the crucial research questions,
to identify the types of experiments to be conducted, to assess the availabil-
ity of existing data, and to identify the locations and types of ecosystems
that should receive the highest priority for immediate attention. As such,
the recommendations are more specific than those found in the report of the
Committee on Earth Sciences (1989), but less specific than some research
plans, e.g., the International Global Atmospheric Chemistry (IGAC) pro-
gram (Galbally, 1989~.
The research needs discussed in this chapter are intended to be comple-
mentary to IGAC, a core project of the IGBP, and to address a crucial gap
in understanding the fluxes of trace gases and materials to and from terres-
trial systems. The focus of IGAC is principally on global atmospheric
chemistry, with plans currently under development to include in the program
the study of terrestrial sources of trace gases.
RESEARCH NEEDS
Trace Gases
Carbon Dioxide
Atmospheric carbon dioxide concentrations have been rising at 0.4 to 0.5
percent per year, apparently faster than ever before in the earth's history.
Recently, they have increased even more rapidly. During the last decade,
these increases have been associated with increasing amplitude of the an-
nual cycle of atmospheric carbon dioxide and possibly surface air tempera-
ture of the earth. It is necessary to know the causes and effects of the
accelerated rate of increase in atmospheric carbon dioxide, because it is a
radiatively active greenhouse gas that has contributed to global warming
and will continue to do so and because it has direct effects on ecosystems.
Research Priorities. The following research questions, listed in approxi-
mate order of priority, must be addressed to determine the causes and con-
sequences of increasing atmospheric carbon dioxide. The priorities reflect
the perceived importance of each research topic in reducing the uncertainty
with which we can predict future changes in carbon dioxide. Chapter 7
addresses ocean-atmosphere interactions.
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RESEARCH STRATEGIES FOR THE USGCRP
· How might climatic warming and associated changes in precipitation
and nutrient status alter the biological carbon storage in ecosystems, espe-
cially those with large pools of stored soil carbon, through the redistribution
of terrestrial ecosystems and through effects of enhanced carbon dioxide
concentrations? Profitable approaches to carbon dynamics and global bud-
get will be field and laboratory experiments, whole-ecosystem manipula-
tions, and modeling, especially in tundra, boreal forest, and peat bog ecosystems,
where there are large stores of organic carbon and where relatively large
temperature changes may occur.
· How do increased atmospheric carbon dioxide and associated changes
in moisture, temperature, and nutrients affect plant litter quality and the
associated changes in soil respiration and nutrient mineralization? What are
the effects of the resulting changes in nutrient availability on plant and
microbial processes and on the sensitivity of intact ecosystems to enhanced
carbon dioxide? This issue is best approached with a combination of field
and laboratory experiments. These studies should be done in a range of
ecosystems (e.g., wet versus dry and fertile versus infertile) where the strength
of feedbacks between nutrient cycling, litter quality, and plant response to
carbon dioxide might be expected to differ. The role of soil nutrient status
is important in the context of global change, because industrial and agricul-
tural pollution have dramatically increased the nitrogen availability of some
ecosystems. Interactions of water availability and carbon dioxide fertiliza-
tion must be studied, because projected climatic changes will involve changes
in both parameters.
· How do ecosystem processes and different functional groups of plants
(or specific key species) belonging to different ecosystems respond directly
to changes in carbon dioxide and temperature in terms of rates of photosynthesis,
allocation and net carbon balance, and indirectly in terms of competitive
ability and such secondary processes as resistance to pathogens and herbivores?
How are these carbon dioxide responses altered by interactions with other
environmental stresses (e.g., drought, ozone, and nutrients)? Which ecosystems
are the most sensitive? This research item differs from the item above in
that it is plant-oriented rather than ecosystem-oriented and, as such, requires
experiments at the level of individual plants.
· How would altered hydrological regimes predicted by global climatic
models affect ecosystem carbon balance through changes in productivity
and respiration in the short term and the characteristics of and the geo-
graphical distribution of ecosystem types in the long term? This issue must
be approached through modeling and field and laboratory experiments.
