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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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170 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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172 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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174 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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192 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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194 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. REFERENCES Andreae, M.O. 1985. The emission of sulfur to the remote atmosphere: Back- ground paper. Pp. 5-26 in J.N. Galloway et al. (eds.), The Biogeochemical Cycling of Sulfur and Nitrogen in the Remote Atmosphere. D. Reidel, Dordrecht, The Netherlands. Andreae, M.O., and H. Raemdonck. 1983. Dimethyl sulfide in the surface ocean and the marine atmosphere: A global view. Science 221:744-747. Andreae, M.O., et al. 1988. Biomass burning emission and associated haze layers over Amazonia. J. Geophys. Res. 93:1509-1527. Andreae, M.O., H. Berresheim, H. Bingemer, D.J. Jacob, and R.W. Talbot. 1990. The atmospheric sulfur cycle over the Amazon Basin. 2. Wet season. J. Geophys. Res., in press. Beaulac, M.N., and K.H. Reckhow. 1982. An examination of land use-nutrient export relationships. Water Res. Bull. 18:1013-1024. Billings, W.D. 1987. Carbon balance of Alaskan tundra and taiga ecosystems: Past, present and future. Quaternary Science Reviews 6:1265-1277. Bormann, P.H., and G.E. Likens. 1979. Pattern and Process in a Forested Ecosys- tem. Springer-Verlag, New York.
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Representative terms from entire chapter: