Agricultural and Food Science, Vol. 14 (2005): 57–69.

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

Vol. 14 (2005): 57–69.

A comparison of nitrogen and carbon reserves
in acid sulphate and non acid sulphate soils in

western Finland
Maija Paasonen-Kivekäs

Helsinki University of Technology, Laboratory of Water Resources, PO Box 5200,
FI-02015 HUT, Finland, e-mail: maija.paasonen@hut.fi

Markku Yli-Halla
MTT Agrifood Research Finland, FI-31600 Jokioinen, Finland

Previous studies suggest that nitrogen (N) loads from acid sulphate soil (AS soil) catchments in Finland are
higher than those from other agricultural catchments. This study seeks to explain this difference by measur-
ing carbon (C) and N profiles in both an AS soil and a neighbouring non AS soil. In Lapua, western Finland,
two adjacent fields (Dystric Cambisols), subjected to similar agricultural practices, were analysed to the
depth of 240 cm for pH, total C (Ctot), total N (Ntot), NH4

+-N, NO3
--N, sulphur and bulk density. Field A, an

AS soil, contained sulfidic materials and 0.9% Ctot below 170 cm, while Field B, not an AS soil, had 0.3%
Ctot in the subsoil and no sulfides. In these soils, the groundwater level declined below 200 cm in summer,
subjecting the subsoil to oxidation. This study revealed large stocks of Ctot, Ntot, and mineral N in the sub-
soil, particularly in the AS soil. At 20–240 cm, Field A contained 292 tons of Ctot ha

-1 and 25 tons of Ntot
ha-1, while Field B had 152 tons of Ctot ha

-1 and 11 tons of Ntot ha
-1. Field A contained up to 435 kg of min-

eral N ha-1 in autumn, while in Field B there was only up to 137 kg of mineral N ha-1. In Field A, NH4
+-N

dominated strongly, while NO3
--N dominated in Field B. It is suggested that the greater concentration of

mineral N in the AS soil is due to 1) a greater stock of total (mineralizable) N and 2) the slower rate of ni-
trification resulting in substantial NH4

+-N retention on cation exchange sites.

Key words: carbon, nitrogen, nitrification, mineralization, organic matter, acid sulphate soils, subsoil, drain-
age

Introduction

Soils developed in materials sedimented in lakes
and small bays of the sea often contain plenty of

organic matter in the subsoil. Organic matter origi-
nates from the biota of the water and has been
thoroughly mixed with the mineral matter, mostly
fine or medium textured. In the Nordic countries,
these mud materials are traditionally called gyttja

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

Paasonen-Kivekäs, M. & Yli-Halla, M. N and C reserves in acid sulphate and non acid sulphate soils

(e.g., Wiklander et al. 1950) and they closely re-
semble the limnic materials of Soil Taxonomy
(Hansen 1959). They often contain substantial
amounts of sulphur (S), accumulated as iron
sulfides in the sediment under reducing conditions.
The parent material of acid sulphate soils (AS
soils), which was deposited predominantly during
the Litorina period (7500–3000 BP) in the Baltic
basin, is a typical example of these soils, occurring
particularly on the western coast of Finland
(Purokoski 1958, Erviö 1975). These former sedi-
ments have been brought into an oxidized environ-
ment by the isostatic land uplift and agricultural
drainage. Small shallow lakes and wetlands have
also been drained to serve as agricultural land. Ac-
cording to the FAO system (FAO 1988), these soils
are commonly classified as Gleysols (Öborn 1989,
Yli-Halla 1997, Joukainen and Yli-Halla 2003),
while the best-drained ones are also classified as
Cambisols (Öborn 1989).

Inventories of carbon (C) in soil and lake sedi-
ments have recently been carried out in Finland,
resulting in estimates of that element in forest soils
(Kauppi et al. 1997, Liski and Westman 1997), in
peatlands (Minkkinen 1999) and lake sediments
(Kortelainen and Pajunen 2000). For agricultural
lands, there are plenty of data on the C content in
the plough layer but fewer dealing with the sub-
soil. Inaccurate information about the areas of the
different soil types also contributes to the fact that
a detailed inventory of C in Finnish agricultural
soils is still missing.

In organic forms, C and nitrogen (N) are inti-
mately linked. In mineral soils, over 95% of the
total nitrogen (Ntot) is generally contained in soil
organic matter. The major part of these N reserves
is usually in the top layer due to accumulation of
crop residues and humus. Investigations of AS
soils (Wiklander et al. 1950, Öborn 1989, Yli-Hal-
la 1997, Bärlund et al. 2004) have indicated a high
content of Ntot in the subsoil also. This N stock is
potentially subjected to mineralization, which is
controlled by several factors, such as composition
of the substrates, soil temperature, moisture and
pH. Mineral N fractions, ammonium nitrogen
(NH4

+-N) and especially nitrate nitrogen (NO3
--N)

are liable to leaching via subsurface drainage flow

or deep percolation of groundwater. Indeed, within
the Finnish network of small representative catch-
ments, the N load from catchments with AS soils
has been observed to be higher than the loads from
other agricultural catchments in Finland (Reko-
lainen 1989, Vuorenmaa et al. 2002).

