Jpn. J. Limnol., 52, 4, 263-281, 1991

Review

Gas Exchange in Ecosystems:

Framework and Case Studies

Eitaro WADA, John A. LEE, Makoto KIMURA, Isao KOIKE,

William S. REEBURGH, Jose G. TUNDISI, Tadashi YOSHINARI,

Takahito YOSHIOKA, and Margret M. I. VAN VUUREN

Abstract

This report is an extended summary of the symposium on "Carbon and Nitrogen

Cycles in the Biosphere and Geosphere" in INTECOL '90 held at Yokohama on 28

August 1990. The general ideas and aspects of nitrogen and carbon dynamics in

ecosystems are described along with atmospheric nitrogen supply, and behavior of

biogenic gases such as carbon dioxide, methane, molecular nitrogen and nitrous oxide.

Increases of the radiatively active gas concentrations (CO2, CH4, and N2,O) in the

atmosphere are described in connection with human impacts such as energy require-

ments and food demand. The increase in atmosheric carbon dioxide has been caused

by fossil fuel combustion, that is, a man-made short circuit between atmosphere and

C-O-Fe-S cycle. Increase in crop production dominates the fluxes of mineralization

processes of organic matter both in aerobic and anaerobic conditions, resulting in the

enhancement of methane and nitrous oxide production in the whole biosphere.

Several current efforts to investigate atmosphere-biosphere interactions are sche-

matically summarized for site specific phenomenon such as plant-soil associations and

for some representative ecosystems (heathland, rice rhizosphere, tundra and taiga,

lake, tropical reservoir, coastal and estuarine system, and global N2O cycle).

Key words: Oxygen cycle, human impact, atmospheric deposition, biogenic gases

1. Introduction

One among the principal prerequisites for the

evolution and survival of terrestrial life is the

cycling of bioelements under moderate climate.

Atmospheric and water circulations play a

major part in the operation of biogeochemical

cycling on the Earth's surface. Each bioele-

ment such as hydrogen, carbon, nitrogen, and

oxygen completes its biogeochemical cycle as

being major or minor components in the atmo-

sphere and the circulating water. After the

genesis of life, living organisms evolved new

biochemical pathways that transport the bioele-

ment directly between the atmosphere and the

biosphere. The primary highlight was the

appearance of photosynthetic organisms some

3.5 billion years ago. Two biochemical path-

ways, fixation of carbon dioxide and produc-

tion of molecular oxygen, provided new dimen-

sions for the cycling of matter and the evolution

of the atmoshere as known in the present bio-

sphere. The increase in atmospheric oxygen

initiated some 2 billion years ago (BERKNER and

MARSHALL, 1964) and the framework of biogeo-

chemical cycling of carbon, nitrogen, and sulfur

* Symposium organizer

264WADA, LEE, KIMURA, KOIKE, REEBURGH, TUNDISI, YOSIIINARI, YOSHIOKA and VAN VUUREN

was completed by the evolution of microbial

oxidation-reduction pathways of these ele-

ments. Evolution of the biosphere has then

been closely associated with these cycles. The

C-O-Fe-S cooperating geological cycle keeps

the partial pressure of atmospheric oxygen at

its present level (GARRELS et al., 1976).

There is a considerable interest in the level of

the partial pressure of atmospheric carbon

dioxide during the Precambrian period due to

an apparent contradiction between the

continued existence of life and the early, faint

Sun that is predicted by the present models of

solar evolution (KUHN and KASTING, 1983;

WALKER, 1983; NEWMAN and ROOD, 1977). In this

context, carbon isotopic studies of Precambrian

rocks appear to provide a piece of biogeo-

chemical evidence; the atmospheric carbon

dioxide content was two orders of magnitude

higher during most of the Precambrian than the

present level (MIZUTANI and WADA, 1985). Our

atmosphere is the product of the past 3.8 billion

years of life and the biosphere has lived in a

global green house during the continuous exis-

tence of life. The Earth's greenhouse effect

has changed over geologic time and is changing

even today. At present the Earth's surface

temperature is raised about 35'C above what it

would be if no radiatively active gases were

present (HILEMAN, 1989).Since the pioneering work of monthly mea-

surements of atmospheric carbon dioxide con-

centration at Mauna Loa by Charles KEELING

(KEELING et al., 1976), considerable attention

has been paid to the increases in atmospheric

deposition and radiatively active gases originat-

ing from expanding anthropogenic impacts,

which principally result from the increasing

pressure of human population and can be char-

acterized in terms of energy requirements and

food demand. For example, carbon dioxide

and NOX generally originate from fossil fuel

combustion, while methane and nitrous oxdie

are byproducts or end products in the process of

microbial decomposition of organic matter.

Global dynamics of the latter two compounds

are thus closely correlated with agricultural

crop production of which expanding increase

certainly introduces the increasing occurrence

of oxidation-reduction boundaries in cultivated

fields, wetlands, and aquatic ecosystems. The

anthropogenic impacts are manifest as two

manmade short circuits between atmospheric

carbon dioxide and the geological carbon cycle

(fossil fuel combustion) and between atmo-

spheric dinitrogen and the short term biogeo-

chemical nitrogen cycling (artificial N2 fixa-

tion).

