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Shannon Glenn, Andrew Heyes and Tim Moore. Department of Geography and Centre for. Climate ...... D. Valentine,. K. Bronson, and W. Parton,. Methane and.

GLOBALBIOGEOCHEMICALCYCLES,VOL. 7, NO. 2, PAGES247-257,JUNE 1993

CARBON

DIOXIDE

AND

PEAT

SOILS,

DRAINED

Shannon Glenn,

METHANE

FLUXES

SOUTHERN

Andrew

FROM

QUEBEC

Heyes

and Tim Moore

Department of Geography and Centre Climate and Global Change Research, University, Montreal, Canada

for McGill

Abstract. Fluxes of CO2 and CH4 were determined by a static chamber technique at eight drained swamp peatland sites, with crop and forest covers. Over a 6month period (May - October, 1991), CH4

found that translate

fluxes

data suggest that processes other than direct oxidation, such as shrinkage and aeolian erosion, are the major contributor to the surface lowering of the peat.

and

ranged

were

temperature Integrated

correlated

with

either

commonly

observed

cultivated

soil

or water table position. seasonal emissions were

seasonal surface

CO•. fluxes lowering of

the

peat of about 2 mmyr '•, whereas the

from -5 to 7 mg CH• m'2 d'•

not

the into

-0.40

lowering

peatlands

in

these

is 20 mmyr '•. These

to 0.04 g CH• m'2 over 147 days; the sites with a forest or grass cover were a small sink of CH• whereas the sites with horticultural crops showed no significant flux. Laboratory incubations showed that the highest CH• consumption rates (3 to 9

ug CH4 g'• d'l) occurred disturbed compared

in the least

soils. The results, with CH• fluxes from

when nearby swamps which have been unaffected by drainage, suggest that drainage of temperate peatlands has reduced emissions

of CH• to the

atmosphere

by 0.6

INTRODUCTION

Peatlands play an important role in the global cycle of carbon: they store carbon, fixed by plants from atmospheric carbon dioxide (CO2), in slowly decomposing organic materials. Gorham [1991] estimated average historical rates of C accumulation

in northern

peatlands

and Armentano

a mean storage

- 1 x 10 •

temperate

at 27 g C m'2 yr '•,

and Menges

rate

peatlands,

[1986]

estimated

of 48 g C m'• yr '• for but

the

current

rates

g CH• yr '•. CO• fluxes ranged from 0 to 16 g CO• m'• d'• and were correlated with the

may be lower. Armentano and Menges [1986] calculated that peatlands in temperate

seasonal pattern of temperature upper part of the soil profile.

regions stored between 57 and 83 x 10 • g C yr '•, and Gorham [1991] obtained a similar figure of 76 - 96 x 10 • g C yr '• for

seasonal

root

fluxes

for

respiration

contribution

the

in the Integrated

sites

in

which

was an unimportant

were 0.6 - 0.8 kg CO2m'• over

181 days. Aerobic laboratory revealed CO• production rates

incubations of 0.2 - 1.4

peatlands of boreal and subarctic regions. Peatlands are also a major source of atmospheric methane (CH•), with an estimated annual global flux of 115 x 10 •

mg co• g'• d-l, an average of 5 times the

g yr '• [Fung et al.,

rate bulk

emissions controlled by the balance between CH4 production and consumption in the peat profile [Cicerone and Oremland, 1988]. Over the past two centuries, large areas of peatlands have been drained, primarily in temperate and boreal regions, for the production of crops and the exploitation of peat for either

under anaerobic conditions. density and loss-on-ignition

Copyright 1993 by the American

Geophysical

Paper number 93GB00469. 0886-6236/93/93GB-00469510.00

Union.

Using data,

we

horticultural

1991] and with

amendments

or

as

a

source

of

248

Glenn

energy. Armentano and Menges estimated the area of drained

[1986] have peatlands in

temperate regions as 20 x 106 ha, from an original area of 349 x 106 ha. Gorham [1991]

estimated

the

drained

boreal and subarctic ha, from an original In

southern

of

the

Quebec

wetlands

area

of

peatlands as 12 x 106 area of 346 x 106 ha. and Ontario,

have

been

20 -

50%

drained

[National Wetlands Working Group, 1988; Parent et al., 1982]. The drainage of peatlands for crop production frequently results in a lowering of the peat surface, with rates

varying

from 10 to 76 mmyr 'l [e.g.,

Armentano and Menges, 1986; Eggelsmann, 1976; Millette, 1976; Millette et al., 1982; Parent et al., 1982; Richardson and Smith, 1977; Tate, 1980; Volk, 1972; Weir, 1950]. This lowering can be attributed to several processes, which include shrinkage and compression of the peat as the water content is reduced, aeolian and fluvial erosion of the peat surface; and increased rates of decomposition caused by the replacement of anaerobic by aerobic conditions throughout most of the peat profile. Assessments of the relative importance of these processes are uncommon. Oxidation of the peat is reported to account for 58 -

90%

of

the

subsidence

rate

observed

in

peatlands in Florida [Shih et al., 1978; Volk, 1972] and 85% in the Netherlands [Schothorst, 1977]. Tate [1980] noted that as the temperature of the peat decreased, so

did

the

rate

of

microbial

oxidation

the organic contribution

matter and thus the of oxidation to peatland

subsidence.

