Agricultural and Food Science in Finland


491

A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D

Vol. 7 (1998): 491–505.

© Agricultural and Food Science in Finland
Manuscript received May 1998

A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D

Vol. 7 (1998): 491–505.

Effect of soil wetness on air composition and
nitrous oxide emission in a loam soil

Antti Jaakkola and Asko Simojoki
Department of Applied Chemistry and Microbiology, PO Box 27, FIN-00014 University of Helsinki,

Finland, e-mail: antti.jaakkola@helsinki.fi

Effects of cropping (bare fallow, grass), heavy irrigation and N fertilization (0, 100 kg ha-1) on soil
air (at depths of 15 and 30 cm) and N

2
O emission were studied in a factorial two-year field experi-

ment in southern Finland. The responses of soil mineral N, dry-matter yield and uptake of N were
also determined. Irrigation was performed during two periods in 1993 and one period in 1994. Dur-
ing sampling periods, the soil moisture ranged from 11% to 45% (v/v) and soil temperature from 0°C
to 21°C. Unirrigated bare fallow contained 14–21% O

2
, 0.1–2% CO

2
 and 0.2–100 µl l-1 N

2
O (1993

maximum 27 µl l-1) in the soil air. Cropping and irrigation lowered O
2
 (minimum 3–7%) and raised

CO
2 
(maximum 9%) in soil air, but fertilization had no effect. Irrigation raised N

2
O in the soil air if

nitrate was present abundantly. Consequently, fertilization increased N
2
O especially in the irrigated

bare soil, which still contained plenty of nitrate in autumn 1993. Cropping decreased N
2
O. The var-

iation in soil air composition was partly explained by that in soil air-space. The average daily N
2
O-N

emission amounted to 0–40 g ha-1 (mean 7 g ha-1) and correlated positively with N
2
O concentration in

the soil air.

Key words: carbon dioxide, denitrification, oxygen, soil air composition

Introduction

Soil moisture affects the gas composition of soil
air in different ways. Soil organisms affected by
moisture consume and produce gases which al-
ter the composition of soil air. Such changes are
counteracted by the gas exchange between the
soil and the atmosphere. Increasing moisture
causes decreasing air content. Because gases are
conducted almost entirely through air-filled
pores, gas exchange between the atmosphere and

soil pores will slow down with increasing mois-
ture.

Excess soil moisture is known to be detri-
mental to the growth of various field crops. The
change in soil air composition may play an im-
portant role (Glínski and Stépniewski 1985). Low
O

2
 concentration in soil air has been shown to

retard plant growth independently of soil wet-
ness in experiments where soil air composition
is artificially regulated (e.g. Jaakkola et al. 1990).

Soil air composition has been measured in a
few field studies in the Nordic countries (Lind-

mailto:antti.jaakkola@helsinki.fi




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Fertilization and sowing was performed on
24 May 1993, with a combine drill being used
to place the fertilizer (calcium ammonium ni-
trate) in rows 25 cm apart and 8 cm deep in the
middle of every second sowing-row interval.
The seed consisted of a mixture of winter rye
(Secale cereale), Italian rye grass (Lolium mul-
tiflorum), Persian clover (Trifolium resupina-
tum), timothy (Phleum pratense) and meadow
fescue (Festuca pratense). Plants from the bare
fallow plots were removed by hand as they
emerged.

Using a tractor-mounted sprayer producing
approximately 10 mm water per hour, the
ploughed layer was saturated with water during
three periods at different stages of the growing
season. In 1993 the field was irrigated with 120
mm of water between 15 June and 2 July, and
with 110 mm of water between 27 July and 10
August. In 1994 84 mm of water was given dur-
ing 18–22 August.

Porous cups made of sintered polyethylene
(pore size Ø 100 µm), one for each depth (15
and 30 cm), were inserted into holes made in
each plot with an auger (Ø 3 cm) immediately
after sowing and fertilization. The air-filled space
around and inside the cup was about 20 ml. Sam-
pling of soil air was performed about once a week
during the growing period of 1993 and about
once a fortnight during the growing period of
1994, mostly between 6 and 8 p.m. A 4 ml sam-
ple was taken with a glass syringe through a sil-
icon rubber septum connected to the cup with a
narrow Teflon tube (volume approximately 1 ml).
After discarding the first sample, 5 ml was tak-
en for analysis in the same way. The air samples
were stored for no more than two days in the
glass syringes and then analyzed for N