In this report the global cycling of bioele-

ments is summarized to address atmosphere-

biosphere interactions with emphasis on current

global environmental problems. Atmospheric deposition, ecosystem processes, and biogenic

consumption and production of gases are

presented for several representative ecosystems involving forests, tundra and taiga systems,

lacustrine environments, coastal and estuary

areas, and open oceans.

2. Biogeochemical cycles

From the biogeochemical standpoint, bioele-

ments are characterized as having stable forms

of molecules in three phases such as gas, liquid

and solid. This fact makes it possible to circu-

late each element through biosphere, hydros-

phere, and atmosphere. In this context, the evolution of biochemical reactions concerning

gaseous substrates and products plays a deci-

sive role in biogeochemical cycling in the pres-

ent biosphere. These biochemical processes

are CO, and N2 fixation, and N2, N2O, CH, and

H,S production. In the history of the bio-

sphere, practical operation of these processes is

accepted to have evolved some 2 billion years

ago when oxygen partial pressure was raised up

to 10-2 of PAL (present atmospheric level).

The accumulation of atmospheric oxgen is

generally accepted by the occurrence of conti-

nental "red beds" (JUNGE et al., 1975). After

the increase in PD, in the atmosphere, opera-tions of all biological processes have been pri-

mary governed by the existence of molecular

oxygen in the atmosphere. At present molecu-

lar oxygen is the most efficient electron accep-

tor in our respiratory energy yielding system

and also determines the stability of chemical

Gas Exchange in Ecosystems265

forms of other bioelements in oxidation-reduc-

tion reactions. In this sense, oxygen places a

terminal on the cooperating biogeochemical

cycles in the present biosphere (Fig. 1).

The cooperation of oxygen in the other cycles

of the bioelements can be expressed as a turn-

over time of atmospheric oxygen as defined by

the following equation:

Turn over time of molecular oxygen in the

atmosphere (Ą, year)

=[total amount of 02 in the atmosphere/

annual rate of a process],

Where the following molar equivalents can be

used in practice; one molar 02=1-0 molar car-

bon=2/5 molar nitrogen=4/3 molar Fee-=1/2

molar sulfur.

Several kinds of organic matter can be classi-

fied in terms of turn over time with respect to

decomposition into gaseous products in the

biosphere. Organic matter via annual primary

production such as grass, crop, and leaf, can be

regarded as representative of a short turn over

time of one year. In many ecosystems almost

all the organic matter produced is decomposed

by consuming equivalent amounts of molecular

oxygen. The Ą(O2) of this process accounts

for ca. 2•~103 (38•~1018/17,000•~1012) years.

Although soil organic matter, dissolved organic

matter in oceans, and peat are not indicated in

Fig. 1, these constitute the second large pool of

organics (320•~1015g atom C, BOLIN et al., 1979)

in the global organic carbon inventory. If we

adopt an average turn over time of 103 years for

this pool (JENKINSON and RAYNER, 1977), then we

can estimate ƒÑ(O2) of 105 years (38•~1018/320

•~ 1015•~10-3) for this group. Molecular oxygen

in the atmosphere has been accumulated

through the geological C-O-Fe-S cycle. The

largest organic pool in sedimentary rocks has

been formed by this cycle, in which net produc-

tion of molecular oxygen can be estimated from

the accumulation of sedimentary organics, as

Fig. 1. Oxygen cycle with emphasis on biogenic gas productions. Numerical values of fluxes are

1012 mole•yr-1. The values are all equivalent to molar O2 Revised after SODERLUND and

SVENSSON (1976), and GARRELS et al., (1976). Data from OHTAKE (1978), and BOUWMAN

(1990) are also included. Nitrification rate was estimated by assuming total inorganic

nitrogen used for annual primary production was nitrate. Details, see text.

266WADA, LEE, KIMURA, KOIKE, REEBURGH, TUNDISI, YOSHINARI, YOSIIIOKA and VAN VUUREN

one molar buried carbon produces one mole of

molecular oxygen. It is now accepted that the

partitioning of photosynthetically produced 02

between free (atmospheric 02) and the bound

reservoir (Fe2O3 plus SO42-) at present is 0.05:

0.95 (SCHIDLOWSKI et al., 1975; WADA, 1990).

The Ą(O2) value by this process is ca. 107 years

(38•~1018/2.5•~1012). Because of the stability

of the triple bond of molecular nitrogen, the

largest pool of nitrogen exists in the atmo-

sphere as a major component. The nitrogen

cycle only cooperates with the oxygen cycle in

the processes of nitrification and denitrifica-

tion. The maximum nitrification rate can be

estimated from nitrogen uptake during global

primary production and its C/N ratio (OHTAKE,

1978). We assume here ca. 60% nitrate utiliza-

tion of the total nitrogen uptake. The annual

nitrification rate is estimated to be ca. 2•~1014g

atom N•yr-1 (WADA and HATTORI, 1991), which

equals to 500•~1012 mole O2•yr-1. A part of

nitrate produced in this process could be incor-

porated into denitrification. r(O2) by the

nitrification then becomes ca. 105 years (38

•~1 018/5•~1014) (Fig. 2).

