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Published online 20 February 2008
Published in J Environ Qual 37:599-607 (2008)
DOI: 10.2134/jeq2006.0445
© 2008 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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Soil–Atmosphere Trace Gas Exchange in Semiarid and Arid Zones

Ian E. Galballya,*, Wayne V. Kirstineb, C. P. (Mick) Meyera and Ying Ping Wanga

a CSIRO Marine and Atmospheric Research, P.B.1 Aspendale Victoria, Australia 3195 & CSIRO Marine and Atmospheric Research & Cooperative Research Centre for Greenhouse Accounting, P.B.1 Aspendale Victoria, Australia 3195
b School of Applied Sciences and Engineering, Monash University-Gippsland, Northways Rd, Churchill Victoria, Australia 3142

* Corresponding author (ian.galbally{at}csiro.au).

Received for publication October 12, 2006.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Trace Gas Exchange in...
 Conclusions
 REFERENCES
 
A review is presented on trace gas exchange of CH4, CO, N2O, and NOx arising from agriculture and natural sources in the world's semiarid and arid zones due to soil processes. These gases are important contributors to the radiative forcing and the chemistry of the atmosphere. Quantitative information is summarized from the available studies. Between 5 and 40% of the global soil–atmosphere exchange for these gases (CH4, CO, N2O, and NOx) may occur in semiarid and arid zones, but for each of these gases there are fewer than a dozen studies to support the individual estimates, and these are from a limited number of locations. Significant differences in the biophysical and chemical processes controlling these trace gas exchanges are identified through the comparison of semiarid and arid zones with the moist temperate or wet/dry savanna land regions. Therefore, there is a poorly quantified understanding of the contribution of these regions to the global trace gas cycles and atmospheric chemistry. More importantly, there is a poor understanding of the feedback between these exchanges, global change, and regional land use and air pollution issues. A set of research issues is presented.

Abbreviations: P, precipitation • PET, potential evapotranspiration • WFPS, water-filled pore space


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Trace Gas Exchange in...
 Conclusions
 REFERENCES
 
HUMAN changes to the Earth's biosphere have profoundly influenced the concentrations of climatically active and other trace gases. The atmospheric effects of deforestation, the expansion of ruminant animal numbers, and some other changes to the biosphere are well quantified. In other cases, such as the loss of vegetation and soil organic matter from the unsustainable use of semiarid regions, the atmospheric consequences are poorly understood. The purpose of this paper is to review and analyze the contribution of agriculture and natural processes via soil–atmosphere exchange in the world's semiarid and arid zones to global atmospheric composition and change.

Trace gases are defined as those present in the atmosphere in minute quantities but having a major role in climate and atmospheric chemistry. In this review the focus is on methane (CH4) and nitrous oxide (N2O), which are major greenhouse gases undergoing substantial changes in their atmospheric concentrations over the last century, and nitric oxide (NOx) and carbon monoxide (CO), which are important components driving the chemistry of the lower atmosphere. Carbon dioxide and volatile organic compounds are omitted from this review—CO2 being a greenhouse gas and volatile organic compounds being the other major component driving tropospheric chemistry—because the primary drivers for the exchange of these gases in semiarid and arid zones are not soil–atmosphere exchange.

There are two ways of classifying semiarid and arid areas. One is based solely on precipitation (P), and the other is based on the ratio of precipitation to potential evapotranspiration (PET). Under the first approach, semiarid and arid regions have an annual precipitation of <300 to 400 mm. Under the second approach, the ratio of P/PET, called the "degree of aridity" (UNESCO, 1977), is used.

The definitions of the semiarid, arid, and hyperarid zones (using the second approach mentioned previously); the areas covered as a fraction of the global land area; and the populations of these areas are summarized in Table 1 . Together, the semiarid and arid zones cover about 4.0 x 109 ha globally or about 27% of the global land area (Leemans and Kleidon, 2002), and a further 6% of the global land area is in the hyperarid zone, which is subsequently included in the arid zones. About one fifth of the world's human population lives in these semiarid and arid regions.