2
, O

2
, CO

2
,

CH
4
, C

2
H

4
 and N

2
O. Two interconnected gas

chromatographs (Hewlett Packard 5890) were
used. One of them was equipped with a Molecu-
lar Sieve 5A packed column (1.8 m) for N

2
,

O
2
+Ar, CH

4
 and C

2
H

4
 and a Porapak Q packed

column (1.8 m) for CO
2 
. Helium was the carrier

gas (35 ml min-1). The oven temperature was
80°C. The detectors (200°C) were TC for N

2
,

O
2
+Ar and CO

2
, and FI for CH

4
 and C

2
H

4
. The

other GC had a Porapak Q packed column
(1.8 m) and an EC detector (300°C) for N

2
O. The

carrier (95% Ar, 5% CH
4
) flow was 35 ml min-1

and the oven temperature 40°C. The Ar concen-
tration in air was assumed to be 0.9% for calcu-
lating the O

2
 concentration. When calculating the

results the sum of determined gas concentrations
was adjusted to 100%.

Steel cylinders, 16 cm in diameter and 25 cm
in height, were inserted 10 cm deep into the soil
in nine plots (Fig. 1) at the beginning of the ex-
periment in order to monitor the emission of N

2
O

from the soil. One cylinder was placed on each
unfertilized plot, but two cylinders on each N
treated plot in order to cover the fertilizer rows
and the space between them representatively. At
each sampling of the soil air each cylinder was
covered with an air-tight rubber sheet for 40–60
min. The daily emission of N

2
O was calculated

assuming a linear increase of gas concentration
in the closed chamber from the measured mean
ambient level (0.322 µl l-1) to the concentration
measured at the end of sampling period.

Soil moisture in the 0–20 cm layer was mon-
itored by TDR (Tektronix 1502B) plotwise in
blocks I-III (Fig. 1) as often as the soil air was
sampled. The soil temperature at depths of 15
and 30 cm was monitored with Pt100 probes in
three plots (Fig. 1) in connection with air sam-
pling.

Soil samples were taken at depths of 0–15
cm and 15–30 cm from the area between unirri-
gated and irrigated plots on 15 June 1993, just
before the first irrigation. The same soil depths
were sampled on 4 July plotwise in the blocks I,
II and III. All plots were sampled at the above-
mentioned depths on 2 September 1993. For de-
termination of mineral nitrogen the samples were
extracted with 2 M KCl. Ammonium and nitrate
in the extract were determined colorimetrically.

The plant stand was cut from the cropped
plots on 1 September 1993 and 14 June 1994,
taking plotwise a sample from an area of 0.45
and 0.25 m2, respectively. The plant samples
were dried at 70°C and weighed. Total nitrogen
was determined using the common Kjeldahl di-
gestion procedure.



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Jaakkola, A. & Simojoki, A. Effect of wetness on soil air

Statistical analysis

The treatments were partly arranged systemati-
cally in the blocks (Fig. 1). However, no sys-
tematic change in soil properties was apparent.
Therefore, in comparing the treatments, an anal-
ysis of variance for a blockwise randomized de-
sign was made. In cases where the interactions
were significant, individual treatment means
were compared by Tukey’s test. Correlation anal-
ysis was performed between the plotwise N

2
O

emission data and corresponding N
2
O concen-

trations in the soil air.
In order to reduce the random variation of

gas concentrations in the soil air samples, aver-
ages over three subsequent samplings were sta-
tistically analysed. Soil moisture data were ana-
lysed similarly. A logarithm transformation was
used for the N

2
O concentrations to approach a

normal distribution.

Results

Nitrogen application increased the crop yield and
the N uptake in the first year (Table 1). Irriga-
tion increased the first-year yield significantly

only when nitrogen was applied. The nitrogen
uptake did not respond to irrigation.

Mineral nitrogen in the top 30 cm of soil did
not significantly respond to nitrogen application
or cropping in the middle of June three weeks
after fertilization and sowing (Table 2) although
the mean concentration was generally higher in
the fertilized plots. About three weeks later
(4 July) nitrogen application resulted in a sig-
nificant increase, while cropping had a decreas-
ing effect. Only nitrate in the topmost layer (0–
15 cm) was affected. Irrigation did not have any
effect. The crop reduced the nitrate concentra-
tions in late summer (2 September), as did irri-
gation, but to a lesser extent. Nitrogen applica-
tion still had a small increasing effect. Concen-
trations in the cropped soil were rather low.