· Why don't the perceived sources and sinks match the interhemispheric
carbon dioxide studies? Much is known about the major sources and sinks
for carbon dioxide and the global pattern of carbon dioxide transport in the
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TERRESTRIAL TRACE GAS AND NUTRIENT FLUXES
171
atmosphere, but currently the perceived sources and sinks for carbon diox-
ide do not match the interhemispheric carbon dioxide gradient. In addition,
the processes affecting trace gases (e.g., carbon dioxide, methane) in the
paleoecological record must be reconciled with current understanding of
carbon dioxide sources and sinks (see chapter 3~. These issues should be
addressed through the continuation and expansion of current programs to
collect information that will most effectively differentiate among possible
sources and sinks of carbon dioxide, e.g., enhanced plant growth and soil
organic matter accumulation, fossil fuel burning, tropical burning associated
with land clearing, and enhanced decomposition in boreal ecosystems. An
expanded network for measuring atmospheric carbon dioxide and detailed
isotopic measurements are needed to localize the major current sources and
sinks for atmospheric carbon dioxide and to validate models that deal with
the seasonal effects of terrestrial vegetation on atmospheric carbon dioxide.
What are the consequences of landscape conversions, such as that of
tropical forest to grassland, in terms of changes in stored soil carbon,
evapotranspiration, energy balance, carbon balance, and nutrient status? Field
measurements in appropriate ecosystems and modeling are needed to ad-
dress this question.
· What is the effect of climatic change on episodic events such as fire
frequency, the amount of carbon released, the resultant change in vegeta-
tion, and consequent changes in albedo, evapotranspiration, and plant pro-
duction? How would the effects of human activities relate to those caused
by climatic change? These effects should be addressed with satellite monitoring
of disturbances such as fires and then related to surface moisture, tempera-
ture, and biomass. Patterns in natural and modified savannas, the boreal
forest, and the tropics are of particular interest.
· What are the pools of biomass and soil carbon, net primary produc-
tion, and ecosystem respiration in the world's ecosystems? All of the con-
siderable field data need to be adequately collated and related to vegetation
and soil maps for inclusion in global climate models. New data must be
acquired by remote sensing of surface temperature, surface moisture, atmo-
spheric water vapor concentration, and indicators of vegetation production
and biomass.
All of these research questions address carbon balance at the ecosystem
or global level and therefore are readily incorporated into ecosystem, re-
gional, and global models. The major challenge will be designing models
and experiments that link studies at the ecosystem level with inputs and
predictions at regional and global levels (see chapter 5~. It is important to
recognize that as climate changes, so will the structure and species compo-
sition of these ecosystems. Thus models of ecosystems for today's circumstances
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RESEARCH STRATEGIES FOR THE USGCRP
will be inadequate for ecosystems for tomorrow, and therefore the models
and analyses must be adaptable to changing ecosystem characteristics. The
evolutionary approach described in chapter 2 is critical to success.
Methane
The concentration of methane is increasing in the atmosphere at a rate of
about 1 percent per year and has approximately doubled in the past few
hundred years. Methane is a greenhouse gas that, on a molecule-for-mol-
ecule basis, is about 20 times more effective than carbon dioxide in trapping
heat. In addition to its role as a greenhouse gas, methane is an important
sink for the hydroxyl radical in the atmosphere. The hydroxyl radical is the
primary agent responsible for the oxidation and subsequent removal from
the atmosphere of many reduced radiatively, chemically, and biologically
important atmospheric gases. Depending on atmospheric nitrogen oxide
concentrations and other chemical parameters, methane increases can change
the atmospheric concentrations of the hydroxyl radical and hence change
the atmospheric lifetimes and concentrations of several important gases,
which would lengthen the time over which a species like methane contributes
to radiative forcing of the climate system. Also, methane is an important
source of water vapor in the stratosphere, and increases in stratospheric
water vapor can have other significant global consequences.
Although the major sources of atmospheric methane are for the most part
known, there is great uncertainty about the relative importance of these
sources and which combination of sources and sinks is responsible for the
rapid buildup of this gas in the atmosphere. The mechanism of methane
production is fairly well known and results from anaerobic microbiological
processes in wetlands, rice fields, and ruminants. Less well known are the
environmental, physical, and biological processes that control the release of
methane to the atmosphere. There are also major uncertainties about the
anaerobic and aerobic sinks of methane in soils and sediments. One of the
major questions that needs to be addressed is how changes in climate (e.g.,
warming in the northern high latitudes) may affect the global methane cycle.