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  		Table 1. Areas and populations of the world's semiarid and arid zones defined using the classification of precipitation (P) divided by potential evapotranspiration (PET) (UNDP/UNSO, 1997). The hyperarid zone is included for completeness.

 
The vegetation and soils of these semiarid and arid regions are fragile and are prone to desertification, particularly through the loss of soil nutrients and soil organic matter and through the invasion of woody shrubs (Leemans and Kleidon, 2002). The contribution of these arid zones to the global cycles and budgets of trace gases and the changes effected by land use modifications in these regions have not been extensively researched. This is a climatological land classification. Within any of these zones, the full range of land cover types (and similarly land uses) from barren to forest can occur, although shrub land and sparsely vegetated land cover prevail in the arid zone. The semiarid and arid zones vary from the cold steppe grasslands of central Asia to the hot sandy deserts of northern Africa.


   Trace Gas Exchange in the Semiarid and Arid Zones
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Trace Gas Exchange in...
Conclusions
 REFERENCES
 
The observations and models of trace gas exchange from semiarid and arid land systems are reviewed and discussed, each gas separately, in this review. The authors have attempted to identify all the relevant literature, although some papers may have been overlooked. When considering the semiarid zones, only studies from sites with precipitation <400 mm per annum are included. Data from an unpublished year-long field study of mature native vegetation and a nearby wheat crop in the semiarid zone of Australia (34.5° S, 141.5°E; annual rainfall 275 mm; P/PET = 0.1) conducted by the authors (Galbally et al., unpublished) is included in the review.

Methane
Methane (CH4) is a potent greenhouse gas, and its abundance in the atmosphere makes it a significant component of the global carbon cycle. Soils are a significant sink for CH4 and account for a global annual uptake of 30 Tg yr–1 of a total sink of 586 Tg yr–1 (Prather et al., 2001). There may have been a change in the global CH4 uptake rate of soils during the last century due to the changing atmospheric concentration of CH4 and the effect of land use change on this uptake rate (Ojima et al., 1993).

There are seven extensive studies of soil uptake of CH4 in semiarid and arid zones (Mosier et al., 1991; Striegl et al., 1992; Mosier et al., 1996; Mosier et al., 1997; Mosier et al., 2002; Wang et al., 2005; Galbally et al., unpublished data). Methane uptake in soils occurs first through CH4 diffusion into soils and then through its consumption by soil microorganisms. Methanotrophic microorganisms (methane-oxidizing bacteria) that live in soils are able to use CH4 as a source of energy and carbon (Le Mer and Roger, 2001). The controlling factors on this CH4 uptake, as summarized by Del Grosso et al. (2000b), are (i) soil gaseous diffusivity, which transports the CH4 from the atmosphere to the methanotrophic microorganisms within the soil; (ii) soil water content, which at its extreme lower limit causes the microorganisms to cease metabolizing and at its extreme upper limit prevents gas phase diffusion in the soil; (iii) temperature; (iv) soil texture; and (v) agricultural effects.

Although in temperate and savanna systems there seems to be negligible CH4 uptake when soil moisture is below 5% water-filled pore space (WFPS) (Otter and Scholes, 2000) or 2% volumetric water content (Del Grosso et al., 2000b), these systems are adapted to higher soil water contents. Striegl et al. (1992) found CH4 uptake rates of about 4 ng CH4–C m–2 s–1 in dry "desert" (arid) conditions. In their study, soil moisture varied from 0.6 to 12% by weight, and CH4 uptake "was not consistently related to soil moisture and/or soil temperature" (Striegl et al., 1992). Adding water enhanced CH4 uptake. These observations indicate that CH4 uptake continues at low soil moisture content, as was observed in the Australian native scrub at soil moisture contents down to 0.4% by Galbally et al. (unpublished), presumably because some types of methanotrophic microorganisms have adapted to the low soil moisture of semiarid and arid soils. The influence of higher soil moisture content and temperature on CH4 uptake in semiarid lands has been recently documented by Wang et al. (2005). Methane uptake rates are roughly halved in going from 10 to 20% by weight moisture. Uptake rates increased from zero to around 50 µg CH4–C m–2 h–1 when soil temperatures rose from –20 to 25°C (Wang et al., 2005).