Nitrogen application did not significantly
affect the soil moisture or the response of soil
air composition to other treatments. Therefore,
averages over both N rates representing cropping
and irrigation treatments are given in Figures 2
and 3, as well as in Tables 3, 4, 5 and 6.

Variations of soil temperature during both
years were rather similar, considering the dis-
similar observation periods (Fig. 2). The soil
moisture varied during the first year between
16% and 44% in the non-irrigated soil. The soil
was dry when the experiment started (beginning
of June), gained moisture for a couple of weeks

Table 1. Crop (C
1
) yield and uptake of N in unirrigated (I

0
) and irrigated (I

1
), as well as in unfertilized (N

0
)

and fertilized (100 kg ha-1 N, N
1
) soil.

C
1
I

0
N

0
C

1
I

0
N

1
C

1
I

1
N

0
C

1
I

1
N

1

Yield, D.M. kg ha-1

1993 1377 a 2736 b 2123 ab 3611 c

1994 4175 a 4472 a 3082 a 3821 a

Total 5551 ab 7207 b 5205 a 7431 b

N uptake, kg ha-1

1993 35 a 83 b 37 a 79 b

1994 50 a 58 a 39 a 50 a

Total 85 a 141 b 76 a 129 b

Means in the same row followed by a common letter do not differ significantly (P=0.05)
D.M. = dry matter



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Table 2. Mineral nitrogen in soil layers 0–15 cm and 15–30 cm, mg kg-1 D.M. Treatments: C
0 
bare soil, C

1
 cropped; I

0
 no

irrigation, I
1
 irrigated; N

0
 no fertilizer, N

1
 100 kg ha-1 N.

Depth 0–15 cm Depth 15–30 cm 0–30 cm
NH

4
–N NO

3
–N Total NH

4
–N NO

3
–N Total Mean

15 June 1993
Treatment
C

0
I

0
N

0
3 20 23 1 6 7 15

C
0
I

0
N

1
13 35 48 2 5 7 28

C
1
I

0
N

0
4 21 25 1 5 7 16

C
1
I

0
N

1
9 42 51 2 6 8 30

Effect of treatment
Cropping –1 4 3 0 0 0 2
N-Fertilization 7 18 25 1 0 1 13

4 July 1993
Treatment
C

0
I

0
N

0
2 24 26 1 9 10 18

C
0
I

0
N

1
2 49 50 2 10 11 31

C
0
I

1
N

0
2 17 19 1 12 13 17

C
0
I

1
N

1
2 42 44 1 10 12 28

C
1
I

0
N

0
3 6 8 2 6 8 8

C
1
I

0
N

1
2 23 26 1 8 9 18

C
1
I

1
N

0
2 1 3 1 7 8 6

C
1
I

1
N

1
2 11 13 2 9 11 12

Effect of treatment
Cropping 0 –23 *** –23 *** 0 –3 –3 –13 ***

Irrigation 0 –7 –8 0 2 1 –3
N-Fertilization 0 19 ** 19 ** 0 1 1 10 **

2 September 1993
Treatment
C

0
I

0
N

0
2 9 11 1 15ab 16b 14

C
0
I

0
N

1
2 33 35 1 30c 32c 33

C
0
I

1
N

0
1 4 6 1 4ab 5ab 6

C
0
I

1
N

1
1 18 19 1 17b 18b 19

C
1
I

0
N

0
1 0 2 2 0a 2a 2

C
1
I

0
N

1
4 3 6 1 2a 3a 5

C
1
I

1
N

0
1 0 1 1 0a 2a 2

C
1
I

1
N

1
2 0 2 1 0a 2a 2

Effect of treatment
Cropping 0 –15 ** –15 ** 0 –16 *** –16 *** –16 ***

Irrigation –1 * –5 –6 0 –6 ** –6 ** –7 *

N-Fertilization 0 10 * 11 * 0 7 ** 7 ** 9 **

Means in columns with significant treatment interactions followed by a common letter do not differ significantly
(P = 0.05)
Significance of effects: * = P<0.05, ** = P<0.01, *** = P<0.001
D.M. = dry matter

and dried again to the initial moisture level in
July. Thereafter, due to rain, the soil became
moister, reaching a maximum at the end of Au-

gust. Irrigation raised the moisture content at
most to 45%, probably saturating the topsoil (0–
20 cm) at that time. The increases produced by







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Jaakkola, A. & Simojoki, A. Effect of wetness on soil air

being largest in the spring. Thus, even the crop
had its biggest effect at the beginning of grow-
ing season. Irrigation performed in late summer
also had only a small but significant effect on
soil air O

2 
and CO

2 
(Tables 4 and 5).