Methane sources in the tundra and wetland regions of the subarctic are
major natural sources of atmospheric methane. A better understanding of
ecosystem processes that control methane fluxes to and from the atmo-
sphere and the impact that environmental changes may have on these processes
is required if reasonable predictions of future atmospheric concentrations of
methane are to be made. In addition, much of the methane in the high-
latitude north is sequestered in permafrost and sediments as clathrates, which
could serve as very significant sources of atmospheric methane if warming
occurs. Similarly, a rise in sea level or warming of the oceans could also
release marine clathrates. While this methane sink has been identified, its
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TERRESTRIAL TRACE GAS AND NUTRIENT FL=ES
173
characterization, e.g., the temperature dependencies of the chemical reac-
tions, needs to be better quantified. Significant research is in progress on
methane, and new studies should be coordinated with projects proposed by
NASA, the International Global Atmospheric Chemistry Program, and IGBP.
Research Priorities. The research activities required to better define bio-
geochemical budgets and cycling of methane were detailed in a recent Dahlem
conference (Schimel et al., 1989~. The major research activities required
under the USGCRP are as follows:
Process studies that relate methane production, consumption, and fluxes
to environmental parameters, to human activities such as burning and live-
stock farming, and to changes in ecosystem structure and function need to
be conducted for ecosystems of known or potential methane sources (e.g.,
wetlands, tundra, rice agriculture, and landfills). The study of the response
of high-latitude northern ecosystems to environmental change should be
studied through large-scale manipulative field experiments. In major rice-
growing areas (e.g., India and China) the effect of cultivation practices on
methane production, destruction, and atmospheric fluxes requires attention.
Also, atmospheric pollutants could affect trace gas fluxes. Thus process
studies must include interactions with pollutants.
Improved instrumentation for the direct measurement of methane fluxes
over small- and large-scale regions must be developed in order to improve
our understanding of the relationship between fluxes and ecosystem processes
and dynamics. Emphasis should be placed on the integration, or scaling, of
information obtained from simultaneous chamber, tower, and aircraft flux
measurements.
.
Better spatial and temporal coverage of atmospheric methane concen-
trations and isotopic composition (carbon and hydrogen) in source regions
must be obtained in order to apportion the global sources of atmospheric
methane and understand the ecological and environmental controls of meth-
ane releases to the atmosphere. More extensive studies of isotopic compo-
sition of methane as a function of source and production and destruction
processes should be made in order to use atmospheric isotopic information
to better understand the biogeochemical budgets and cycle of methane. With
such data it will be possible to more accurately model and determine the
regional fluxes of methane to the atmosphere.
· In order to fully understand the atmospheric methane cycle, improved
estimates of the atmospheric oxidation by the hydroxyl radical must be
obtained. This requires a more complete understanding of atmospheric
photochemistry than is currently available. Specifically, it is necessary to
either directly or indirectly determine the concentration of hydroxyl radical
in the atmosphere and the chemical processes that control this concentra-
tion. Thus it will be necessary to determine if a portion of the methane
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RESEARCH STRATEGIES FOR THE USGCRP
increase results from a reduction in hydroxyl radical concentrations (and
hence a diminished oxidizing capacity of the atmosphere) through increases
in the atmospheric concentrations of hydroxyl radical sinks, e.g., methane,
carbon monoxide, and volatile organic compounds.
Volatile Organic Compounds
Depending on oxides of nitrogen concentrations, a number of volatile
organic compounds (VOCs) are photochemical sources or sinks of tropo-
spheric ozone a toxic gas and a greenhouse gas and as such they may
play a significant role in global warming (see the section "Tropospheric
Ozone" below). In addition, VOCs compete for oxidation by the hydroxyl
radical with other atmospheric species, in particular methane; changes in
the VOC budget could therefore affect the methane budget. The importance
of the terrestrial biosphere as a source of VOCs is still poorly understood
(Logan, 1985~. Only a fraction of the large number of biogenic VOCs have
been identified in the atmosphere, and few data on emission rates are available.
To identify and quantify the role of VOCs in atmospheric processes, it
will be necessary to establish a comprehensive inventory of biogenic VOCs
in the atmosphere, their emission rates from different types of ecosystems,
and the environmental variables determining these emission rates. Addi-
tional studies of the chemistry of biogenic VOCs need to be made in the
laboratory.