The effect of cropping and tillage practice in reducing CH4 uptake is well known in moist temperate systems (Dobbie and Smith, 1996; Jacinthe and Lal, 2005). Methane uptake in semiarid systems is reduced, compared with paired sites, by cropping, N fertilization, and grazing (Table 2 ). The one exception to this is the study of Galbally et al. (unpublished), which is discussed below. Mosier et al. (1997) demonstrated that tillage alone reduced CH4 uptake for the 3 yr that the site was studied after the plowing.


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  		Table 2. Methane uptake rates from extended field studies of soils in semiarid regions.

 
Galbally et al. (unpublished) observed CH4 uptake in winter in undisturbed native vegetation and a paired wheat field. In summer there was CH4 uptake in the wheat field, but some of the sites in the undisturbed native scrub had CH4 emissions that were accompanied by enhanced soil respiration. Because there is CH4 production within the guts of subterranean termites (Zimmerman et al., 1982), the observed CH4 emissions were attributed to subterranean termites who forage the fallen litter, which, in this system, is greatest in summer. The explanation of this phenomenon is that (i) there was ongoing CH4 uptake in soils due to the action of methanotrophic microorganisms; (ii) there was extensive CH4 production by foraging subterranean termites, which produced CH4 as a byproduct of cellulose digestion; and (iii) the termite emissions counterbalanced the soil CH4 uptake producing net uptake or net emissions, depending on the availability of the food supply for the termites. Burning of the above-ground biomass, harvesting, or overgrazing the above-ground vegetation removes a food source for these termites, and cultivation destroys their nests (Black and Okwakol, 1997), thus reducing emissions and tipping the balance in favor of CH4 uptake. This phenomenon has been described for biomass burning and termites in wet/dry savanna soils by Poth et al. (1995). These processes are shown in schematic form in Fig. 1 .


Figure 1
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  		Fig. 1. Schematic of the methane exchange between the atmosphere and soil in semiarid regions in the wet and the dry seasons showing different net exchange associated with different environmental conditions in the dry season. The vertical arrow on the right in each panel is the net contribution determined as the sum of the metanotrophic CH4 consumption and subterranean termite CH4 production within the soil.

 
The CH4 uptake rates from the seven available extended studies in semiarid and arid regions indicate an average CH4 uptake of 6.5 ± 3.6 ng(C) m–2 s–1 (range, 1–15 ng(C) m–2 s–1) (Table 2). The CH4 uptake rates in semiarid regions are lower, on average, than those in forest and savanna systems and are comparable with or slightly larger than those in grasslands and cultivated lands throughout the world (Smith et al., 2000; Boeckx and van Cleemput, 2001). Because of the large area of the semiarid and arid zones (Table 1), as much as 40% of the global soil sink for CH4 may occur in these regions, where little data exist. The paucity of data for this significant uptake of CH4 in semiarid and arid zones has been commented on by Potter et al. (1996a) and Smith et al. (2000).

The processes controlling the CH4 uptake in semiarid and arid zones are soil microbial activity coupled with soil moisture, subterranean termites, and above-ground biomass and its loss through burning, drought, grazing, and harvesting. It is a challenge for current models that incorporate CH4 exchange (e.g., Del Grosso et al., 2000b) to include all of these processes. An even greater challenge is to understand how soil microbial and invertebrate community structure might change in response to various environmental changes and affect global soil–atmosphere trace gas exchange, including CH4 (Schimel and Gulledge, 1998). It seems that methanotrophic microorganisms have adapted in semiarid and arid regions to functioning at much lower soil moisture contents than in moist temperate regions. If climatic boundaries change (as is suggested by Prather et al. [2001]) and land uses change, then there will be a feedback through changes in the global CH4 uptake rate. More critical experimental studies and associated developments in modeling are necessary to understand and quantify such changes in CH4 uptake in semiarid and arid zones.