3. Expanding anthropogenic impacts

One way to outline human impacts during the

recent past is to compare the temporal change

of primary factors with the resulting increases

in atmospheric concentrations of biogenic

gases. Since the changes in one part of the

system are always interlinked with the change

in other part of the cycles, we could not expect

similar figures of the variation among all vari-

ables but could expect qualitative general

trends to occur in the present biosphere. The

main events along expanding anthropogenic

impacts can be represented by the following

processes.

i) Population growth (PG)•¨Energy require-

ment (ER) plus Food demand (FD),

ii) ER•¨Fossil fuel combustion (FFC)•¨CO2

plus Aerosol,

iii) FD•¨Crop production•¨Decomposable

organic matter•¨CO2 plus CH, plus N2O

plus N2 plus burning products, and

iv) ER+FD•¨Deforestation+Cattle•¨CO2

plus CH4

The anthropogenic impacts for the

radiatively active gases (CO2, CH4, N2O) pro-

duction were accelerated after World War II.

The present concentration of the CO, (340

ppm) is currently increasing more than 1 ppm

every year (KEELING et al., 1976, 1979), which is

lower than the percentage annual growth rate

of human population (Fig. 3) (OFITAKE, 1978).

At present, fossil fule burning annually adds

some six billion tons of CO, to the atmosphere.

A nearly equal amount is generated from vege-

tation and soil destruction. Of the COQ

released annually, about 2.5 billion tons of car-

bon remains in the atmosphere (HILEMAN, 1989).

The lower percentage increasing rate of CO2 in

the atmosphere than the population growth

results from a variety of sinks of CO2 such as

terrestrial biota, regrowth of harvested forest,

and the buffering action of the ocean.

Of the biogenic green house gases, the level of

methane in the atmosphere is changing the

fastest (PEARMAN and FRASER, 1988; BOUWMAN,

1990). Each year, an additional 50 million tons

of released methane remains in the atmosphere

representing an annual increase of more than

1%. Main sources of methane are wet lands,

paddy fields, intestinal fermentation and the

pyrolysis of organic matter (biomass burning)

(CICERONE and OREMLAND, 1988). The last three

of these are closely related to the crop produc-

tion. Consequently, the increase in the meth-

Fig. 2. Schematic illustration on residence time of

atmospheric oxygen (Ą(O2)).

Gas Exchange in Ecosystems267

Fig. 3. Temporal variations of atmospheric radiatively active gases, human population, rice crop

production, and fertilizer production. CFCS: chlorofluorocarbons.

ane concentration is correlated with the popula-

tion growth.

Nitrous oxide exhibits a 0.3% annual

increase which is not as fast as methane, rising

from 280 ppb at the turn of this century to 303

ppb at present (RASMUSSEN and KHALIL, 1986; BOUWMAN, 1990). This results from the injec-

tion of 5 million tons of N2O into the atmo-

sphere each year. Its tropospheric turn over

time is rather long (ca. 100 years) as compared

with methane (ca. 10 years). The major

sources of the increased nitrous oxide are fertil-

ized soils, and biomass and fossil fuel burning.

Most of the methane is removed from the tropo-

sphere by oxidation with hydroxyl radicals,

while the only known atmospheric sink for NZO

is its breakdown to NO in the stratosphere by

ultraviolet light (YOSHIDA and MATSUO, 1983).

Differences in the annual increase rate between

methane and nitrous oxide might result from

the different modes of production during de-

composition of organic matter in ecosystems.

The detailed microbial, ecological and biogeo-

chemical studies will be required to elucidate

this point.

Perturbations of human activity on gas

exchange of ecosystems are shematically illus-

trated in Figure 4 with emphasis on fossil fuel

combustion and crop production. At this

point, we would like to emphasize a new dimen-sion to approach this problems. Stable iso-

topes of light elements such as carbon and

nitrogen undergo fractionation in chemical and

biological reactions. Biogenic materials in

global cycles of the light elements have their own isotopic composition, so-called "dynamic

stable isotope (SI) finger print". In principle,

a fluctuation of a molecule's isotope ratios

268WADA, LEE, KIMURA, KOIKE, REEBURGH, TUNDISI, YOSIIINARI, YOSHIOKA and VAN VUUREN

Fig. 4. Schematic illustration of the perturbation of carbon and nitrogen cyclings by the human

impact.

provides information about its origin, path-

ways, and the reaction mechanisms concerning

its formation. On a macroscopic level , SI chemical structure of an ecosystem can provide

a key to solve biogeochemical phenomena

involving transport of organic matter and per-

turbation in an ecosystem. For example, the

CO2 increase in the atmosphere and its carbon

isotope ratio follow a single mixing (KEELING et

al., 1979) and every one ppm of the CO, added

to the atmosphere introduces the ratio changes

of about -0.02 %o (MOOK et al ., 1983). Several examples will be shown in the following case

studies.