N application did not have any significant
effects on O

2
 concentration (Table 4) or CO

2
 con-

centration (Table 5) in soil air at either depth.
The O

2
 concentration in the soil air decreased

in the ploughed layer to below 15% only when
the volumetric soil moisture exceeded 30%
(Fig. 4).

The concentration of CH
4
 in the soil air var-

ied between 0 and 43 µl l-1 independent of treat-

ment or sampling date (data not shown). The
concentration of C

2
H

4
 did not exceed the detec-

tion limit of 0.5 µl l-1 in any sample (data not
shown).

The concentration of N
2
O in bare, unirrigated

soil (control treatment, C
0
I

0
, Fig. 5) varied at

various depths and N rates between 0.4 and 27
µl l-1 in the first year. In the second year, the range
was between 0.4 and 100 µl l-1, but the concen-
trations did not exceed 7 µl l-1 after May. The
peak concentration in 1993 took place by the end
of August; higher values were found deeper in
the soil. N application raised the peak. The irri-
gation in June 1993 raised the concentrations for

Table 4. Average concentration of O
2
 in the soil air at two depths during various periods, and concentration

increase due to treatments, %.

27.6.–11.7.1993 1.8.–15.8.1993 22.8.–19.9.1994

Treatment 15 cm 30 cm 15 cm 30 cm 15 cm 30 cm

Control 20.6 20.4 19.6c 18.8c 20.4 20.3
Irrigated 18.2 17.0 13.7b 11.0b 19.6 18.8
Cropped 20.4 20.3 17.3bc 16.3bc 19.9 18.8
Cropped + irrigated 17.0 16.4 8.7a 4.1a 18.9 17.0

Effect of treatment
Cropping –0.7 –0.4 –3.6 *** –4.7 * –0.6 * –1.6
Irrigation –2.8 *** –3.6 *** –7.3 *** –10.0 *** –0.9 *** –1.7
N-fertilization –0.1 –0.1 0.3 –0.6 0.1 0.5

Significance of effects: * = P<0.05, ** = P<0.01, *** = P<0.001

Table 5. Average concentration of CO
2
 in the soil air at two depths during various periods, and concentra-

tion increase due to treatments, %.

27.6.–11.7.1993 1.8.–15.8.1993 22.8.–19.9.1994

Treatment 15 cm 30 cm 15 cm 30 cm 15 cm 30 cm

Control 0.58 a 0.73 a 1.16 a 1.31 a 0.40 0.47
Irrigated 1.60 ab 1.59 ab 3.61 b 3.09 a 0.63 0.88
Cropped 0.77 a 0.94 a 3.26 ab 3.44 a 0.92 1.21
Cropped + irrigated 2.99 b 2.60 b 8.15 c 8.26 b 1.56 2.02

Effect of treatment
Cropping 0.79 *** 0.61 ** 3.32 *** 3.65 *** 0.73 *** 0.94 ***

Irrigation 1.62 *** 1.26 *** 3.67 *** 3.30 *** 0.44 * 0.61 **

N-fertilization –0.05 0.05 –0.38 –0.18 –0.20 –0.25

Treatment means followed by a common letter do not differ significantly (P=0.05)
Significance of effects: * = P<0.05, ** = P<0.01, *** = P<0.001







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CO
2
 caused by compaction in the trial of Simo-

joki et al. (1991) was 5% at depths of 25 and 50
cm, but it lasted longer deeper in the soil. In the
experiment of Hansen and Bakken (1993) a max-
imum CO

2
 concentration of almost 5% at depths

of 7–12 cm was caused by soil compaction.
Soil respiration consuming O

2
 and produc-

ing CO
2
 was, no doubt, the most important phe-

nomenon altering soil air composition in the
present experiment. Respiration in cropped soil
was probably 2–3 times higher than in uncropped
soil (Currie 1975). In the first year, irrigation and
N fertilization improved plant growth, which in
turn probably also increased soil respiration. On
the other hand, if plant water uptake had in-
creased air-filled porosity, the enhanced gas ex-
change would have counteracted the effects of
respiration. But since cropping and N fertiliza-
tion had only minor effects on soil moisture in
this experiment, the significant effects of crop-
ping on O