In

southern

Quebec,

of

Parent

et

al. [1982] suggested that 75% of the subsidence may be related to aeolian erosion. Measurements of CO2 flux from drained peatlands are also uncommon. Silvola [1986] estimated the CO2 flux from undisturbed Finnish peatlands as 100 - 150

mg m'2 h'l, but lowering from

0

-

10

cm to

40

of the water table -

60

cm increased

this flux to 300 - 400 mg m'• h'l; from a net C storage of 25 g C m'• yr '• in undisturbed

peatlands

conditions,

lost

C at

the

the

rate

drained

of

about

250

g C m'• yr '•. Ombrotrophic bogs in temperate regions are generally minor sources of atmospheric

CH4, with except

in

annual fluxes sections

of < 1 g CH4m'2,

where the

peat

is being

degraded [Bubier et al., in press; Moore and Knowles, 1990]. Swamps, commonly drained for horticultural crops, may be more important sources of atmospheric CH•, with annual fluxes ranging from 1 to 40 g

CH• m'• yr 'l, though the lowering

of the

water table during the summer, through an imbalance between precipitation and evapotranspiration and runoff, reduces the annual flux [e.g. Harriss et al., 1982; Moore

and Knowles, 1990; Roulet et al., 1992; Wilson et al. 1989]. Drained peatlands may be a significant sink for atmospheric methane, as several swamps consume atmospheric methane when the water

et

al.

Carbon

Dioxide

and Methane

Fluxes

table drops to a depth of 0.5 - 1.0 m during the summer [e.g. Harriss et al., 1982; Moore and Knowles, 1990]. In this paper, we examine the seasonal

pattern of fluxes of CO2 and CH• from eight peat soils in southern Quebec; these soils represent a range of sites from undisturbed

forest

to

drained seasonal

for crop pattern

site

environmental

to

those

that

production. of gas flux

have

been

We relate the within the

variables,

such

as

temperature and water table position, and the pattern between sites to the influence of drainage and land use changes. We present data on the ability of peat samples to produce and consume CH• and to produce CO• in laboratory slurry incubations. losses

Finally,

from

the

we compare

drained

soils

CO•

with

the

observed rates of subsidence of the peat surface, to evaluate the role of organic matter oxidation in the lowering of the soil

surface.

METHODS

Sites

Measurements of gas flux were made at eight sites, eastern temperate forested swamps [National Wetlands Working Group, 1988] and peatlands drained for horticultural crops (Table 1). Sites 1 to 5 (45 ø 08' 18" N, 73 ø 26' 12" E) were located

on

and

near

a

commercial

horticultural farm L•gumes du Quebec)

(Les near

south

Sites

of

Montreal.

Distributeurs Napierville, 1 and

2,

de 70 km which

differ in their peat depth and crop cover, were located on a peatland that has been intensively cultivated for 10 and 20 years, respectively, and chambers were established on rows along a 140 m transect from a drainage ditch at the edge of the peatland. There were no differences in gas flux along the transect so the measurements

were

Sites 3 and 4, of sites 1 and

combined

located 2, had

at

each

site.

about 0.5 km west been drained in the

past 5 years. Site 3 was ploughed but not planted and thus remained bare for most of the summer. Site 4 retained the original forest further

but had been 0.3 km west

drained. of sites

Site 5, a 3 and 4, was

a relatively undisturbed forest swamp, though its water table has probably been lowered by the regional drainage changes in this area of intense agricultural activity, when compared with other swamps in areas remote from agricultural activities [Moore and Knowles, 1990]. The remaining sites were located at and near the Agriculture Canada Experimental Farm at Ste. Clotilde (45 ø 09' 46" N, 73 ø 40' 38" E), 50 km west of Napierville. Site 6 was an organic soil that had been used for vegetable crops for 30 yr, but which was regularly ploughed and kept bare during the summer, until celery was planted in August. Site 7, 100 m from site 6, had been left as a grass fallow for the past 22 years. Sites 6 and 7 are fields 1 and 3 in Figure 1 of Parent et al. [1982]. Site 8, 1 km east of sites 6 and 7, was a

Glenn

et

al.