This study seeks to explore the origins of this
increased N load by comparing soil profile con-
centrations of C and N to depths of 240 cm in an
AS soil and a neighbouring non AS soil, both un-
der crops, in western Finland. The aims of the
study were (1) to investigate the vertical distribu-
tion of C and Ntot and mineral nitrogen (Nmin) re-
serves in an AS soil compared with a non AS soil
and (2) to evaluate the role of native organic mat-
ter as a source of Nmin. Concentrations of Nmin in
AS soils have not been published before; this is the
first study to monitor Nmin below 120 cm in any
Finnish agricultural soil. This study contributes to
our knowledge of the stock and distribution of N
and C in AS soils. This basic information is needed
for a more detailed understanding of the observed
net transport of N to watercourses and to develop
sound management practices for AS soils, as well
as to assess the impacts of different practices in-
volving these soils.

Material and methods

Study site
The experimental site is located at Lapua (62°51'N,
23°15' E) in Ostrobotnia, western Finland (Fig. 1).
The site is 60–62 m above the sea level and the
surface slope is about 1%. The study was carried
out for two field sections (Fields A and B), which
have separate subsurface drainage systems. The
distance between the experimental areas is about
100 m. The area of Field A is 2.47 ha and that of
Field B is 0.59 ha. The soil texture is fine sand to a
depth of about 0.5 m, while deeper horizons con-
sist of silt. The clay content of the top layer (0–30
cm) is 11–15%, increasing downwards to 27–29%
at the depth of 50–100 cm.

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

Vol. 14 (2005): 57–69.

Drainage of the fields was accomplished with
open ditches in the 1960s and with subsurface
drainage pipes in the early 1970s. The drain depth
in Field A varies from 1.0 to 1.5 m. The field drains
discharge into a main collector pipe having an out-
let to a main ditch flowing between the fields. In
Field B, the drain depth is about 1 m and the drains
discharge directly to the main ditch. The initial
drain spacing was 20 m, but the spacing was halved
to 10 m in spring 1993 for a controlled drainage
experiment.

The crop was mainly starch potato (Solanum
tuberosum, L.) several years before and during the
study period. The annual N fertilizer rate was 70–
83 kg ha-1. No manure or other types of organic
fertilizers were applied. Agricultural practices in
the two fields were the same and representative of
those employed in the region.

Soil sampling and groundwater
observations

Soil sampling and monitoring of groundwater lev-
el were carried out during the period 1994–1996.
The soils were sampled down to 240 cm taking
samples at 20–40 cm increments with a 25 mm di-
ameter manual drill. Upon sampling, the soil was

sealed in airtight plastic containers, which were
stored in cool boxes. The sampling dates were 1)
after snowmelt and before fertilization in May and
2) after harvest in November. The number of sam-
pled profiles was 1–3 per field and date. The total
number of profiles sampled was 10 for Field A and
7 for Field B.

The depth of the water table was recorded in
observation wells in each field in the middle of two
drain lines. There were twelve wells in total, with
depths ranging from 1.6 to 2.5 m. The wells were
made of 30 mm diameter PVC pipe surrounded by
a filter. Manual observations using a graded water-
detecting gauge were carried out on a two-to-four-
week basis. Furthermore, automatic pressure sen-
sors (Jensen Ltd. Type PSL) with a 15-min meas-
urement interval were installed in three wells in
Field A. Due to technical problems, continuous
water table data series were not achieved. The field
site and measurements have been described in
more detail by Paasonen-Kivekäs et al. (1997).

Soil analyses
Mineral N was determined from all sampled pro-
files. The soil samples, stored at +4°C, were ana-
lyzed within 3–4 days after sampling. Ten grams of
fresh soil was extracted by shaking with 50 ml of 2
M KCl for 1 h. The suspension was centrifuged for
5 min and then filtered using a Schleicher & Schuell
595 filter. The concentrations of NO3

--N and NH4
+-

N in the extract were measured by a flow injection
analyser (Tecator, FIAstar 5010 Analyzer) accord-
ing to the Tecator Application Notes ASN 65-31/84
and ASN 65-32/84 (Emteryd 1989). The contents
of NO3

--N and NH4
+-N were calculated as mg g-1 of

dry soil. Water content in the fresh soil samples was
determined gravimetrically by drying at 105ºC.
The soil samples were analysed for pH(H2O) in the
field immediately after sampling (MacLean 1984;
pH meter ORION SA 520).