4. Atmospheric nitrogen supply and

ecosystem process

During the Twentieth Century deposition of

combined nitrogen from the atmosphere has

increased markedly in many regions of the

Northern Hemisphere. For example estimates

for nitrate deposition show increases between

two and four fold depending on location (LEE, J . A., unpublished).

In many near pristine environments nitrogen

supply can be shown to be a major determinant

of plant growth, and increases in the nitrogen

content of plant litter can accelerate decompo-

sition and mineralization processes. Thus

increases in combined nitrogen deposition from

the atmosphere may be having marked and

widespread effects on ecological processes.

Evidence is now accumulating to support the

view that terrestrial ecosystems are being per-

turbed markedly by anthropogenic nitrogen

emissions. These include ombrotrophic mires

and heathland ecosystems where atmospheric

deposition represents an important source of

Gas Exchange in Ecosystems269

bioelement. In the most polluted regions these

ecosystems can be shown to be adversely

affected by atmospheric nitrogen deposition.

However as nitrogen accumulates in plants and

soil organic matter the potential for latent

change in ecosystems which are less susceptible

to an alteration in the aerial supply of elements

or in less polluted regions becomes more pro-

nounced. This change is difficult to assess, but

may of considerable and increasing potential

importance.

5. Case studies

5-1. The impact of dominant plant species

upon nitrogen cycles in heathland

ecosystems (M. I. MARGRET)

Heathlands in the Netherlands have devel-

oped on sandy soils and used to be dominated

by ericaceous shrubs like Calluna vulgaris or

Erica tetralix (Plate 1). At the beginning of

succession, nitrogen supply through mineraliza-

tion is very low in these ecosystems. However,

the amount of soil organic matter increases by

the input of plant litter, which gradually leads

to higher nitrogen mineralization rates

(Beizexose, 1990). Nowadays, the increased

atmospheric deposition of nitrogen accelerates

the succession process. As a result, there has

been a strong increase of perennial grasses such

as Molinia cacrulea (Plate 2).

The increase in nitrogen availability created

suitable conditions for the spread of Molinia.

However, once this species has become domi-

nant, the nitrogen cycle is strongly affected by

the species itself. The above- and below-

ground biomass production and the amount of litter is greater in Molinia as compared to Erica

or Calluna dominated heathlands (Aioers and

BERENOSE, 1989; Ar;Rrrs et al., 1989). Although

Molinia litter has a relatively low nitrogen

concentration, which leads to a net nitrogen

immobilisation in the first stage of decomposi-

tion, it decomposes much faster than litter

produced by Erica or Calluna due to its relative-

ly low lignin content. As a consequence, a

greater amount of nitrogen will be supplied through mineralization, instead of accumulat-

ing in the soil organic layer. It was concluded

that the dominant plant species have a clear

influence on the rate of nitrogen cycling in these

Plate 1. Erica lctralix.

270WAnA. LjAc, KIMURA, KOIKE, REEBURGH,TUNDISI, YOSHINARI, YOSHIOKA and VAN VIIREN

Plate 2. Molinia caerulea.

ecosystems.

5-2. Production of biogenic gases in rice rhizospheres and their transfer to the

atmosphere (M. KIMURA)The fact that paddy rice grows in the submer-

ged (reduced) environment characterizes the kinds and amounts of biogenic gases produced

in the rhizosphere and their transfer to the atmosphere.

Contrary to the general understanding, the

rice rhizosphere is not always oxidative. It

becomes reduced from the maximum tillering

(MT-) stage in growth, and the biogenic gases

are produced sequentially as growth proceeds;

the production of CO,, and N,O and N, by denitrification occur in the early growth stage

and the denitrification terminates before the

MT-stage. Hydrogen production is observed

around the MT-stage. Methane production

begins after that.

The root system consists of roots of different

ages, and old roots produce the reducing envi-ronment where H, and CH, production and

SO,'- reduction occur. Hydrogen is produced

in/on the roots, and the main sites of CH4

production and SO42- reduction are in the rhizosphere soil. Hydrogen produced in/on

roots is selectively utilized for sulfate reduction

in the rhizosphere.

Thus, the production of each biogenic gas is

different to one another in time and space, and

is controlled by the growth stage of paddy rice

(CICERONE and SHETTER, 1981; CICERONE et al.,

1983; DE BONT et al., 1978; HOLZAPFEL-PSCHORN et

al., 1986; HOLZAPFUL-PSCHORN and SEILER, 1986;

MINAMI and YAGI, 1988; SEILER et al., 1984).

The transfer of CO, and CII, produced in the

rhizosphere is different due to the difference of

solubility to water (Fig. 5).

CO2 is absorbed by rice roots, and transferred

to the shoot and then to the atmosphere

(HR.ucHI et al., 1984; WAUA et al., 1983;

YOKOYAMA and WADA, 1983). Some portion of

absorbed CO, is photosynthesized by the shoots.