Carbon

TABLE 1. Site

Dioxide

and Methane

The Eight

Sites

Characteristic

Fluxes

Studied,

249

Land Use/Vegetation

and Peat Peat

Land Use/Vegetation

Chambers

Depth,

drained, drained,

cropped cropped bare forested

drained,

drained,

forested

drained, drained, forested

cropped cropped

1

onions celery

0.5

occasional

and Carbon

Field Measurements dioxide and methane

soils.

The

chambers

from which the base covered with A1 foil inside

the

were

bottles

chamber

fitted

12 12 6

as

1.5 1 1.5

6 9 6 8

for

site

4

celery grass trees (Prunus spp.); (Covuus stolonifera, herbs (Solida9o spp.,

0.053 m2) with

a

rubber stopper containing a glass tube fitted with a serum seal through which air from inside the chamber was sampled with a 20-mL syringe. At sites 1, 2, and 6, with row crops, the chambers were located between the rows, so that root respiration was a minor component of the CO2 flux. The chamber was pushed gently 5 to 10 cm into the peat surface; an initial sample of air taken and another sample after 0.75 to 1.0 hour. Fluxes of CO2 and CH4 were based on differences in gas concentration between the initial and final chamber samples, exposure period and the volume of air in the chamber. Between 6 and 12 replicate chambers were employed at each site (Table 1) and randomly located in the site each week to account for the spatial

1.5

shrubs Spirea Aster

spp.); spp.)

variability. between

Measurements 09.00

and

may overestimate et al., 1985]. Gas within

17.00

the

were hours,

daily

concentrations were 48 hr of collection

made and

flux

thus

[Silvola

determined on a Shimadzu

Mini-2 FID gas chromatograph, using a 5-mL sample, a 1-mL injection loop, a Poropak-Q column (80/100 mesh, 3m x 3 mm) at 40 ø C and He was used as the carrier gas at a

rate

of 30 mL min '•. Concentrations

of CH•

in the gas sample were analyzed on the GC and CO2 was determined by using a Shimadzu Methanizer MTN-1 which converted CO2 to CHq. Standards of 2 and 500 ppmv CH• and 50 and 2050 ppmv CO2 were employed. Analysis of replicate analytical

flux

had been removed, to reduce heating and

1.5 1

6

18-L

(covering

herbs

Used

1.5

measurements were made at approximately weekly intervals from early May to September 1991 and then at 2 to 3 week intervals until early November. A static chamber technique was used to measure gas fluxes. Wha!en et al. [1992] have recently compared static chamber and other methods for measuring CH4 consmption in forest

polycarbonate

shrubs,

1

m

trees (Betula spp., Thuja occidentalis, Acer rubrum); shrubs (Alnus ru9osa); herbs (Carex spp., Aster spp.)

forested swamp, whose water table has been lowered by a nearby drainage ditch. The regional climate is characterized by a mean annual temperature of 7 ø C and a mean annual precipitation of 950 mm. Mean monthly temperatures at Ste. Clotilde ranged from 8.8 ø C in October to 19.9 ø C in July and for the period May - October, 1991 averaged 15.4 ø C, close to the 30year normal mean of 15.2 ø C. The 30-year normal precipitation for May - October averaged 527 mm, but in 1991, 460 mm fell. Although May was wetter than average, precipitation for June, July, and August was only 68% of the normal. Flux

Depth

samples error for

concentrations

ppmv,

of

5

respectively,

suggests that CO2 and CH• -

10

which

and

0.05-0.10

convert

into

detectable fluxes of 50 - 100 mg CO• m'• d'• and 0.2 - 0.4 mg m'2 d'•. The latter figures are similar to the detection limits quoted by Crill [1991] and Wha!en et al. [1991]. On the date of sampling, the thermal profile at each site was determined at depths of 5, 10, 20, 30, and 50 cm, using a

thermistor

set

and

multimeter.

Water

table position was determined in 2.5-cm diameter PVC tubes inserted into the peat, though the record was terminated prematurely at the cultivated sites when the crop was harvested. Laboratory Incubations To establish the potential of the peat to produce and consume CO2 and CH4, soil samples from the surface layers (0 - 10 and 10 - 20 cm) were collected from all sites, except 2, and profiles to a depth of 60 cm were sampled at sites 1, 3, and 4. Samples were collected in August 1991 and stored at their original moisture content at 4 ø C prior to their incubation

in December - March 1991-1992. production determined 5 mL water Erlenmeyer

CH• and CO2

under anaerobic conditions by placing 5 g of wet peat into triplicate 50-mL flasks, evacuating the air

were and

250

Glenn

et

al.