Total S (Stot), Ctot and Ntot were determined us-
ing air dry samples taken in May 1994. These con-
centrations were assumed to be so stable that they
were determined only once from three soil profiles
from Field A and two from Field B. A Leco CN

Fig. 1. Location of the Lapua study area in western Fin-
land.

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

Paasonen-Kivekäs, M. & Yli-Halla, M. N and C reserves in acid sulphate and non acid sulphate soils

2000 dry combustion analyzer was used for the de-
termination of Ctot and Ntot. The detection limit
(blank mean + 3 × standard deviation of the
blanks), calculated on the basis of the results over
a long time, is 0.12% and 0.09% for Ctot and Ntot,
respectively. Mean deviation between the two rep-
licates of each sample was on average 1.5% for Ctot
and 3.2% for Ntot. All C was assumed to be organ-
ic. Total S was digested with concentrated HNO3
(SFS 3044) and determined by plasma emission
spectroscopy. Subsoil samples (110–240 cm) were
tested for the presence of sulfidic materials by the
method of Soil Survey Staff (1999) using an aero-
bic incubation of 8 weeks. After incubation, the
pH(H2O) was determined and sulphate sulphur
(SO4

2--S) was extracted from the soil samples with
0.01 M CaCl2 and determined by plasma emission
spectroscopy. Mean deviation between the two
replicates was on average 1.5% for SO4

2--S and
1.8% for Stot.

Bulk density (BD) was determined for the
depths 0–30 cm, 40–50 cm and 60–70 cm by the
core sampling method (Blake 1985a). Bulk density
for deeper horizons was calculated on the basis of
C content using the formula presented by Howard
et al. (1995): BD = 1.3 – (0.275 × log C%). Parti-
cle density was measured employing a pycnometer
(Blake 1985b). Porosity was calculated from the
BD and particle density (Vomocil 1985). Total po-
rosity of the plough layer was about 55 vol-% in
both fields. The porosity of the silty subsoil below
the fine sand layer was about 51 vol-% in Field A
and about 48 vol-% in Field B.

The concentrations of Ctot, Ntot, NO3
--N and

NH4
+-N were converted to quantities, expressed as

kg ha-1, for the individual soil layers using the BD
values. The measured BD values were used for the
upper three layers and the estimated values
(Howard et al. 1995) for the deeper ones. The
amount of Nmin was calculated as the sum of NO3

--
N and NH4

+-N quantities.

Plant sampling and analyses
To estimate the N content of the harvested crop
and in the above-ground plant residues, the plants

were also analysed for N. Ten potato plants were
collected randomly over each experimental area.
The crop density was determined by measuring the
number of potato plants per 10 m of ten randomly
selected beds in each area. The plants were divided
into leaves, stems and tubers. The samples were
oven dried (12 h, 105ºC) and their dry weight was
determined. The nitrogen concentration of each
plant organ was determined by using a Leco CHN-
900 analyzer (Kleemola and Teittinen 1996).

Results

Soil profiles
The three studied profiles in Area A had sulfidic
materials in the subsoil, which started at the depth
of 140 cm to 200 cm and had its average upper
boundary at 170 cm, see Table 1. According to Soil
Taxonomy (Soil Survey Staff 1999), fresh pH
> 4.0 and a decrease of soil pH by at least 0.5 units
to values below 4.0 indicates the presence of
sulfidic materials. The Stot concentration of these
horizons was, however, too low (< 0.75%) to qual-
ify as sulfidic materials of the FAO system (FAO
1988). Morphologically, these horizons were grey
and massive, indicating predominantly reduced
conditions. The oxidized horizons above (80–140
cm) had a pH of between 3.5 and 4.0, no decrease
of pH upon incubation and a well developed struc-
ture, stabilized by plenty of iron hydroxide. The
concentration of SO4

2--S at 140–170 cm was just
above 0.05% (Table 1). As the soils were sampled
in spring, soon after the snow melt waters had
leached the soil profile, higher values of SO4

2--S
may have been measured in summer. Area A did
not have a sulphuric horizon (pH < 3.5; Table 1),
but the criteria of the sulfic attribute of Soil Tax-
onomy were barely met.

Deviating substantially from Area A, Area B
did not have sulfidic materials within the investi-
gated depth (–240 cm). The pH of the incubated
subsoil samples remained above 5.2. Also the con-
centration of SO4

2--S was very low throughout the
two profiles sampled, and the concentrations of Stot

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

Vol. 14 (2005): 57–69.

in the subsoil of Area B were only 3–10% and Ctot
38–53% of those in the corresponding horizons of
Area A.