The mechanisms of transfer of CO, through

plants is still controversial. Some scientists

consider it to be associated with the flow of

transpiration, and otheres consider it to occur

by diffusion in gaseous state through intercel-

lular spaces of the aerenchyma in the root

Gas Exchange in Ecosystems271

Fig. 5. Schematic illustration showing production of biogenic gases in rice rhizoshere and their

t ransfer to the atmosphere.

cortex.

CO2 remaining in the soil is carried to the

subsoil in the form of HCO3- with the move-

ment of leaching water. Rice plants are

known to influence this CO2 leaching (TSUCHIYA

et al., 1984).

Due to the low solubility of methane in water,

it is not leached downward. It is considered to

be present in bubbles, and it is carried to the

atmosphere partly by ebullition, but there is

evidence that transfers occur mainly through

root-shoot pathway (INUBUSHI et al.,11989).The emission rates of methane from rice

paddies were not altered by cutting the rice

plants above the water level. So the transfer is considered to be due to the gaseous diffusion

272 WADA, LEE, KIMURA, KOIKE, REEBURGH, TUNDISI, YOSHINARI, YOSHIOKA and VAN VUUREN

not due to the transpiration. The main route

of methane emission is considered to be through

old roots to the shoots. Some portion of meth-ane may be oxidized on its way to the atmo-

sphere.

5-3. The role of tundra and taiga systems in the global methane budget (W. S.

REEBURGH and S. C. WHALEN)

Tundra and boreal forest soils contain some

30% of the terrestrial soil carbon reservoir

(Pose, et al., 1982). A large fraction of this soil carbon is immobilized in permafrost and peat.

This soil carbon reservoir could be susceptible to biogeochemical conversion to CO, and CH,

under warmer, wetter climates. Both of these

gases are radiatively active, and could be responsible for a positive climate feedback .

We evaluated the importance of tundra and

taiga systems in the present global CH, budget

and studied processes important in modulating emissions under present and modified climates

with weekly measurements of CH, flux at a set

of permanent sites (WHALEN and REEBURGH, 1988, 1990a). Our CH, flux time series at tun-

dra sites in the University of Alaska Arboretum

covers over 3.5 years and indicates that water table level, transport by vascular plants and

microbial oxidation at the water table are

important in modulating tundra CH, emissions.

Emissions of CH, essentially cease during fro-zen periods. Our study at permanent taiga

sites involves one set of flux measurement in

October, 1989, and weekly measurement from May to October 1990. All taiga sites except

those adjacent a river consumed atmospheric

methane, indicating that these soils are sink for

atomspheric CH4.

Integrated annual emissions from the sea-sonal time series measurements and results

from a detailed survey along the Trans Alaska

Pipeline Haul Road lead to independent esti-mates of the global tundra CH, flux of 19-33

and 38 Tg•yr-1, respectively (WHALEN and

REEBURGH, 1990b; REEBURGH and WHALEN, 1990). The road transect estimate for the global taiga

CH, contribution is 15 Tg•yr -1.Microbial oxidation of methane is an impor-

tant modulator of tundra fluxes (REEBURGH and

WHALEN, 1990; WHALEN and REEBURGH, 1990b).

The process is restricted to the region adjacent

the water table in waterlogged soils, but occurs

at high rates in moist soils, which equilibrate

rapidly with the atmosphere. Methane oxida-

tion is expected to become more important

under warmer, drier conditions. Preliminary

work suggests that the process could not only

control tundra methane emissions, but also

serve as a sink for atmospheric methane (Fig.

6).

5-4. Metabolism of gaseous components in

lacustrine ecosystems (T. YOSHIOKA)

Aquatic ecosystems exchange gaseous com-

ponents resulting from biological activities with

the atmosphere. Concerning their stable iso-

tope ratios, the equilibrium isotope fractiona-

tion on gas exchange between aqueous and

gaseous phases may be the most important step.

However, the content and the isotope composi-

tion of biogenic gases in lacustrine environ-

ments will be governed not only by interations

with the atmosphere, but also by the biological

production and consumption processes of the

gas below the air-water interface, because the

biological activities in lakes largely vary with

time and with water depth.

Stable carbon isotope ratios of dissolved in-

organic carbon (ƒÂ13CDIC) in Lake Suwa, a eutro-

phic shallow lake in Japan (surface area, 13.3

km2; max. depth, 6.8 m), were lower than those

observed for waters at equilibrium with atmo-

spheric CO2, particularly in the summer season

(TAKAHASHI et al., 1990). In eutrophic lakes,

pH values of surface waters often rise about 10

and the partial pressure of CO2 in water

becomes quite low because of the intense photo-

synthesis by bloom forming phytoplankton,

such as Microcystis. Under these conditions,

the S13C,,,c value may be largely affected by the

chemical enhancement of CO2 invasion from

atmosphere, which shows the large negative

kinetic fractionation, compared with the equi-

librium CO2 exchange. The supply of 13C de-

pleted CO2 from respiration and decomposition

processes seem to contribute also to the low 813

CD,G. Since the fractionation of photosynthetic

CO2 assimilation during the summer season in

Gas Exchange in Ecosystems273

Waterlogged-Saturated Soils

Fig. 6a. Schematic diagram showing controls on methane oxidation in waterlogged and saturated

solis. Oxidation occurs in the vicinity of the water table, which is the oxic: anoxic

boundary. Transport of methane is controlled by aqueous phase molecular diffusion.