Carbon

Dioxide

and Methane

Fluxes

MayiJune I julyI AUg'l Sep. iOct. iNov.May I June I JulyiAug. II Sep. !I Oct. Ii Nov ,0 Site 1

Site 5 25 50

................

7!5

100

-4

••

-8





,_2-125 o

_Site2

_ Site 6 =.

_

75 -4

ß

i

i

i

I

E

1oo -I125

I

0

Site

x 8•Site 3 ::3

7 25

_

50

0

0 -

'--

-4

!

-8

7!5

100

I

I

125

0

Si 25 50

'--

-

7!5

-4

I 150

-8

150

200

250

300

JULIAN

100 125

200

250

300

DAY ß WATER

0 CH4 FLUX

TABLE

DEPTH

Fig. 1. Seasonal pattern of CH4 flux and water table depth at the eight sites.

Vertical

bars

indicate

the

standard

deviation

around the mean flux

at each

sampling date. At sites 6 and 7, the water table was at a depth greater than 1.25

under

m.

vacuum 3 times,

backfilling

with

N2

and incubating at room temperature (20 ø22 ø C). Rates of CH• consumption and CO2

production

under aerobic

determined

by placing

5 mL water

into

conditions

were

5 g of wet peat

triplicate

50-mL

and

Erlenmeyer

flasks,

an

concentration

initial

adding CH4 to produce of

= 1000

ppmv,

and incubating at room temperature under continuous shaking to avoid the development of anaerobic pockets in the

peat.

Samples of the air

were taken at

Glenn

et

al.

Carbon

Dioxide

and

TABLE 2.

Site

Methane

Fluxes

Temporally Weighted Fluxes of the Sites, May - November

CH4 Flux,

1

mg m'2

CO• Flux,

222)

223) 112)

5

110)

1.11

-+0.473,

(-0.011,

-+0.279,

109)

(7.33,

-+0.86,

169)

(3.06,

-+0.48,

0

111)

Figures

in

interval

141)

represent

number

of

individual

air sample removal, an equal amount of N• was added to the flask. CH4 concentrations under

aerobic

incubations

CO2 concentrations

anaerobic

under

incubations

increased. The rates of gas production consumption were calculated from the change in concentration from day 0 to the

volume

of

the

or day

flasks,

the dilution caused by N• addition during sample removal and the oven dry weight of the peat sample, determined at the end of the

incubation.

Soil

Analyses During sample collection from sites 1, 3, and 4, bulk density was determined by taking replicate cylindrical cores, weighing the sample fresh and then again after oven drying at 100 ø C for 48 hours. The weight loss gave the gravimetric fieTd moisture content, expressed as a percentage of the fresh weight of the sample. Bulk density was calculated from the oven dry weight and the volume of the core. Loss on ignition was measured in a muffle

furnace

at

850ø

C

for

0.5

hr.

Soil

pH was determined in a 1:1 soil:water mixture on the oven dried samples.

-

the

mean,

95 % confidence

chamber

Methane

fluxes

Methane

fluxes

ranged

1).

made

from

-5

to

The high spatial

7 mg

and the

fluxes to the the standard

limit deviation

closeness

of

of

mean that mean flux

detection of the

the

commonly spans zero flux, though in most cases, the mean flux from the replicate chambers is greater or less than the 0.2

to 0.4 mg CH• m'• d'• estimated

limit

of

detection. There was no strong seasonal pattern in CH• emission from the eight sites. The water table was at depths greater than 50 cm below the peat surface at all sites after the spring period (Figure 1). Sites 4, 5, and 8 exhibited a small positive flux of CH• in the spring in 1991 (late May), but resampling of these sites in the spring of 1992 revealed that positive fluxes were of short duration. At each of the sites, there were no significant relationships between the daily CH• flux and either water table depth or soil temperature at 5, 10, or 20 cm, or a combination of these variables, over the 1991 season (single and multiple linear regression, p > 0.05). The

mean

each site

of

all

ranged

chamber

measurements

from -2.50

to 0.06

at

mg CH•

m'2 d '• and the 95 % confidence the means spanned zero flux,

intervals of except for sites 5 and 7 (Table 2). There were no significant differences between the means of the sites, except for site 5, with a fluxes

S

CH4 m'• d'• (Figure

measurements

variability

mean flux RE SULT

126)

(mg CH4 m'• d'• and g CO• m'2 d'l)

intervals up to 4 days and CH• and CO2 concentrations determined as above; after

for

-+2.09, -

-+0.360,

brackets

and

at each site

corrected

(9.30,

-96

(-0.391,

185)

0.74

-+0.277,

8

113)

0.59

-84

(-0.451,

121)

-

-393

(-2.500,

7

4,

-+0.72,

298)