In the Lapua area, soils have a cryic tempera-
ture regime (Yli-Halla and Mokma 1998). In spite
of a relatively high content of organic matter, the
soils were not very dark (moist colour 7.5YR 4/2,
dry colour 7.5YR 6/2) and they consequently had
ochric Ap horizons. According to Soil Taxonomy
(Soil Survey Staff 1999) and given the require-
ment that acid sulphate characteristics occur within
150 cm of soil surface, Area A was a Sulfic
Cryaquept and Area B a Typic Cryaquept. Accord-
ing to the FAO system (FAO 1988), both areas rep-
resented Dystric Cambisols, because the more
stringent criteria of acid sulphate characteristics of
the FAO system (pH < 3.5 and total S of sulfidic
materials > 0.75%) were not met.

Total carbon and nitrogen
The Ctot- and Ntot-concentrations, shown in each
sampling layer in Table 2, indicated clearly that
Field A was richer in both elements, the relative

difference being largest below the depth of 50 cm.
In both fields, the amount of Ctot and Ntot varied
clearly with the depth (Table 3). The amount of Ctot
in the plough layer accounted for 40% of the re-
serves in the whole profile of 0–240 cm in Area A
and 53% in Area B. The horizons of 100–240 cm
formed 36% of Ctot in Area A and 27% in Area B.
Concerning Ntot, the plough layer had only 25%,
while the depths below 100 cm had as much as
52% in the profile of the 0–240 cm in Area A. The
corresponding proportions in Area B were 38%
and 42%, respectively. The mean C/N ratio at 0–20
cm was about 23 in both fields. The ratio decreased
sharply below the depth of 50 cm (Table 2).

Mineral nitrogen
On average, Nmin (NO3

--N and NH4
+-N) accounted

for 1.4% and 0.8% of Ntot in the whole profile (0–
240 cm) in Area A and in Area B, respectively. The
amount of Nmin and its fractions and their vertical
distribution in the soil profile clearly differed be-
tween the fields (Table 4). There was over three
times more Nmin in Field A than in Field B. In both

Table 1. The pH values and sulphur concentrations of soil samples taken from different depths of the
experimental fields in Lapua. The concentrations of SO4

2--S were analysed from incubated samples.

Depth pH pH SO4
2--S Total S

cm fresh incubated % %

Field A
0–20 5.7 n.d. n.d. 0.05

20–50 5.3 n.d. n.d. 0.02
50–80 4.3 n.d. n.d. 0.03
80–110 3.9 n.d. n.d. 0.05

110–140 3.9 3.9 0.015 0.12
140–170 4.1 3.7 0.050 0.17
170–200 4.5 3.7 0.082 0.26
200–240 6.2 3.5 0.206 0.41

Field B
0–20 6.2 n.d. n.d. 0.04

20–50 4.9 n.d. n.d. 0.02
50–80 5.0 n.d. n.d. 0.01
80–110 5.3 n.d. n.d. 0.01

110–140 5.4 5.2 0.002 0.01
140–170 5.4 5.3 0.002 0.02
170–200 5.4 5.3 0.002 0.01
200–240 6.0 5.2 0.003 0.02

n.d. = not determined

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Paasonen-Kivekäs, M. & Yli-Halla, M. N and C reserves in acid sulphate and non acid sulphate soils

Table 2. Mean concentration of total carbon (C) and nitrogen (N), C/N ratio and bulk density of soil
samples taken from different depths in Fields A and B at the Lapua site.

Depth Total C Total N C/N Bulk density
cm % % ratio g cm-3

Field A
0–20 6.65 0.29 22.8 1.07

20–50 1.92 0.10 19.5 1.54
50–80 0.56 0.05 11.2 1.35
80–110 0.91 0.09 10.1 1.31

110–140 0.90 0.09 9.9 1.31
140–170 0.86 0.09 9.5 1.32
170–200 0.87 0.09 9.6 1.32
200–240 0.78 0.08 9.8 1.33

Field B
0–20 5.71 0.25 23.3 1.14

20–50 1.27 0.06 21.1 1.52
50–80 0.22 0.02 11.0 1.66
80–110 0.22 0.02 10.9 1.48

110–140 0.34 0.03 11.3 1.43
140–170 0.45 0.04 11.1 1.40
170–200 0.46 0.04 11.6 1.39
200–240 0.38 0.04 9.4 1.42

Table 3. Mean stock of total carbon (C) and nitrogen (N)
in different depth intervals and in a 0–240 cm profile in
Fields A and B in Lapua.

Depth, cm Total C, tn ha-1 Total N, tn ha-1

Field A
0–30 172 7.8

30–50 59 3.0
50–100 46 4.4

100–150 59 6.0
150–200 57 5.9
200–240 42 4.3

0–240 435 31.4

Field B
0–30 150 6.5

30–50 39 1.8
50–100 17 1.6

100–150 24 2.1
150–200 32 2.8
200–240 21 2.3

0–240 283 17.1

fields, there was a prominent pool of Nmin in the
deeper soil layers. In Area A, 75% of the Nmin re-
serve was at a depth of 100–240 cm. The corre-
sponding value in Area B was 57%.