Methane fluxes to the atmosphere are positive (REEBURGH and WHALEN, 1990).

Moist Soils

Fig. 6b. Schematic diagram showing controls on methane oxidation in moist soils. Oxidation

occurs throughout the moist soil interval. Transport is controlled by gas phase molecu-

lar diffusion. The moist soil interval equilibrates with the atmosphere at rates that are

fast relative to biological consumption rates. Moist soils are capable of consuming

atmospheric methane (REEBURGH and WHALEN, 1990).

274WADA, LEE, KruuRA, KOIKE, REEDURGII, TUNDISI, YOSHINARI, YOSIIIOKA and VAN VUUREN

Lake Suwa was very small, the 13C enrichment

in the remaining DIC seemed to be negligible.

These phenomena are originally due to the

intense photosynthesis by Microcystis in the

euphotic zone of Lake Suwa. During he long

duration of N-deficient condition in Lake

Suwa, the new nitrogen was introduced into the

lake ecosystem through the biological N2 fixa-

tion process by blue-green algae, Anabaena

spp. Since the fixed nitrogen by this process

shows the characteristic low ƒÂ15N value (%0) ,

the flow of the new nitrogen in lake ecosystem

can be traced by the ƒÂ15N values (Fig. 7).

In Lake Kizaki, a mesotrophic lake in Japan

(surface area, 1.4 km2; max. depth, 29.8 m),

both nitrification and denitrification are regard-

ed to be the N20 accumulating processes (YOH

et al., 1988a, b). Preliminary survey showed

that the ƒÂ15N value of N2O seemed to be lower

than that of NO3- in the oxic hypolimnion and

the remaining N03 in anoxic hypolinion had

very high 15N value (WADA et al., submitted;

YOSHIOKA et al., 1988). These observations

suggested that the isotopically light nitrogen

might emit to atmosphere as N20 and N2 dur-

ing the whole overturn period of Lake Kizaki .

The amounts of hypolimnetic N2 and N2O

released to the atmosphere across the lake

surface during the overturn period depend on

the activities of these processes in hypolimnion,

the extent of the anoxic layer, and the timing of

whole overturn of lake, when denitrification

will cease. The nitrogen isotope ratios of in-

organic nitrogen (NO3-, N2O and so on) might

also depend on these factors.

5-5. Gas exchange in tropical aquatic eco-

systems (J. G. TUNDISI and E. WADA)

The number of reservoirs in many part of

tropical forests has increased during recent past

with the purpose of producing hydroelectric

power and water supply. These reservoirs in

Brazil are distributed in a range of geographic

regions with diverse human impacts and pro-

vide excellent opportunities for assessing inter-

actions between environmental factors and

human communities.

Newly constructed dams in the Amazon have

a large flooded forest area where rapid mineral-

ization of organics can take place under rela-

tively high temperatures. Accumulation of

tree leaves and microalgae and dense under-water vegetation produce microanaerobic sites

Fig. 7. Schematic illustration of gas exchange in Lake Suwa .

Gas Exchange in Ecosystems275

near the bottom surface of the reservoir, where

large amount of bubbles are emitted with high

methane concentrations (>30%) (TUNDIsI and

WADA, unpublished).

The Samuel reservoir constructed in October

1988 is the third dam in the Amazon water

basin. It is located 100 km west of Port Velho,

Rondonia. The Samuel reservoir extends to

over 150 km long and has a maximum width of

20 km with a total flooded area of 1300 km2.

The maximum depth is 35 m and the deeper

layers of the reservoir became anaerobic at the

time of construction because of the loading of

huge amounts of organic materials from terres-

trial plants flooded by the dam construction.

In July, 1989, half a year after construction of

the dam, biological materials and bubble gases

were collected for nitrogen and carbon isotope

studies. Particulate organic matter (POM) on

a GFC fillter exhibited dramatically low ƒÂ13C

values down to -37%0. This was caused by

the recycling of mineralized CO, by phytoplan-

kton. The surface POM showed high 615N

values of +13%, suggesting the release of

nitrogen from upper areas of the reservoir with

subsequent occurrence of nitrification -

denitrification processes. A smell of hydrogen-

sulfide-like compounds was easily detected at

the dam site at this time (Fig. 8). Thus human

impact such as dam construction, deforestation

(cutting and burning of forest), and sand min-

ing with subsequent burial of plant materials in

downstream areas, enhanced the release of

biogenic gases such as CO, CH,, N, and N,O.

Consequently, s15N of the remaining available

nitrogen became higher and low s13C values for

aquatic algae were recorded. Such effects

caused by dam construction commonly last for

at least 3 years.