-

-+0.501,

6

while

(4.16,

-102

(-0.282,

and

-+0.59,

271)

0.64

-+0.483,

4

aerobic

(6.52,

-20

(0.022,

decreased,

-+0.54, 0.83

-+0.152,

3

flasks

(4.14,

-5

(-0.152,

kg m'•

0.72

-+0.226,

2

the

CH4 and CO2 From 1991

37

(0.056,

in

25!

of -2.50

determined

mg CH• m'• d'•. The mean at

each

date

were

used

to estimate the temporally weighted CH4 flux from each site over the sampling season (147 days), by wighting for the

252

Glenn

(37 mg CH4 m'2) at site at

(i.e.,

sites

uptake

4,

5,

7,

and

CH4),

8,

84 to -376 mg CH• m'2 (Table Carbon

Dioxide

Carbon

especially

with

fluxes

of

-

16 g CO• m'• d'• (Figure

ranged

2).

from

0 to

of

181

d

seasonal

amounted

to

2).

soil

The largest

representing

organic

flux

CO•

the oxidation

matter.

CO• production rates (0.7 - 1.4 mg CO• g'• d'l) were observed in the surface layers (0

flux

between

of CO2 0.59

- 10 and 10 - 20 cm) of sites 6, 7 and 8. Sites 1, 3, 4, and 5 showed lower rates of

and

MayIJuneJ Ju,¾ Iu.l Sep.Ioc,.i,ov. MayIu.e I u'v ß

i Se,.IOc,.INov

ß

20

20

.,

lO

10

o

0

I

-lO

"' 'o

Fluxes

Laboratory Incubations The results of the laboratory incubations to determine the potential of the soil samples to produce CO• and consume CH• under aerobic conditions are presented in Figures 3 and 4. The largest

table.

The integrated

and Methane

Most sites

exhibited a strong seasonal pattern of CO• flux, reaching a peak during mid-summer. In contrast to the CH• fluxes, there was a significant (p < 0.05), positive relationship between CO• flux and peat temperature within each site, but not between flux and position of the water over

(Table

m'2, primarily

fluxes

Dioxide

bare soil at sites 1, 2, 3 and 6), fluxes averaged 0.70 kg CO•

2).

Fluxes

dioxide

Carbon

was recorded at site 5, presumably through the contribution of root respiration from the undisturbed plant cover. At the sites where root respiration into the chambers was assumed to be minor (the row crops or

1 to negative of

al.

1.11 kg CO• •

period over which the measurement was made. Seasonal fluxes (late May to October) ranged from a small positive flux

fluxes

et

i

i

I

!

20

i,

-

i

-10

Site 6

20

E

.?

lO

10

x u.

0

o -10

I ,

I



i

i

20

I

i

i

Site7

-10

20

lO

10

o

I

i

I

i

150

200

250

300

-10

150

200

250

300 JULIAN

Fig.

2.

Seasonal

bars indicate date.

pattern

of

CO• flux

the standard deviation

at

DAY

sites

1,

2,

3,

5,

around the mean flux

6,

and 7.

Vertical

at each sampling

-10

Glenn

et

al.

Carbon

Dioxide

and Methane

Fluxes

253

AEROBIC CO2 PRODUCTION (mgg'• d'•) 0

0.5

0

I

I

I

I

AEROBICCH,•CONSUMPTION(p,gg'• d'•)

1.0

I

I

I

I

I

I

1.5

I

I

I

I

0

--

I

0

i

I

I

I

i

5

I

i

I

I

10

I

lO

20

20

'• ,30.

,,o

[ ,,o

50

50

60

60

70

70

Ol

©3

a4

&5

v6

v7

O1

D8

Fig. 3. Pattern of CO2 production in peat samples from seven sites incubated under aerobic conditions, expressed as the average daily rate over a 4 day period. Each value represents the mean of duplicate samples.

CO2production

(0.2 - 0.7 mg CO2g-1 d-l).

There was a pronounced decrease in CO2 production rate beneath 20 cm at sites 1 and 3, whereas site 4 showed little trend with depth. Unlike rates of CH4 consumption, these differences do not appear to be realted to site differences in land use, drainage history or water table position. Ratios between aerobic and anaerobic rates of CO2 production averaged 4.8 (standard deviation 3.1), with the largest ratios generally in the uppermost soil

samples. None of the

significant

samples

was able

to

produce

amounts of CH4 (< 0.1 ng CH, g-i

d'l) under anaerobic incubation conditions. Most samples, however, did exhibit an ability

to

consume CH• under

incubations

in

flasks

aerobic

spiked

with

an

original concentration of about 1000 ppmv CH• (Figure 4). CH• consumption rates were

greater

(0.3 - 5 Hg CH4g-i d-l) in the

surface layer of sites (3, 4, 5 and 8) that were undrained or had recently been drained. The consumption rates were

smallest

(0 - 1 fig CH4 g-i d-l) at sites

6, and 7, cultivated with

the

surface

which have been drained for the longest period, water

for

table

the

consumption rates the soil profile, where

the

the soil occurred

water

1 m beneath

summer.