At the depth of 0–100 cm, NO3
--N formed over

80% of the Nmin pool in both fields. In Field A, the

amount of NH4
+-N sharply increased and NO3

--N
decreased below the 100–150 cm layer. The
amount of NH4

+-N at 150–240 cm was 256 kg ha-1,
accounting for 96% of the Nmin in this horizon. In
Field B, NO3

--N remained the dominant fraction at
all depth intervals, while the proportion of NH4

+-N
remained below 17%, even in the deepest layers.
The vertical distribution of the Nmin fractions with-
in each field gave the same type of pattern at all
sampling dates.

The amount of Nmin was relatively consistent
between the different sampling profiles and dates
within each field. The average amount of Nmin in
spring was 429 kg ha-1 in Field A and 122 kg ha-1 in
Field B. In November, there was only 6.1 kg ha-1
more Nmin at 0–240 cm in Field A than in May and
14.8 kg ha-1 more in Field B. The change in the
quantity of NH4

+-N and NO3
--N in Field A was 6.7

kg ha-1 and –0.6 kg ha-1, respectively. In Field B,
the quantity of NO3

--N increased by 23.0 kg ha-1,
whereas NH4

+-N decreased by 8.2 kg ha-1. The big-
gest changes from spring to autumn were detected
at the depths below 100 cm. The coefficient of
variation (CV) of NH4

+-N ranged from 19% to
80% within single soil layers. The lowest variation
was observed in the deepest layers of Field A. On

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

Vol. 14 (2005): 57–69.

Table 4. Content of ammonium, nitrate and mineral nitrogen (NH4
+-N, NO3

--N and Nmin, respectively) in different depth
intervals and in a 0–240 cm profile in Fields A and B in Lapua. Mean, minimum (min) and maximum (max) value,
standard deviation (std) and number of profiles studied (n).

Depth NH4
+-N, kg ha-1 NO3

--N, kg ha-1 Nmin, kg ha
-1

cm mean min max std mean min max std mean min max std n

Field A
0–30 4 1.4 11 3 34 13.1 56 14 38 17 57 13 10

30–50 2 0.0 3 1 20 10.2 38 11 23 12 40 11 10
50–100 7 1.7 16 5 39 27.5 73 13 45 29 89 17 10

100–150 40 15.7 70 17 22 8.3 33 8 61 48 88 12 10
150–200 106 68.5 160 30 7 2.0 24 6 114 74 165 33 10
200–240 150 98.3 180 29 2 0.2 11 3 152 100 180 29 10

0–240 309 185.6 440 124 61.3 234 433 280 619

Field B
0–30 3 1.2 7 2 15 10.9 19 3 18 12 21 3 6a/7b

30–50 2 0.4 5 1 9 4.9 18 5 11 5 20 5 6a/7b

50–100 4 1.1 7 2 23 16.1 35 7 27 19 40 7 6a/7b

100–150 3 0.6 5 2 27 21.4 31 4 29 22 33 4 6a/7b

150–200 2 0.0 4 2 25 19.5 29 5 27 21 33 5 6a/7b

200–240 3 0.1 6 2 15 7.7 20 5 18 14 22 4 6a/7b

0–240 17 3.4 34 114 80.5 152 130 93 169

a NO3
--N and Nmin

b NH4
+-N

average, the amount of NO3
--N showed more con-

sistency at a depth of 0–200 cm. The highest CV
(165%) was in the 200–240 cm layer in Field A,
where the reserve of NO3

--N was very small.

Nitrogen uptake by crop
The average fertilizer rate and N uptake of crops
are presented in Table 5. The N uptake between the
samples varied more within Field A (CV 40% on
average) than within Field B (CV 24% on aver-
age). The N content of potato stems and leaves
varied from 22.6 kg ha-1 to 61.9 kg ha-1 one week
before harvest depending on the area and year. Po-
tato tubers stored 5–93.6 kg ha-1 more N than the
amounts applied as fertilizer N.

Depth to groundwater table
Temporal variation of depth to groundwater table
is shown in Fig. 2. Since variation across the fields
was found to be small, the mean depths are pre-

Table 5. N fertilizer rate and mean N uptake by crop in
Fields A and B in Lapua. Residual N = fertilizer N – N in
the tubers.

Fertilizer rate
kg ha-1

N in tubers
kg ha-1

Residual N
kg ha-1

Field A
1994 76 106 –30
1995 83 148 –65

Field B
1994 76 81 –5
1995 83 177 –94

sented for each point of time. The water table in
Field A usually remained shallower than in Field B
due to the slightly lower topographic position and
control drainage measures of Field A. Soil in Field
B was also more effectively drained owing to its
closer vicinity to the main ditch compared to Field
A. Controlled drainage temporarily raised the wa-
ter table and retarded its decline in Area A.