2
 and CO

2
 concentrations in soil air

were mainly due to differences in respiration.
Hansen and Bakken (1993) also found no effect
of N fertilization in sandy loam under ley. In
contrast, Stépniewski (1977), working with sev-
eral plant species, found that doubling the min-
eral fertilizer dose improved soil aeration status
in a cropped loamy sand soil.

Soil air composition deviated from atmos-
pheric air composition most during a period of
very high moisture content in soil simultaneously
with high temperature, occurring in July-August
1993. In wet soil, under a vigorously growing,
oxygen-consuming plant, the O

2
 concentration

in soil air at the bottom of plough layer dropped
below 4%. The concentration increased again
during the second half of August and thereafter,
although no marked increase in air-filled poros-
ity occurred. Obviously the decreased consump-
tion of oxygen allowed this increase. A similar-
ly decreasing deviation from atmospheric air
towards the end of growing season was observed
e.g. by Simojoki et al. (1991) in a pot experi-
ment with barley. In their study, decreasing res-
piration could have been related mainly to the
developmental stage of the plant. However, in
the present study the influences of oxygen defi-
ciency, due to its low content in soil air, and of
simultaneously decreasing temperature were
most obvious, because similar changes were ob-
served in both bare and cropped soils. In addi-
tion, the respiration rate in grass does not change
remarkably with development stage as in cere-
als. Soil respiration is generally regarded as an
exponential function of temperature (Glinski and
Stépniewski 1985). Yearly variations in soil res-

Table 6. Average (geometric mean) concentration of N
2
O in the soil air at two depths during various periods, µl l-1, and ratio

(effect of treatment) between treated and untreated soils.

27.6.–11.7.1993 1.8.–15.8.1993 22.8.–5.9.1993 22.8.–19.9.1994

Treatment 15 cm 30 cm 15 cm 30 cm 15 cm 30 cm 15 cm 30 cm

Control 0.75 0.98 1.09 ab 1.38 b 6.86 10.02 0.91 0.83
Irrigated 2.67 3.36 4.44 b 6.54 c 2.09 3.91 0.83 1.27
Cropped 0.79 1.08 0.77 a 0.98 ab 0.73 0.54 0.48 0.63
Cropped + irrigated 2.56 3.14 0.44 a 0.26 a 0.33 0.47 0.41 0.49

Effect of treatment
Cropping 1.01 1.01 0.26 *** 0.17 *** 0.13 *** 0.08 *** 0.51 *** 0.54 ***

Irrigation 3.40 *** 3.17 *** 1.52 1.12 0.37 * 0.58 0.89 1.09
N-fertilization 1.41 * 1.35 * 1.46 1.60 2.54 * 2.51 1.15 1.07

Treatment means followed by a common letter do not differ significantly (P=0.05)
Significance of effects: * = P<0.05, ** = P<0.01, *** = P<0.001



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Jaakkola, A. & Simojoki, A. Effect of wetness on soil air

piration mainly due to temperature fluctuations
are well known (Currie 1975).

The decrease of O
2
 was connected with an

increase in CO
2
. However, they were not equiv-

alent, the former being bigger than the latter. This
difference has been observed in many other stud-
ies (e.g. Russell and Appleyard 1915, Glinski and
Stépniewski 1973, 1985) and is explained by the
rather high solubility in water

 
of CO

2 
as com-

pared with that of O
2
. If there had been strong

anaerobic production of CO
2 
in the soil, the sum

of O
2
 and CO

2
 would have exceeded 21% (Glin-

ski and Stépniewski 1973). Probably, CH
4
 con-

centration would have also increased. In the
present study, increases in the CH

4
 concentra-

tion and in the sum of O
2
 and CO

2
 concentration

were never observed.
The difference of O

2
 concentration between

depths was many times larger than the corre-
sponding difference of CO

2
 concentration. This

is partly explained by the better solubility of CO
2

in water, but the more rapid diffusion of CO
2
 in

soil water may also play a role (Greenwood
1970).