At

were retained and at sites table

the

site

remained

!,

soil

low CH4

through 3 and 4, closer

surface, higher consumption at depths of up to 70 cm.

Properties Soil profiles (Table 1) indicate

1,

and and

to

rates

Soil

for sites 1, 3, that the soils

and 4 are

Fig.

4.

©3

z•4

Pattern

of

&5

v6

•'7

CH• consumption

[:]8

in peat

samples from seven sites incubated under aerobic conditions, expressed as the average daily rate over a 4 day period. Each value represents the mean of duplicate samples.

slightly acid (pH 5.2 organic matter content 83 - 96%). Absorbance extract

of

the

soil

-

6.4) (loss of the

with a large on ignition pyrophosphate

increases

toward

the

surface, suggesting an increased degree of decomposition [Levesque and Mathur, 1979]. Bulk densities generally increased from the base of the profiles (0.13 - 0.16 g

cm'3) toward

the

surface.

At site

1, which

has been intensively cultivated for 10 yr, a 40-cm-thick compact layer with plough pan has developed, with bulk densities of

0.23

- 0.30

g cm'3. Surface

bulk densities

are smaller at sites 3 and 4, which have not been so intensively cultivated. Although gravimetric water contents (expressed as a percentage of the fresh

weight of the surface

of the sample) peat were high layers of site

desiccation, 41%

in

with

in the basal layers (75 - 80%), the ! revealed

water

contents

of

23

-

mid-June.

DISCUSSION

Methane

Temperate peatlands, especially swamps, are a substantial source of CH4, though when the water table drops during dry summer periods, these wetlands may become a sink,

rather than a source, of atmospheric CH4 [Harriss et al., 1982; Moore

and

Knowles,

1990;

Roulet

et

al.,

1992; Wilson et al., 1989]. The two sites (5 and 8) chosen as references in the present study have low water tables (depth > 60 cm) for all of the summer. Measurement of water table position at two swamps 40 km north of Napierville, which have been unaffected by human activities,

254

Glenn

revealed that the water table rarely fell below 60 cm (sites 6 and 7 in the work by Moore and Knowles [1990] ). This suggests that the forested swamps near Napierville and Ste. Clothilde have been affected by changes to the regional drainage pattern in this intensively cultivated part of southern

These

Quebec.

forested

swamps (sites

5 and 8)

exhibited the largest rates of CH4 consumption during the summer period,

with

minimum fluxes of -5 mg CH4 m'2 d" and 6month fluxes of -0.1 to -0.4 CH• g m'2. There was some evidence of positive fluxes of CH• to the atmosphere during the spring. 1991 was an exceptionally dry summer in southern Quebec, so caution must be exercised when utilizing the results from the two forested sites. Similar CH• consumption rates have been reported recently from sites such as well-drained temperate forest soils [Born et al., 1990; Crill, 1991; Steudler et al., 1989; Yavitt et al., 1990], grassland soils [Mosier et al., 1991], and taiga and tundra soils [Whalen and Reeburgh, 1990; Whalen et al., 1991]. In drained forest soils at Wally Creek, northern Ontario, al. (Methane flux from

N.T. Roulet et drained northern peatlands: the effect of a persistent water table reduction on flux, submiited to Global Bioqeochemical Cycles, 1993) herein referred to as N.T. Roulet et al., submitted manuscript, 1993) observed decreases in CH• emission rates as a function of the lowered water table, but that, even at the driest sites, consumption rates were very small (-0.1 to

-0.4

mg CH4m'2 d").

A similar

pattern

has

been observed in drained peatland soils in Finland [Martikainen et al., 1991]. Drainage and cultivation of these peatlands soils in southern Quebec appears

to have reduced CH• exchange with the atmosphere. Of the four sites (1, 2, 3, and

6)

in this study that have been and have a small root biomass, seasonal fluxes ranged from -20 to 37 mg

drained

CH• mø2, close to the anticipated and

limits

of

detection

errors

around

zero

flux.