The water table periodically rose until the drain
depth or higher during snow melt and rainy peri-
ods in summer and autumn. However, after the

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

Paasonen-Kivekäs, M. & Yli-Halla, M. N and C reserves in acid sulphate and non acid sulphate soils

rainfalls, the water table rapidly declined below
the drain depth in both fields. From July to No-
vember 1995, the depth to water table remained
below 2 m from the soil surface for about 80 days
in Field A and about 125 days in Field B. From
September to November 1996, the water table was
below 2 m for about 65 days in Area A and 88 days
in Area B. The water table even dropped to depths
exceeding the depth of the deepest observation
wells (2.5 m in Field A, 1.9/2.2 m in Field B, see
Fig. 2).

Discussion

Field A is a typical example of cultivated Finnish
AS soils. Horizons formed of non-sulfidic materi-
als exclusively cover the sulfidic materials, which
have been partly oxidized, a process accelerated
by increased drainage efficiency. Leaching of wa-
ter-soluble products of oxidation has resulted in a
subsoil rather low in soluble salts and a pH at 3.5–
4.0. Often the AS characteristics are not harsh
enough or they are too deep to meet the criteria of

international classification systems regarding AS
soils (Yli-Halla et al. 1999). On the basis of the
surveys by Erviö (1975) and Puustinen et al.
(1994), these kinds of soil are typical on the coasts
of Finland, the latter giving an estimate of as much
as 300,000 ha AS soils in Finland. These soils may
not cause problems to agriculture anymore and at
the top they may not be distinguished from non-
AS soils such as those of Field B, but they may
still produce hazardously acidic drainage waters
(Joukainen and Yli-Halla 2003). The fact that both
soils of the present study have, according to the
FAO classification system, the same name while
the sulfidic nature of the subsoil A is not recog-
nised, indicates the inadequacy of that system to
cope with this environmentally relevant character-
istic of effectively drained AS soil.

It is evident that Field B, with its subsoil pH
between 4.9 and 6.0, has never contained consider-
able concentrations of sulfide within the investi-
gated depth. This conclusion is supported by the
fact that, after oxidation of sulfide and washing out
the solutes, the pH of the soil remains at about 4,
which is not raised by leaching with water (Har-
tikainen and Yli-Halla 1986). Therefore we con-

Depth of pipe drains

0.0

1.0

2.0

3.0

1.1.1994 31.5.1994 28.10.1994 27.3.1995 24.8.1995 21.1.1996 19.6.1996 16.11.1996

G
ro

un
dw

at
er

d
ep

th
(

m
)

Field A
Field B
Depth below max. observation depth (2.5 m) in field A
Depth below max. observation depth (1.9/2.2 m) in field B

Fig. 2. Groundwater depth in Fields A and B in Lapua. No observations were available from 8 Nov 1995
until 7 April 1996. The depth of pipe drains represents the average depth in the fields.

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

Vol. 14 (2005): 57–69.

clude that the difference between the two adjacent
fields is caused by the large native heterogeneity
indicated clearly in the AS soil material of Puus-
tinen et al. (1994).

The concentration of Ctot in the subsoil in Field
A was similar to the values observed at corre-
sponding depths in, for example, the soils of Ylis-
taro (about 1%, Yli-Halla 1997) and Ilmajoki (0.6–
1%, Joukainen and Yli-Halla 2003), both AS soils
less than 100 km from the present experimental
field, and in similar soils of Kungsängen (1.3%,
Kirchmann 1991), Ängesby and Ersnäs (0.9–1.3%,
Öborn 1989) in Sweden. These concentrations are
much lower than in the subsoils of the more severe
AS soils of western Finland, such as Laitila (2.5–
3%, Yli-Halla 1997) and Mustasaari (1.6–2.2%,
Joukainen and Yli-Halla 2003), which probably
occupy a relatively small area (Yli-Halla et al.
1999). The much lower concentrations of Ctot in
Field B resembled the values (0.1–0.5%) measured
at similar depths in other soils without any AS
characteristics, several examples of which can be
found in Yli-Halla et al. (2000).

As compared to other AS soils with similar
concentrations of organic matter in their subsoil,
the concentrations of Ntot at 80–240 cm in Field A
(0.08–0.09%) were higher than the values at
Liminka (0.04–0.06%,) but lower than those in
Ylistaro (0.18–0.24% at 50–150 cm, Yli-Halla
1997) and in Ilmajoki (0.23–0.27% at 170–300
cm, Bärlund et al. 2004). Field A had clearly lower
concentrations of Ntot than more severe AS soils
with higher concentrations of organic matter in
their subsoil, such as Mustasaari (0.33–0.36% at
50–200 cm, Bärlund et al. 2004) or Laitila (0.32–
0.58% at 50–150 cm, Yli-Halla 1997). In Field B,
the concentrations of Ntot (0.02–0.04%) in the sub-
soil were in the range of those measured in the ag-
ricultural silt and fine sand soils of Finland (0.027–
0.068% at 40–100 cm, Sippola and Yläranta
1985).