5-6. Gas production in coastal and estuar-

ine environments (I. KoIKE)

Due to the increasing inflow of nutrients from

the populated coastal region, microbial activ-

ities, including nitrogen metabolism, have been

stimulated in both the water column and the

sediments of near-shore environments to a

considerable extent. Denitrification is the lar-

gest gas evolving process within the nitrogen

cycle in the biosphere and is responsible for

returning fixed nitrogen to the nitrogen gas

reservoir.

In coastal environments, the surface layer of

sediments is the most active site of denitrifica-

tion. Recent observations indicating the occur-

rence of a close coupling between nitrification

and denitrification in sediments are consistent

with the above view (NisHio et al., 1983).

Although production of nitrogen gas from

coastal environments has a significant effect on

the nitrogen budget of the system, evolved

Fig. 8. Schematic illustration of gas exchange in Samuel dam.

276WADA, LEE, KIMURA, KOIKE, REEBURGH, TUNDISI, YOSHINARI, YOSHIOKA and VAN VUUREN

nitrogen gas is rather chemically inert and even

long term changes in atmospheric content can

hardly be detected because of the large atmo-

spheric reservoir.

Both nitrifying and denitrifying bacteria have

the capacity for nitrous oxide production. In

near shore environments, the concentration of

nitrous oxide is mainly controlled by the supply

of inorganic nitrogen as well as oxygen to the

sites of microbial activity. Eutrophication of

coastal waters stimulates production of nitrous

oxide by accelerating the process of nitrifica-

tion and denitrification (KOIKE and SORENSEN,

1988; SuMI and KOIKE, 1990; KOIKE, 1990).

Figure 9 shows the possible micro sites in

coastal waters, where close coupling of

nitrification and denitrification is to proceed.

Both microbial nitrification and denitrification

is mainly responsible for the evolution of

nitrous oxide, a potent source of ozone destruc-

tion in the atmosphere.

Fig. 9. Micro sites for nitrous oxide and nitrogen gas evolution in coastal and estuarine environ -

ments. Figure in the middle column indicates vertical profiles of major oxidants in

sediments and the effect of biological processes on the profiles (ALLER, 1982). Figure in

the right side of bottom column indicates the injection of oxygen (Umole•El-1) into

sediments during irrigation activity of benthic animals (JORGENSEN and REVSBECH, 1985).

Gas Exchange in Ecosystems277

5-7. Stable isotope composition in nitrous

oxide from various sources (T.

YOSHINARI).

Nitrous oxide (N2O) in the atmosphere is a

natural source of stratospheric NO, which is

important in ozone chemistry, and a greenhouse

gas, which contributes to the increase of aver-

age surface temperature of the Earth by absorp-

tion of infrared radiation. The increase of

tropospheric N20 concentration at the rate of

approximately 0.2-0.4% annually (BoUWMAN,

1990) is attributed to release mainly due to

expanding anthropogenic processes. Also, this

gas is one of key elements in the nitrogen cycle

on the Earth since it is an intermediate of

biological denitrification as well as the by-

product of biological nitrification. The use of

the stable isotope signature of N20 as a tracer

promises to provide the information which can-not be obtained by the conventional

approaches. The long-term goal of this study

is to establish a broad database of the isotope

fingerprint of N20 in the atmosphere and vari-

ous other environments. Through this effort,

it is hoped to gain insight into the relative

importance of different processes of N2O pro-

duction and decomposition, such as nitrification

and denitrification in the soil and oceanic envi-

ronments, the fossil fuel combustion and

biomass burning, and the photolysis of N20 in

upper troposphere.

For the determination of both nitrogen and

o xygen isotope composition of N2O, the sam-

ples in the atmosphere and in a few different

sources were collected by the modified method

of YOSHIDA and MATSUO (1983). Subsequently,

N2O purified by gas chromatography was

converted quantitatively to N2 and CO, in the

presence of graphitic carbon at 450•Ž for 24 hr

in sealed quartz tube (YosIIINARI, 1990). The 15

N/14N and 18O/16O ratios in N2 and CO2 respec-

tively, were determined by mass spectrometry.

Based on the N20 isotope data from the

present study and from the literature, a qualita-

tive aspect of N20 biogeochemistry and global

budget can be described.

The schematic diagram (Fig. 10) shows (1)

terrestrial and oceanic source strengths of N20

Fig. 10. Schematic diagram of the sources and sink of atmospheric N20, and the ranges of

nitrogen and oxygen isotope values of N20 in the atmosphere, land and ocean.