At the four sites (4, 5, 7, and 8) that retained a forest or grass cover, seasonal CH• consumption was larger (-84 to -393 mg

CH• m'2) than at the cultivated

sites.

suggests

and

microbial

that and

root

activities

chemical

environments

This within

the rhizosphere may play a role in stimulating CH4 consumption. CH• consumption rates obtained from the peat samples were at the lower end of the range observed from other peatlands (1 - 100 ug

CH• g'• d'•; Moore and Knowles, 1990; Moore et

al.,

Methane

emissions

from

wetlands,

southern Hudson Bay Lowland, submitted to Journal of Geophysical Research, 1993; N.T. Roulet et al., submitted manuscript, 1993). The failure of the peat samples to produce CH• in anaerobic laboratory incubations may be partly a function of a lag response extending over 5 days, as has been observed in a cultivated peat soil [Megraw and Knowles, 1987 ].

et

al.

Carbon

Dioxide

and Methane

Fluxes

There is evidence that nitrifying bacteria are capable of oxidizing CH• [e.g., B•dard and Knowles, 1989; Megraw and Knowles, 1989]. Patterns of nitrogen mineralization may affect CH4 consumption rates, with a decrease in CH• consumption observed in grassland and forest soils accompanying the addition of nitrogen [e.g. Mosier et al., 1991; Steudler et al., 1989]. Rapid nitrification rates under these high soil pH values (5.8 6.4) could contribute to the decreased rates of CH• consumption in the drained soils, compared to those that are less disturbed.

The

lack of evidence for strong of environmental variables, such as temperature and water table position, on CH• flux is not surprising, given the small fluxes and the high errors at each sampling date. Relations between CH• consumption and temperature were not observed by Born et al. [ 1990] and Steudler et al. [1989], though Crill [1991] was able to establish a relationship in a New England forest. While there may be an initial period during the spring when microbial consumption of CH4 is temperature limited, later in the summer the consumption rate may be limited by the ability of CH• to diffuse from the atmosphere to the sites of oxidation in the soil profile [Crill, 1991]. Most of the drained soils have low moisture contents in the surface layers during the summer (Table 3) and occasionally require irrigation and thus CH• consumption is unlikely to be limited by slow diffusion through small soil controls

pores. An

estimate

can

be

made

of

the

effect

of temperate peatland drainage to the global CH• cycle, using the drained area of 12 - 20 x 106 ha of Armentano and Menges [ 1986] and Gorham [ 1991]. The major uncertainty in this estimate is the annual emission of CH• by undrained peatlands,

which ranges from 0 to 40 g CH4m'2 yr '•, as noted above. The present study suggests that drained horticultural peatlands are neither a significant source nor sink of atmospheric CHq. Assuming that CH• flux from the peatlands prioir to drainage

averaged 5 g CH• m'2 yr '•, the drainage

of

12 - 20 x 106 ha would result in a reduced emission to the atmosphere of 0.6 - 1 x

10•2 g CH• yr '•. This is a very small proportion of the current estimated global emission of CH4 to the atmosphere of about

500 x 10 •2 g CH• yr '• [Fung et al., Carbon

Dioxide

The CO2 fluxes

(0.6

1991].

at

these

peatland

sites

- 1.1 kg m'•) are lower than those

measured in other temperate wetlands and forests [e.g., Crill, 1991; Reiners, 1968], though similar to that estimated for a drained Finnish peatland [Silvola, 1986]. It is likely that the November April period would add only minor increases

to

these

seasonal

fluxes:

Crill

Glenn

et

al.

Carbon

TABLE 3.

Dioxide

Properties

Depth,

and Methane

of

the

Water

cm Site 0-10 10-20 20-30 30-40 40-50 50-60 60-75

1

Site 0-10 10-20 20-30 30-40 40-50 50-65

3

Site

4

0-10 10-20 20-30 30-40 40-50 50-60 60-70

Soil

Bulk

Content,

%

Fluxes

Profile

density

Sites

1,

3,

and 4

Loss

pH

on

ignition,

41 23 27 52 68 78 84

0.227 0.284 0.300 0.265 0.154 0.130 0.134

6.3 6.4 6.0 5.8 5.2 5.2 5.2

90.3 89.2 89.4 89.2 94.5 96.1 82.4

67 76 79 77 80 78

0.183 0.163 0.151 0.189 0.151 0.159

5.8 5.8 5.8 6.3 6.4 6.4

92.0 96.4 95.0 89.2 83.4 92.5

nd 72 76 77 77 80 78

nd 0.179 0.177 0.154 0.148 0.149 0.132

5.8 5.9 6.1 6.1 6.0 5.8 6.2

93.4 91.3 93.6 92.5 83.2 92.9 93.3

Absorbance

% at 550 nm•

0.145 0.132 0.124 0.066 0.051 0.049 0.036

0.227 0.139 0.082 0.068 0.062

0.078

0.189 0.287 0.277 0.101 0.103 0.241 0.165

• The absorbance measured at a wavelength

of 550 nm of a filtered

pyrophosphate

[Levesque

extract

of the

peat

sample

identified

the

control

of

water

table

position on CO2 flux [e.g. Hogg et al., 1992; Moore and Knowles, 1989]. For example, Moore and Knowles [1989] demonstrated for a swamp soil, similar to the ones used in the present study, that lowering of the water table increased CO2 flux, in an approximately linear manner, with

from

(g cm'3)