The reserves of Ntot in the 0–100 cm layer in
both fields were similar to reserves determined in
other agricultural soils in Finland. The values com-
monly range from 6300 to 10 900 kg ha-1 at 0–60
cm (Sippola 1981) or up to 13 300 kg ha-1 at 0–100
cm (Linden et al. 1992), usually being higher in

clay soils than in coarse mineral soils. Indeed, the
marked differences between the two soils of the
present study were below 100 cm, where the stock
of Ntot of the AS soil was much greater.

The Nmin amount at 0–100 cm in both fields
was equivalent to that of other Finnish soils. The
Nmin in agricultural mineral soils in spring before
fertilization has generally varied from 7 to 150 kg
ha-1 at 0–60 cm or 0–100 cm profiles (Sippola and
Yläranta 1985, Leppänen and Esala 1995, Kuisma
2002), but Nmin amounts up to 290 kg ha

-1 at 0–60
cm or 0–90 cm in spring and autumn have also
been observed in some fields (Leppänen and Esala
1995, 1999). The highest values were connected
with animal manure application and cultivation of
grass and vegetable crops.

Even though the Ntot concentrations in the sub-
soil samples were generally small, they seem to be
quite accurate. This conclusion can be drawn from
the small mean deviation of the replicates and the
small variation of the C/N ratio in the subsoil sam-
ples. The biggest inaccuracy of this study is prob-
ably the estimation of the BD values of the subsoil
with the help of the Howard et al. (1995) equation.
Indeed, that equation gave much lower BD values
than were actually measured for the soil samples
taken from 40–50 cm, probably compacted by ag-
ricultural operations. However, the close match
between the calculated and measured BD values of
the soil samples taken from 0–30 cm and 60–70
cm gives us confidence in the calculated values of
the deeper layers, which are not affected by com-
paction. We therefore think that the estimates of
the Ctot and Ntot in the deeper layers are reasona-
ble.

In both years of the experiment, the N uptake
by potato tubers was higher than the amount of N
fertilizers applied, an observation also made in
other experiments (Kuisma 2002). Consequently,
fertilization does not explain the substantial pool
of Nmin in the subsoil. Moreover, the fields were
managed similarly. We therefore conclude that the
fact that there was a higher Nmin pool in Field A
than in Field B is explained by the larger amount
of organic matter and organic N. The fact that the
C/N ratio was clearly below 20 at 50–240 cm in
both fields indicates a net mineralization tendency

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

Paasonen-Kivekäs, M. & Yli-Halla, M. N and C reserves in acid sulphate and non acid sulphate soils

leading to an accumulation of Nmin (Stevenson and
Cole 1999). These horizons have been continu-
ously waterlogged before agricultural drainage,
which probably enhances the present N minerali-
zation by providing aeration of subsoil. The de-
cline of the water table well below the drains for
several weeks in summer and autumn has also
been observed in other studies of AS soils of the
region (Joukainen and Yli-Halla 2003). The de-
cline at the Lapua site is attributed to low precipi-
tation, evapotranspiration and deep percolation to
the neighbouring open channel. The oxidised hori-
zons in the fields have prominent iron-hydroxide-
lined cracks and remnant root channels that sub-
stantially increase hydraulic conductivity of the
fine textured soils. A rough estimate of annual N
mineralization is 1.5–3.5% of the amount of or-
ganic or total N (Brady and Weil 1999). In Finland,
N mineralization at 0–100 cm was about 0.6% of
Ntot in silty clay soil during the growing season of
barley (Sippola 1986). These results correspond
well with ours in terms of the ratio of the stocks of
Nmin and Ntot.

NO3
--N is usually the dominant Nmin fraction in

well-drained neutral-to-slightly-acid mineral soils.
The predominance of NO3

--N at 0–100 cm in Field
A and at 0–240 cm in Field B reflects favourable
conditions for the nitrifying micro-organisms. The
abundant accumulation of NH4

+-N below 100 cm
in Field A indicates retarded nitrification. It can be
attributed to 1) the higher water table, 2) a higher
content of organic matter consuming oxygen dur-
ing decomposition, 3) the strongly acidic condi-
tions in the aerobic subsoil and, consequently, high
concentrations of dissolved metals such as Al, 4)
the incidence of H2S in the reduced subsoil and 5)
low soil temperature (e.g., Paul and Clark 1989,
Scheffer and Schachtschabel 2002).