Research Priorities. The following research needs are listed in order of
. .
prlorlty:
Accurate techniques for identifying and measuring individual and cu-
mulative VOCs fluxes and atmospheric concentrations (down to the pptv
range) must be developed. A top priority should be to develop analytical
instrumentation that can be operated from aircraft or better sampling and
Reconcentration techniques, since concentrations seem to be affected by
sample storage. Once such instrumentation is available, large-scale field
studies of atmospheric concentrations should be conducted to determine the
regional and continental budgets of biogenic VOCs. Particular focus should
be placed on tropical and mid-latitude forests, as biogenic VOCs may be
strong modifiers of atmospheric photochemistry over these regions (Logan,
1985; Tingey et al., 1979~.
· Improved measurements of fluxes are needed. The two methods cur-
rently used are (1) branch enclosure measurements and (2) inversion of
measured atmospheric concentrations using chemistry-transport models. These
methods have provided valuable information, but they are not fully satisfac-
tory. Branch enclosure measurements are intrusive, and the resulting emission
data will be biased to the degree that the biological functioning of the
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TERRESTRIAL TRACE GAS AND NUTRIENT FLUXES
189
mechanisms that control production and deposition and for predicting the
changes in controlling factors and the resulting fluxes that may accompany
different types of global change.
For considerations of global changes in trace gases, it may not always be
necessary to produce detailed, mechanistic models at this level. For ex-
ample, for gases of primarily anthropogenic origin and readily identifiable
sources, mesoscale models may suffice. But at least for some gas species
with unknown biogenic sources and/or with moderate half-lives in the at-
mosphere, detailed studies of the near-surface dynamics will be essential.
A likely example is carbon monoxide, which appears to be produced in
significant quantities in at least some tropical forests and to exhibit vertical
changes in concentration between the soil and the canopy.
Mesoscale-Fluxes within patches of similar ecosystem type. At the
scale of approximately 1 km, it should be relatively easy to use remotely
sensed and ground-based data to classify ecosystem types, including heavily
human-modified ones such as different kinds of agricultural, suburban, and
urban systems. It should also be practical, for example, to monitor spatial
and temporal variation in gas concentrations at this scale (i.e., in the lower
atmosphere above the vegetation). What is needed are predictive process
models to characterize the sources, sinks, chemical transformations, and
fluxes of gases and other materials that occur within three-dimensional cells
at this scale. Moreover, we need to know, for example, how much biologi-
cal detail is needed to characterize fluxes from physiognomic types of veg-
etation.
Figure 6.3 illustrates the five essential components of a mesoscale model:
(1) vertical exchange with the soil, water, or vegetation that covers the
earth's surface (inputs characterizing these production and deposition pro-
cesses come from the microscale models, appropriately aggregated if necessary
to account for surface heterogeneity); (2) vertical exchange with the upper
atmosphere; (3) horizontal exchange, via wind, with adjacent patches of the
same or different ecosystem type; (4) circulation and chemical reactions
within the cell that affect the concentration and flux; and (5) environmental
forcing functions, such as changes in vegetation, temperature, cloud cover,
or the concentrations of other materials, that are likely to change the dy-
namics of the gas or nutrient species in question.
Models must be customized to account for the unique features of each
gas nutrient or pollutant species. The local production and deposition com-
ponents and the circulation and chemical reactions within the cell will tend
to be species specific.
· Mesoscale and macroscale Linking trace gas process models to earth
system models. The final state in modeling global fluxes is to aggregate the
mesoscale cells and incorporate the trace gases and other materials into
atmosphere-biosphere interface models. This is necessary not only to account
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190
RESEARCH STRATEGIES FOR THE USGCRP
exchange with
the upper atmosphere
input loss
horizontal
exchange
import
export
~ /
I \
exchange production
with the surface I
a
/
internal circulation
and
chemical reactions
-
-
\
environmental
forcing functions
l
1
1
~1 1
---1-
dep:
Finks
FIGURE 6.3 Main ingredients of a mesoscale model for trace gas fluxes.
for regional variations in concentration and long-distance transport of an-
thropogenic gases and other materials, but also to understand the global
fluxes of any species for which the primary sources and sinks may be
spatially isolated (e.g., between tropical forests and oceans).