Carbon Monoxide
Carbon monoxide is central to the chemistry of the background atmosphere. Carbon monoxide is the major reactant with hydroxyl radicals (OH) in the atmosphere, converting them to hydroperoxyl radicals (HO2) and, along with nitric oxide, regulating the ratio of OH to HO2 radicals in the background atmosphere (Seinfeld and Pandis, 1998). This is of primary importance to atmospheric chemistry because the OH radical, sometimes known as the cleansing power of the atmosphere, initiates the oxidation of many compounds of importance in the climate and chemistry of the atmosphere, including CH4, ozone-depleting substances such as hydrochlorofluorocarbons, and dimethyl sulfide (Brasseur et al., 2003).

Total global sources of CO are estimated as 2780 Tg(CO) yr–1 (Prather et al., 2001). The atmospheric CO sources arising from atmospheric oxidation of CH4 and other organic compounds, biomass burning, fossil fuel combustion, and other minor sources are thought to be approximately in balance with the sinks. Globally, the CO emission from soils and above-ground dead plant material is estimated to be 100 Tg(CO) yr–1 (range, 50–170 Tg(CO) yr–1) (Schade and Crutzen, 1999), occurring particularly in the tropical regions including savannas, grasslands, and rainforests. Global soil uptake of CO is estimated to be 190 to 580 Tg(CO) yr–1 by King (1999) and 16 (8–50) Tg(CO) yr–1 by Potter et al. (1996b).

Carbon monoxide is produced in the soil through the thermally and photochemically induced oxidation of humic acids and phenolic materials that are present in the soil and by the decomposition of cellulose, particularly in dead above-ground biomass (Conrad and Seiler, 1985a, 1985b; Schade et al., 1999). The production of CO is positively correlated with the amount of dead above-ground biomass and the organic carbon levels in the soil, and its emission increases exponentially with temperature, rising markedly at temperatures above 35°C. Production of CO increases with sunlight, particularly at UV wavelengths, but is unaffected by soil moisture at low to moderate soil moisture contents (Conrad and Seiler, 1985a, 1985b; Tarr et al., 1995; Zepp et al., 1996; Schade et al., 1999; Gödde et al., 2000). For these reasons, the global semiarid and arid zones are potentially the major regions for soil emissions of CO (Table 3 ).


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  		Table 3. Carbon monoxide emissions from field studies of soils in semiarid regions.

 
Carbon monoxide in the soil can be used as a substrate by soil microbes so that there may be CO uptake. Two studies have demonstrated CO uptake in the field in semiarid regions (Conrad and Seiler, 1982; Conrad and Seiler, 1985b). This CO uptake does not vary substantially with temperature and increases in the transition from a dry to a moist soil. Because the CO production increases with higher temperatures and CO uptake does not, there is an observed strong diurnal variation of net CO exchange, with uptake at night and emissions during the daytime (Conrad and Seiler, 1985b).

In an Australian semiarid study (Galbally et al., unpublished data), where there was a significant seasonal cycle of temperature and solar radiation, low daytime CO emissions were observed in winter, and high daytime CO emissions were observed in summer. The southeastern Australian wheat field and the undisturbed native scrub measurements (Galbally et al., unpublished) are among the highest CO emissions observed, with yearly averaged daytime emissions of 11 ± 18 ng(C) m–2 s–1. Part of the variability may be associated with varying amounts of surface litter on different plots. These emissions have the same temperature dependence as observed by Scharffe et al. (1990). During the daytime, the CO emissions from these Australian semiarid systems are equivalent to 0.1% of the carbon lost from the soil by soil-plus-root respiration. It is unclear, from the few measurements in semiarid and arid zones, whether the soil/litter system is a net source or a net sink for CO.