278WADA, LEE, KIMURA, KOIKE, REEBURGH, TUNDISI, YOSHINARI, YOSHIOKA and VAN VUUREN

and the stratospheric sink, (2) the amount of N2O reserved in each environment, and (3) the

range of nitrogen and oxygen isotope values of

N2O in the troposphere, land and ocean. The

source strengths of N2O from the land and ocean, 8-13 and 4-6 Tg•yr-1, respectively, were

derived from the following consideration. The

estimated range of land origin N2O, which includes the fluxes of N2O from microbial

processes in the soil, fossil fuel combustion and biomass burning, was derived from the addition

of the stratospheric sink strength [11 Tg•yr-1

(CICERONE, 1989)] and annual rate of increase

[3-6 Tg•yr-1 (WEISS, 1981; RASMUSSEN and KHALIL, 1986)], less the total flux of oceanic N2O

, 4-6 Tg•yr-1. The latter was estimated from the results of recent studies in the open ocean

(CLINE et al., 1987; BUTLER et al., 1989) and the contribution from the coastal environments,

which was assumed to be as much as the

amount of N2O from the open ocean.

The range of the nitrogen and oxygen isotope values of N2O shown in this figure were derived

from the literature (KIM and CRAIG, 1990; WAHLEN and YOSHINARI, 1985; YOSHIDA and

MATSUO, 1983; YOSHIDA et al., 1984; YOSHIDA et

al., 1989; YOSHINARI and WAHLEN, 1985) and from

the present study. Since various sources that

produce N2O by different mechanisms o bined, the range for the N2O isotope values

from the land shown in this diagram is very wide.

摘 要

生 態 系に お けるガス 交換:

その全体 像 とケ ースス タデ ィ

この報 文 は,1990年8月28日 横 浜 市 で開 催 さ

れ た国際 生態学 会 の シ ンポジ ウム"生 物 圏 ・大気

圏 の炭素 ・窒素 循環"で 発表 され た論文 を表 題 の

線 に沿 って ま とめた もので あ る。先 ず大 気降 下窒

素 化合 物 の生態 系へ の供給 や炭 酸 ガス ・メ タ ン ・

窒 素 ガス ・亜酸化 窒素 ガ スの生 態系 か らの発生 に

つい ての一般 的 な知見 と考 察 を人 間活 動,特 に化

石燃 料 の消費 や作 物生 産 の増大 に関連 させ て論 じ

た。 す なわ ち大 気 中のCO2の 増加 はC-O-Fe-Sサ

イ クル と大気 の 間の短 絡 に起 因 し,メ タン ・亜酸

化窒 素 の増大 は有 機物 の増産 に起 因 して い る。

生態 系各 論 においてはいつ くかの代表 的な場(ヒ

ース ラ ン ド,水 稲 根圏,ツ ン ドラ とタ イガ,湖,

熱 帯ダ ム湖,河 口 と沿岸域)お よび地球 規模 のN2

Oに 関 す る大気 ・生態 系相互 作 用 に関 す る最近 の

成 果 を ま とめ,い くつか の場 につ いて は模 式 図 を

提 示 した。

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(Authors: Eitaro WADA, Mitsubishi Kasei Insti-tute of Life Sciences, Minamiooya , Machida, Tokyo 194, Japan, Present address: Center for Ecological Research, Kyoto University , 4-1-23, Shimo sakamoto, Shiga Prefecture 520-01, Japan;

John A. LEE, School of Biological Sciences, Univer-sity of Manchester, Williamson Building , Oxford Road, Manchester M 139PL, U. K., Makoto KIMURA,

Gas Exchange in Ecosystems 281

Faculty of Agriculture, Nagoya University, Chi-

kusa-ku, Nagoya, Aichi 464- 01, Japan; Isao KOIKE,

Ocean Research Institute, University of Tokyo, 1-

15-1, Minamidai, Nakano-ku, Tokyo 164, Japan;

William S. REEBURGH, Institute of Marine Science,

University of Alaska Fairbanks, Fairbanks, Alaska

99775-1080, U. S. A.; Jose G. TUNDISI, Center of

Hydric Resources and Applied Ecology, School of

Engineering, University of Sao Paulo CEP 13560,

Sao Carlos, S. P., Brazil; Tadashi, YOSHINARI, New

York State Dapartment of Health, Wadsworth

Center for Laboratories and Research, Empire

State Plaze, P. O. Box 509 Albany, New York

12201-0509, U. S. A.; Takahito YOSHIOKA, Faculty

of Science, Shinshu University, Asahi 3-1-1, Mat-

sumoto, Nagano 390, Japan; Margret M. I. VAN

VUUREN, Centre for Agrobiological Res., P. O. Box

14,6700 AA Wageningen, The Netherlands; 和 田英

太 郎,三 菱 化 成生 命 科 学 研 究所,〒194東 京都 町 田市

南 大 谷11;現 住 所:京 都 大 学 生 態 学研 究 セ ンタ ー,〒

520-01,滋 賀 県大 津 市 下 阪本4-1-23;木 村 真人,名 古

屋 大 学 農 学 部,〒464-01名 古 屋 市 千 種 区 不 老 町;小

池 勲 夫,東 京大 学 海 洋研 究 所,〒164東 京 都 中野 区 南

台1-15-1;吉 岡 崇 仁,信 州 大 学理 学 部,〒390長 野 県

松 本 市 旭3-1-1)

Received: 6 December 1990

Accepted: 22 May 1991