The problems of predicting atmospheric circulation on a scale of ap-
proximately 100 km have not been solved, but there is currently a major
research effort to improve and test GCMs. These models can be used
(Matthews and Fung, 1987) to predict the large-scale dispersal of trace
gases if atmospheric reactions are included. A much more difficult problem
would seem to be the development of techniques for aggregating the outputs
of mesoscale models over heterogeneous landscapes (see chapter 5) to obtain
accurate inputs for the GCMs. For example, construction of an atmospheric
boundary layer model should include (1) drag coefficients that vary with
topography and vegetation especially when topographic variation is relatively
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TERRESTRIAL TRACE GAS AND NUTRIENT FLUXES
191
small, (2) turbulent exchange coefficients that depend on these drag coeffi-
cients and on the thermal structure of the lower atmosphere, and (3) moisture-
balance equations that depend on current and changing vegetation characteristics
(Walker et al., 1990~.
High priority should be placed on developing mesoscale models of trace
gas flux, and then aggregating or synthesizing them over space so that they
can be used as inputs into GCMs. More work on this type of predictive
modeling is required, and for the full suite of gases and materials of inter-
est. To avoid doing such modeling in an information vacuum, it will be
necessary to accompany this effort with systematic monitoring of spatial
and temporal variation in trace gas concentrations and material fluxes as a
function of ecosystem type and with initial descriptive models that quantify
the environmental correlates of this variation.
Lower priority should be placed on developing microscale and macroscale
models, because there is already considerable effort at these levels. But the
pace of research at micro- and macro-levels must also be increased if we are
to produce predictive models of global trace gas fluxes in time to deal with
these and other pressing problems of global change.
Instrumentation for Measuring Fluxes
One of the current key limitations in formulating a predictive under-
standing of global processes is the inability to measure unequivocally the
abundances of many trace species that are centrally involved in those processes.
This is particularly true for the measurement of chemical fluxes (emission
or deposition), where the number of gas species for which it is generally
accepted that reliable techniques exist are only a few.
A wide variety of emission sources, deposition surfaces, and chemical
species are involved in global fluxes. Natural emissions of chemically or
climatically important compounds occur from terrestrial and oceanic sources
(e.g., nonmethane hydrocarbons and methane from wetlands). Deposition
surfaces that figure strongly in major removable processes range from veg-
etative uptake (e.g., of carbon dioxide) to physical attachment (e.g., nitric
acid depositing on wetted soils). The chemical variety of the emitted or
depositing compounds (inert species and reactive radicals) implies that the
likelihood of even semiuniversal detectors is unlikely.
Flux measurements of trace gases (molecules per unit area per unit time)
require a determination of the atmospheric concentrations over time. Mea-
surement of low concentrations of many chemical compounds requires highly
sensitive detectors and rigorous analytical quality assurance. The need to
obtain a representative flux from a large spatial area generally implies use
of remote or aircraft sensing. Many natural emissions are quite sensitive to
moisture, temperature, and other such factors, thereby introducing substan
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RESEARCH STRATEGIES FOR THE USGCRP
tial spatial and temporal variability not well studied in most contemporary
investigations. Because of the challenges that flux measurements pose and
because of the necessity to substantially improve the current capabilities, a
focused program for new methodologies and instrumentation is needed.
To date, flux measurements have been made in enclosures, along gradi-
ents, and via eddy correlation. The enclosure method establishes the flux
from a small area based on increases in concentration of the compound in
the container. The gradient method generally employs towers to determine
differences in the target compound or element across a spatial gradient
(e.g., as a function of altitude, in conjunction with meteorological analy-
ses). The eddy correlation method, often used with towers or aircraft,
relates small-scale concentration variations to variations in air motion. The
usefulness of these methods depends on the research question and the scale
of the investigation.
The key to success in the gradient and eddy correlation methods is the
availability of rapid-response sensors. Detectors are needed that can make
reliable measurements of the concentration of a species at the part-per-
trillion level and with a measurement rate of less than once per second.
Thus the development of new physical and chemical sensors with those
characteristics is the key to improving the status of flux measurements.
Improvements and new innovations in instruments that measure more than
one species simultaneously are especially needed, since covariation pro-
vides key insight into the biogenic processes involved.