Land use change and climate change will affect the amount of above-ground dead biomass through changes to plant growth, grazing, and biomass burning and consequently will affect CO emission. These processes are essential ingredients for understanding and modeling CO exchange and atmospheric chemistry in the troposphere (Schade and Crutzen, 1999), particularly over the semiarid and arid zones. There is a need for more measurements and modeling efforts to improve our understanding of CO exchange in semiarid and arid zones.

Nitrous Oxide
Nitrous oxide is a key greenhouse gas and is one of the six main gases covered by the United Nations Framework Convention on climate change. The other important atmospheric role of N2O is that, in the stratosphere, it reacts to produce nitric oxide (NO), which catalytically destroys ozone, thus participating in the regulation of the ozone content of the stratosphere (Crutzen, 1974). The total global source of N2O is estimated to be 16.4 Tg(N) yr–1, of which 7 to 8 Tg(N) yr–1 is from anthropogenic sources and 2 to 4 Tg(N) yr–1 is from agricultural sources (Prather et al., 2001).

In the soil, N2O and NO are produced as gaseous intermediates during the conversion of NH4+ and NO3– by nitrifying soil bacteria and NO3– to molecular nitrogen by denitrifying soil bacteria (Bremner, 1997) (Fig. 2 ). Hence, there is some overlap between this section and the following section on NO plus nitrogen dioxide (NO2). The microbial processes of nitrification and denitrification account for about 99% of the N2O generated in the soil ecosystem (Webster and Hopkins, 1996). The major factors regulating nitrification and denitrification are the availability of the N substrates (NH4+ and NO3–), soluble soil carbon (required for denitrification), temperature, soil oxygen content, and soil moisture (which, along with soil respiration, regulates soil oxygen content). Soil oxygen is required for the aerobic nitrification process, whereas denitrification is anaerobic.


Figure 2
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  		Fig. 2. Schematic of the nitrogen transformations associated with the terrestrial nitrogen cycle and the production of nitrous oxide and nitric oxide.

 
There are several sources of the inorganic N that undergoes nitrification and denitrification to produce NO and N2O. There are N additions as synthetic fertilizer, decomposition of organic matter, or N deposition. There also are symbiotic and asymbiotic N fixation within the soil and N release due to soil disturbance, which can increase the rate of decomposition of soil organic matter and mineralization and organically bound N. Barger et al. (2005) investigated the N input from N-fixing crusts of cyanobacteria and lichens, which are characteristic of some semiarid and arid regions. These crusts can be an important source of N for N2O and NO production that does not occur outside these regions. Both NH4+ and NO3– are important plant nutrients and are commonly applied to the soil as fertilizers to encourage plant growth.

In hot dry climates, when the soil is moistened through rainfall or irrigation, the production of N2O (and NO) happens with a rapidity and intensity that is not characteristic of temperate agricultural systems. A similar phenomenon has been observed with CO2 respiration in hot dry soils (Xu et al., 2004). This emission pulse could come entirely from aerobic nitrifying bacteria. Levels of NH4+ increase dramatically and then decrease to nearly zero as the proliferation of nitrifying bacteria under the moist conditions convert the NH4+ to NO3– and release NO and N2O (Mummey et al., 1994; Matson et al., 1998). However, in spite of semiarid soils being substantially aerobic, there is a persistent pool of denitrifying enzymes that is able to survive the drought and can become activated rapidly after the soil becomes wet, provided there are sufficient carbon and nitrogen substrates available in the soil (Peterjohn, 1991). A flush of microbial activity could deplete the oxygen within the soil and promote N2O (and NO) production by denitrifying organisms during these re-wetting events. The best evidence (from NO studies; see next section) is that nitrification dominates in these regions. The role of ammonia volatilization as a process causing loss of substrate during this wetting of hot dry soils has not been quantified.