A plant hormone C
2
H

4
 is involved in plant

response to hypoxia (see Jackson 1991). In rela-
tively wet soils concentrations of several µl l-1
have been observed both in field (Dowdell et al.
1972, Smith and Dowdell 1974) and pot experi-
ments (Simojoki et al. 1991), although variation
has been great. In contrast, no C

2
H

4
 was found

in soil air in the present study. In other investi-
gations low concentrations (0.5 µl l-1 or less) have
been measured in aerobic soils (Otani & Ae
1993), but sometimes also in wet soils (Meek et
al. 1986). Taken together the results suggest that
C

2
H

4
 is not a sensitive indicator of hypoxia in

soil.
The N

2
O concentrations (0.2–100 µl l-1) were

in the range reported by Hansen and Bakken
(1993) in the topsoil of Norwegian field experi-
ment on a sandy loam with different soil com-
paction and fertilization treatments. In the
present experiment the highest concentrations
(May 1994) were probably caused by the spring
thaw (see Nyborg et al. 1997).

At the beginning of the experiment a lot of

nitrate was present in both non-fertilized and
fertilized plots. By the end of the first irrigation
period no effect of cropping on N

2
O in soil air

was found; however irrigation had increased the
concentration markedly and N application had
done so to some extent. Soon thereafter crop-
ping started to decrease N

2
O in soil air as a con-

sequence of decreased nitrate in the soil due to
uptake by plants. Cropping still affected N

2
O in

soil air in the second year, while the other treat-
ments did not. Irrigation probably caused losses
of soil nitrate by denitrification and leaching. As
a consequence, the high concentration of N

2
O in

bare soil in the first autumn was markedly low-
ered by irrigation. Heterotrophic denitrification
by bacteria is the most probable source of N

2
O

in the conditions prevailing in the present ex-
periment (see Granli and Bøckman 1994). Nitri-
fication probably also contributed to N

2
O pro-

duction, since the concentration of N
2
O in soil

air was generally higher than the ambient level
even when the soil was not wet (Bremner and
Blackmer 1978). When the soil was very wet due
to irrigation and soil nitrate was depleted by the
crop, lower than ambient concentrations were
found, suggesting that N

2
O was reduced to N

2

more rapidly than N
2
O was produced by denitri-

fication or diffused from the atmosphere to the
soil.

N application increased N
2
O in soil air dur-

ing several periods throughout the first growing
season. The increases were largest in bare soil,
when it was wet either due to irrigation or rain.
The biggest increase during a fortnight was on
average 2.5-fold. This compares well with the
results of Hansen and Bakken (1993) in uncom-
pacted soil. They reported a much higher (100-
fold) increase due to mineral N application in
compacted soil only.

The emissions of N
2
O were in the range re-

ported in other studies made in comparable con-
ditions with similar methods (e.g. Kaiser et al.
1996, MacKenzie et al. 1997). Substantially
higher emissions were observed only occasion-
ally in their studies. The correlation between
emission and N

2
O in soil air was expectedly bet-

ter at a depth of 15 cm than deeper in the soil.



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An air-filled porosity of 10% v/v is common-
ly regarded as critical for the satisfaction of the
oxygen demand of the crop (Wesseling 1974).
However, the actual critical value depends on
oxygen consumption rate, pore size and pore
continuity in the soil, and will therefore change
with microbial activity, temperature, vegetation,
soil type and soil structure. Values from 8% to
15% have been reported (Wesseling 1974, Hodg-
son and MacLeod 1989, Chan and Hodgson
1995). There are also cases where no critical
value could be determined (Chan and Hodgson
1995).

Assuming that the soil was water-saturated
during the rather long period of wetness in the
first year, an estimate (probably an underesti-
mate) of total porosity is 43–45%. It is almost
certain that in the present study plant growth was
not affected by the limited gas exchange when
the soil moisture was below 30% v/v. This cor-
responded to air-filled porosities of at least 13–
15% v/v. Periods of limited gas exchange accord-
ing to the adopted criterion were in the unirri-
gated soil from the middle of July onwards in
the first year and until the middle of June in the
second year. In the irrigated soil there was short-
age of air-filled porosity already in the latter part
of June. The irrigation in August of the second
year caused a period of restricted gas exchange

after the middle of August. The periods referred
to above agree quite well with the periods of
decreased O

2
 and increased CO

2
 concentrations.