It is imperative that rigorous intercomparison experiments precede the
widespread and large-scale application of flux measurement techniques. The
atmospheric chemistry community has developed an approach that provides
a valuable unbiased indication of measurement capabilities. The main fea-
tures of the experiments that have proved the most informative are the
following:
· several different methods for measuring the same species are involved;
· "mature" instruments (i.e., those that have been used in published
investigations) are compared;
· measurements are made at the same time and place and under typical
and documented environmental conditions, insofar as possible;
· the expected accuracy and precision are hypothesized in advance of
the intercomparison; and
· all results and conclusions are published in the open literature.
CROSS-CUTTING ISSUES
Trace gas sources and sinks are affected by the intrinsic characteristics
of ecosystems, by changes in land use, and by changing climatic conditions.
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TERRESTRIAL TRACE GAS AND NUTRIENT FLUXES
193
Understanding both the reservoirs and the fluxes under these three condi-
tions is necessary for documenting and predicting global change. These
studies require careful stratification to ensure a parsimonious set of mea-
sured conditions with the greatest experimental efficiency. Thus trace gas
studies will profit from a comprehensive experimental design that addresses
several gases simultaneously. Similarly, in many cases nutrient flux studies
can be conducted with trace gas studies. Finally, as discussed in chapter 5,
water-energy balance of the biosphere will require instrumented watersheds.
These, too, can be combined with the trace gas and nutrient flux studies.
A global data base of direct measurements of trace gas fluxes to and
from ecosystems is not achievable in the foreseeable future. No technology
is available that would allow such measurements to be made remotely from
satellites, and global surveys using ground-based or aircraft platforms would
involve tremendous costs and logistical difficulties. The best approach at
this time for constructing a global data base of trace gas fluxes is to map the
environmental variables known to regulate those fluxes from each ecosys-
tem. The functional dependences relating trace gas fluxes to these variables
can then be quantified by ground-based and aircraft studies focusing on
specific ecosystems, and the resulting data can then be aggregated as appro-
priate. Laboratory and small-scale experiments must be conducted for the
purpose of relating trace gas fluxes to input variables that can be measured
via remote sensing techniques.
Since measuring flux rates directly is difficult at global scales and grids
of concentration data are much more feasible, it will be necessary to de-
velop mathematical methods for inverting from concentration data to flux
rates, and to be able to do so at local to regional to global scales. Additional
constraints on the inversions may be derived from remote sensing and the
development of large-scale soil and land use data bases. Also, there is a
need to refine statistical techniques that identify adequate sample sizes in
relation to the cost of acquiring data and the required sampling intensity.
Large manipulation experiments will be necessary under selected condi-
tions that represent important types of ecosystems, e.g., agricultural sys-
tems. In other instances, where the initial conditions are variable and het-
erogeneous, comparative measurements may be more reasonable. Moreover,
careful analysis will be required of existing data, both to synthesize what is
already known and for designing efficient experiments. Connecting large-
scale manipulation experiments with experimental Regional Research Cen-
ters will contribute to research economy and assist in the extrapolation of
results to regional and global scales.
Nutrient transfer studies measuring the lateral fluxes of nutrients should
be organized to include hierarchical descriptors of land use arrangements
and other driving variables. Most of the experimental conditions are in
place, and thus the challenge is to establish the field measurements and not
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RESEARCH STRATEGIES FOR THE USGCRP
to develop new large-scale experimental conditions. The most difficult step
in some of the studies will involve the prediction of how these fluxes will
change with alterations in the regional and global climate. Thus the sce-
narios for global climatic change must be solidified as a basis for these
experiments and for subsequent models.
Great economies can be achieved by careful coordination of the nutrient
transfer and Face gas flux measurements. Making the measurements at the
same sites will minimize logistic expenses and assist in the development of
microscale and mesoscale models and of correlative indicators of system
dynamics and responses.
In many instances, the first step in these studies will be to develop initial
models to determine unknown parameters and to identify the experiments
most likely to yield critical important information. Thus models will be
important at all stages of the studies, from beginning synthesis of known
information and experimental design to final synthesis of new information
and for scaling among time and space scales.
These studies on trace gases and the fluxes of materials involve a wide
spectrum of traditional disciplines and will require a significant number of
scientists over one to two decades. In addition, the investigations will
depend on a thorough understanding of human systems, especially in terms
of land use practices. Thus the scientific community must be sure that there
are educational programs that include this wide spectrum of disciplines.
There must also be undergraduate and graduate programs that encourage the
best of our students to participate in these studies.
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Representative terms from entire chapter:
terrestrial ecosystems