There are more than 800 studies of N2O emissions from agricultural systems (Bouwman et al., 2002), and at the time of this review there are 12 studies from semiarid and arid systems, of which eight studies provide a comparison of farmed versus natural vegetation in semiarid regions (Table 4 ). These eight comparison studies indicate a net additional contribution to N2O emissions of 0.07 kg(N) ha–1 yr–1 from cultivation and/or fertilizer compared with the natural land systems (Mosier et al., 1991; Mosier et al., 1996; Mosier et al., 1997; Corre et al., 1999; Xu-Ri et al., 2003; Wang et al., 2005; Galbally et al., unpublished). If these studies are representative of cultivated and fertilized semiarid and arid lands, then about 5% of the global soil emissions of N2O from agricultural disturbance, as given by Prather et al. (2001), may occur in semiarid and arid zones, where little data exist. There are no studies explicitly determining the fraction of applied fertilizer lost as N2O and NO in semiarid and arid areas.


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  		Table 4. Nitrous oxide emissions from field studies of soils in semiarid regions.

 
The aerobic processes controlling the N2O emissions in semiarid and arid zones that occur on re-wetting of dry soil are not characteristic of the temperate zone. Current models of N2O emissions from soils (Wang et al., 1997; Frolking et al., 1998; Del Grosso et al., 2000a; Parton et al., 2001) do not have these processes explicitly included. Understanding this may become more important because of the push for more food production leading to further irrigation of semiarid lands and because one aspect of climate change is suggested to be less frequent, but more intense, rainfall events. These two pressures may drive changes in the global N2O emissions and feedback to climate change, but we cannot adequately quantify or model that feedback because of the lack of understanding of N2O emission processes in semiarid and arid zones.

Nitric Oxide and Nitrogen Dioxide
Nitric oxide and nitrogen dioxide (NO2), jointly described as NOx, are of importance due to their roles in the atmospheric reactive nitrogen cycle, acidification of precipitation, and regulation of the ozone level of the atmosphere (Crutzen, 1979). The major global sources of NOx are fossil fuel combustion, biomass burning, soils, and lightning and total 44 Tg N yr–1 (range, 30–73 Tg N yr–1) (Penkett et al., 2003). The soil source of NOx (which is primarily NO) is estimated at 5 Tg(N) yr–1 (range, 2–20 Tg N yr–1) (Galbally and Roy, 1978; Penkett et al., 2003).

The chief sources of NOx in soils are denitrifying and nitrifying bacteria (for N2O) and the chemical decomposition of NO2 under acidic conditions (Firestone and Davidson, 1989), although on a global scale the latter process is of minor significance (Galbally, 1989). The majority of the emission occurs as NO, and in subsequent discussion NO is used to describe these emissions.

There can be multiple sources of N input in desert soils. Barger et al. (2005) investigated the N input from N-fixing crusts of cyanobacteria and lichens and found that the annual average NO loss varied with fixed N input and accounted for 3 to 7% of the N fixation input. These biological crusts are very susceptible to land use disturbance, including animal grazing and mechanical stress. Also, there are significant distributions of N-fixing plants and trees in the semiarid and arid zones (e.g., Acacia) that provide N input to the system.

In semiarid and arid systems, where soils are well aerated, nitrification is expected to be much more important than denitrification for the production of NO (Smart et al., 1999; Hartley and Schlesinger, 2000). Soil moisture is a critical factor in determining the rate of NO emission. Nitric oxide emissions increase linearly with soil moisture between 5 and 40% WFPS (Hartley and Schlesinger, 2000). The reason for this increase can be associated with the N cycling within the soil, where net mineralization and net nitrification rates increase linearly under the same conditions. The temperature dependence of NO emissions is quite weak in semiarid and arid regions when the soils are drier (Stocker et al., 1993) but is quite pronounced when the soils are moist (2–3% WFPS) (Anderson and Levine, 1987; Hartley and Schlesinger, 2000).