The agreement with elevated N
2
O concentrations

is also reasonably good, if the existence of ni-
trate in the soil is also considered.

In conclusion, it can be stated that in the con-
ditions prevailing in southern Finland the O

2 
in

soil air might be markedly decreased in wet pe-
riods. Especially under crop stands the O

2
 con-

centration may drop substantially to a level
where the plants, if the low concentration per-
sists, may suffer from O

2
 deficiency (see Glín-

ski and Stépniewski 1985, Jaakkola et al. 1990).
Increases of CO

2
, although occasionally very

large, probably do not reach detrimental levels.
In wet soil, denitrification causing losses of ni-
trate N and increasing N

2
O emission is obvious.

Marked increases of CH
4
 or C

2
H

4
 in soil air do

not seem to be probable in the conditions of the
present study.

Acknowledgements. We thank Dr. Delbert Mokma (Michi-
gan State University, USA) for soil classification, the Agri-
cultural Research Centre for analysing the soil extracts, the
Finnish Meteorological Institute for providing the precipi-
tation data, and Henry Fullenwider for revising the text.
The financial support from the Academy of Finland is grate-
fully acknowledged.

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Vol. 7 (1998): 491–505.

SELOSTUS
Maan märkyyden vaikutus ilman koostumukseen ja dityppioksidiemissioon hiuemaassa

Antti Jaakkola ja Asko Simojoki
Helsingin yliopisto

Kaksivuotinen kenttäkoe tehtiin hiuemaalla Etelä-
Suomessa. Faktorikokeen koetekijöinä olivat rankka
kastelu ja typpilannoitus. Osa ruuduista oli kasvitto-
mia, osalla kasvoi heinää. Maan ilman koostumusta
15 ja 30 cm syvyydessä seurattiin yhden tai kahden
viikon välein otetuista näytteistä. Myös maan kos-
teutta (TDR) ja lämpötilaa mitattiin säännöllisesti.
Kasvittoman, kastelemattoman ja lannoittamattoman
maan ilmassa oli 14–21 % happea, 0,1–2 % hiili-
dioksidia ja 0,2–100 µl l-1 dityppioksidia. Lukuunotta-
matta toisen vuoden toukokuun kaikkein suurimpia
dityppioksidipitoisuuksia, suurin pitoisuus oli 27 µl
l-1. Maan kosteus vaihteli välillä 11–45 % ja lämpö-
tila 15 cm syvyydessä välillä 0–21°C. Kasvipeite ja
kastelu vähensivät happipitoisuutta ja lisäsivät hiili-
dioksidipitoisuutta. Happipitoisuus muuttui selvästi
enemmän syvemmällä maassa (30 cm) kuin matalam-
massa (15 cm), sen sijaan hiilidioksidipitoisuuden
muutos oli syvyydestä riippumaton. Kasvipeitteisen
ja kastellun maan pienimmät happipitoisuudet olivat

7 % (15 cm syvyydessä) ja 3 % (30 cm syvyydessä).
Suurimmat hiilidioksidipitoisuudet olivat 9 %. Typ-
pilannoitus ei vaikuttanut merkitsevästi maan ilman
happi- ja hiilidioksidipitoisuuksiin. Kastelu lisäsi
maan ilman dityppioksidipitoisuutta silloin kun maas-
sa oli runsaasti nitraattia. Kasvittomassa maassa oli
runsaasti nitraattia jäljellä vielä elokuussa. Typpilan-
noitus nosti maan ilman dityppioksidipitoisuutta eri-
tyisesti kastellussa kasvittomassa maassa. Kasvipei-
te vähensi dityppioksidipitoisuutta. Maan ilman koos-
tumuksen vaihtelua voitiin osittain selittää arvioidun
ilmahuokoisuuden avulla. Muutamalta ruudulta sul-
jetun kammion menetelmällä mitattu koekentän di-
typpioksidiemissio vaihteli välillä 0–40 g N ha-1 d-1.
Kaikkien mittausten keskiarvo oli 7 g N ha-1 d-1.
Emissio korreloi 15 cm syvyydestä mitatun dityppi-
oksidipitoisuuden kanssa (r=0,80; n=234) ja 30 cm
syvyydestä mitatun pitoisuuden kanssa (r=0,65,
n=234).


	Title
	Introduction
	Material and methods
	Statistical analysis
	Results
	Discussion
	References
	SELOSTUS