[1991] found that 83% of the annual CO2 flux occurred between May and October. CO2 flux patterns were primarily controlled by the thermal regime of the soil profile, as has been found in other studies [e.g., Crill, 1991; Reiners, 1968]. Laboratory studies of soil columns have

255

the

flux

when

the

water

table

a depth of 70 cm beneath the peat being 9 times that when the water was at the peat surface. Lieffers demonstrated

that

rates

of

was

at

surface table [1988]

cellulose

decomposition at a depth of 30 cm in a drained Alberta peatland, with a water table depth of 50 cm, were double that observed in an undisturbed peatland, where the water table was at a depth of about 20 cm. The whole soil profile can produce CO2, but the thermal regime and the greater CO2 production rates of the surface layers in laboratory incubations suggest that most of the CO2 emitted is produced in the upper soil layers [Stewart and Wheatley, 1990]. However, the low soil

and Mathur,

1979]

moisture contents of the surface layer of the drained peatlands during the summer dry periods may slow CO2 production rates [Orchard and Cook, 1983]. Using the bulk density and loss-onignition data for site 1, an estimate can be made of the lowering of the peat surface

by organic

matter

oxidation

alone.

A flux of 0.7 kg CO2m'2 and a bulk density of 0.23 g cm'3 and loss-on-ignition of 90%, result in a surface lowering of 2 mm yr '•, and a similar number is produced for site 3. This is an order of magnitude lower than the reported subsidence rates of these soils [Millette, 1976; Parent et al., 1982]. Other processes, such as shrinkage of the surface layers and compaction of the lower layers may be important, particularly soon after drainage [Mathur and Levesque, 1977], as well as aeolian erosion [Campbell and Millette, 1981]. Parent et al. [1982] estimated measured

that 75% of the in soils at the

Experimental aeolian

Farm

erosion:

unprotected

could

subsidence Ste Clotilde

be ascribed

subsidence

against

aeolian

rates

to on

sites

erosion

averaged 45 mm yr '•, compared to a rate 10 mm yr '• at sites that were protected. The dry, pulverized surface layer drained, cultivated peat soils is susceptible to aeolian erosion,

of

of the

256

Glenn

particularly

in the

summer,

the

when

spring

surface

in

the

ash

content

of

the

surface

layer, left behind after the organic matter is oxidized [Schothorst, 1977]. At site 1, however, loss-on-ignition values in the surface layer are as high as those in the underlying ones (Table 3). Armentano and Menges [1986] estimated that, prior to disturbance, temperate zone peatlands accumulated

between 57 and 83 x 10•2 g C yr '•, with an average storage rate of 48 g C m'2 yr '•. After disturbance, they attributed half the subsidence rate to organic matter oxidation, and used annual subsidence

rates

ranging

of

soils

amounted to 63 x 10 • g yr '•, in

temperate

peatlands

acting

as

a sink of CO2of between 19 and -7 x 10• g C yr '•. The shift in CO• flux of 63 x 10•2 g yr '• is minor compared to the increased terrestrial flux of CO2 associated with fossil fuel combustion or tropical deforestation [e.g., Detwiler and Hall, 1988]. The results from the present study suggest that the estimates of the direct contribution of organic matter oxidation to peatland subsidence may be too large. Our data suggests that upon drainage the peatlands convert from a sink to a source

of C of about 200 g C m'2 yr '•, similar

to

the pattern observed in drained Finnish peatlands by Silvola [1986]. However, if organic matter is removed from drained peatlands by wind, it will accumulate in adjacent fields or drainage ditches, where it will be susceptible to oxidation, so the net effect of CO• production will be the same. On the basis of a peat

subsidence half this

rate

of 20 mm yr '•, of which

is due to shrinkage amounts to a C loss

and compaction, of about 1000 g

m-• yr -•' Acknowledgments. The authors gratefully acknowledge the cooperation of Laurent Deslauriers and the managers of Les Distributeurs de Legumes du Quebec and

Agriculture

Canada

conduct our studies. Mike Dalva assisted

farms

for

Jamie in the

permission

Windsor field

Research

Royal

Canadian

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257

S. Glenn, Department University, Montreal,

(Received revised accepted

A. Heyes, and T. Moore, of Geography, McGill 805 Sherbrooke St. W., Canada,

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