After prolonged hot dry periods, the wetting of dry soils rapidly increases NO emissions by about one order of magnitude; these emissions decay to the original levels over the next few days (Smart et al., 1999; Hartley and Schlesinger, 2000). Hartley and Schlesinger (2000) show a positive relationship between NO fluxes and NH4+ availability. However, denitrification rates in a Chihuahuan desert site were observed to be similar to those measured in temperate and tropical forests (Peterjohn and Schlesinger, 1991), supporting the idea that denitrification may occur in deserts under suitable conditions (Barger et al., 2005). Smart et al. (1999) demonstrated that the NO pulse 1 h after the wetting of a hot dry soil is limited by NH4+ availability and not NO3– or soluble C or both. This indicates that, in their study, denitrification was not the source of the NO pulse. Further studies of this type are needed.

Mosier et al. (2002) found a negative correlation between above-ground biomass (grass) and NOx emissions, which is presumably due to NOx uptake by the biomass. Characteristically, semiarid and arid areas have less above-ground biomass than temperate and tropical humid zones, and hence it is likely that a larger fraction of the soil NOx emissions pass from the soil to the atmosphere in the semiarid and arid areas than in more humid regions. In a 43-mo study using CO2-enriched, open-top chambers, Mosier et al. (2002) found no change in trace gas exchange associated with elevated CO2 concentrations.

There have been six studies of NO emissions in semiarid and arid zones (Anderson and Levine, 1987; Smart et al., 1999; Hartley and Schlesinger, 2000; Mosier et al., 2002; Barger et al., 2005; Galbally et al., unpublished) (Table 5 ). These studies give average NO emissions ranging from 0.06 to 3.5 ng(N) m–2 s–1 and indicate that the semiarid and arid lands could be a major contributor to global soil NO emissions. This has been borne out by the satellite-sensing and inverse modeling study of Jaeglé et al. (2004), who observed strong rain-induced pulses of soil NO emissions lasting 1 to 3 wk after the onset of rain in semiarid sub-Saharan Africa. Jaeglé et al. (2004) extrapolated this finding to all of the tropics and estimated a 7.3 Tg(N) yr–1 biogenic soil NO source, which is perhaps 20% of the global NOx emissions and larger than the previous estimates of the total global NOx emissions from soils (Penkett et al., 2003).


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  		Table 5. Nitric oxide and nitrogen dioxide emissions from field studies of soils in semiarid regions.

 
This NO emission pulse, after re-wetting of dry hot soils, is not included in process-based models of NO emissions. Recently, a statistical model of NO emissions (Yan et al., 2005) has developed an empirical methodology to include this NO pulse activity. There is a need for more surface-based critical studies of NO emissions in the semiarid zone to confirm the satellite observations of Jaeglé et al. (2004). Such studies should seek to provide process understanding so that models may be extended to include this wetting-emission pulse phenomena that significantly affects the soil atmosphere exchange of NO and N2O (and CO2).


   Conclusions
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 NOTES
 ABSTRACT
 INTRODUCTION
 Trace Gas Exchange in...
 Conclusions
REFERENCES
 
The agricultural and natural sources of CH4, CO, N2O, and NOx in the world's semiarid and arid zones and their products through atmospheric chemical transformations are important contributors to (i) the climate and radiation balance of the atmosphere (through gaseous infrared absorbance and scattering of solar radiation) and (ii) the chemistry and oxidizing power of the troposphere through photochemistry and ozone production. The exchanges of these gases in the semiarid and arid zones may make up 5 to 40% of the global exchanges of these gases from these land-based sources and sinks, but these estimates are based in each case on a handful of studies from a few locations.

There is a poor understanding, quantification, and modeling capability for some of the processes that affect these soil–atmosphere trace gas exchanges in semiarid and arid regions, including:

There are larger-scale couplings not addressed in this review that need further understanding. These include the coupling of rainfall and drought, fire, and soil erosion on multi-year timescales, including the consequences of climate change that affect these semiarid and arid zone exchanges and the pressures of global food demand that could see more irrigated cropping in the semiarid lands. New knowledge is necessary to understand how human activities and climate change may affect these exchanges and to model the feedback between them and global climate change, land use, and air pollution issues.


   ACKNOWLEDGMENTS
 
We thank the reviewers of this paper for their helpful suggestions.


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