In spite of the periodically deep groundwater
table, the diffusion rate of oxygen into the deepest
horizons studied is assumed to be slow due to the
massive soil structure and high water retention ca-
pacity of the silt soil. Nitrification is one of the
most pH-sensitive soil reactions; the minimum pH
for the reaction to occur is about 4.5. However, in
acid soils (pH < 4), production of NO3

--N has been
reported to occur probably due to heterotrophic ni-

trifiers (Paul and Clark 1989). They are more acid
tolerant than autotrophic organisms, but the rate of
nitrification is much lower. The impact of the low
pH alone on nitrification in Field A was not clear
because high amounts of NO3

--N also occurred in
horizons which had a pH < 4.5.

Conversion of soil organic N to NH4
+-N has

been observed even at 2ºC, whereas very little
NO3

--N is formed below +5ºC. The rates clearly
increase above +15–20ºC (Karvonen 1992, Ste-
venson and Cole 1999). In Ylistaro, less than 100
km from the present experimental site, the average
monthly soil temperature in the period 1971–1990
at 100 cm in summer was > 10°C in only three
months and never at 200 cm (Heikinheimo and
Fougstedt 1992). At both depths, the soil tempera-
ture was < 5.0°C for six months with minimum
monthly values of 0.4°C at 100 cm and 2.8°C at
200 cm. The temperature conditions indicate that
release of mineral N probably proceeds in the bot-
tom layers of the studied soils at relatively low
rates throughout the year.

In spite of the fact that NH4
+-N occurred in the

subsoil of Field A in larger quantities than found
usually, it represented less than 0.2 cmol kg-1. That
is a negligible amount when compared to the esti-
mated cation exchange capacity of about 14 cmol
kg-1 (see the Ylistaro soil in Yli-Halla 1997). This
calculation thus shows that the subsoil of Field A
was in no way saturated with NH4

+-N, suggesting
that NH4

+-N can to a large extent be retained by
soil. In contrast to NH4

+, NO3
- ions are highly mo-

bile and move readily via drainage water and deep
percolation. A major part of NO3

--N leached below
the drain depth in Field A may be reduced to gase-
ous N forms (N2O and N2) by denitrification. In the
deeper horizons, low aeration and an ample supply
of soluble organic C form favourable conditions
for the reaction (Firestone 1982).

Conclusions
This study dealt with Ctot, Ntot and Nmin reserves in
an agricultural AS soil in western Finland as com-

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

Vol. 14 (2005): 57–69.

pared to a non AS soil. Despite the small amount
of research material, Ctot and Ntot of the experimen-
tal site were in the ranges commonly found in AS
soils in Finland. We therefore believe that we can
also make meaningful conclusions and generaliza-
tions on the basis of the Nmin results of this study.

A large stock of Nmin occurred below the depth
of 100 cm particularly in the AS soil, but also in
the non AS soil. These depths are usually ignored
in soil N investigations. This storage may be insig-
nificant for crops, but can have an environmental
impact releasing soluble N into drain pipes and
deep groundwater. The release of Nmin from the
deeper soil layers is expected to contribute much
more to the Nmin load of watercourses than losses
from fertilization. Significant amounts of Nmin
seem to be released from the large reserves of or-
ganic matter, particularly in previously poorly
drained AS soils, due to their high organic N con-
tent and low C/N ratio. The persistence and trans-
port of NH4

+-N, the dominant form of Nmin in the
subsoil of AS soil, need to be examined in further
studies. Measurements of Nmin in different types of
non AS soils are needed to further explain the dif-
ferences in N losses between AS soils and non AS
soils.

Acknowledgements. The authors thank the laboratory staff
of the two participating institutions for the soil analyses.
Thanks are extended to Dr. Jouko Kleemola and Mr. Matti
Teittinen for crop analyses carried out at the Department
of Plant Production at the University of Helsinki. Mr. Ka-
levi Pelanteri, the farmer of the experimental fields, is ac-
knowledged for the groundwater observations and co-op-
eration. The contribution of Prof. Pertti Vakkilainen and
Prof. Tuomo Karvonen to the research project of the Lapua
site is appreciated. The research was supported by funding
from the Finnish Field Drainage Foundation.

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Vol. 14 (2005): 57–69.

Tavanomaisten viljelymaiden hiilestä ja typestä valtaosa
on sitoutuneena muokkauskerroksen orgaaniseen ainek-
seen. Happamissa sulfaattimaissa on melko runsaasti
orgaanista ainesta myös muokkauskerroksen alapuolel-
la. Tehokkaasti kuivatetuissa maissa näistä varoista voi
mineraloitua melkoisesti typpeä maan toistuvan kuivu-
misen ja kostumisen seurauksena. Tässä tutkimuksessa
verrattiin Lapualla sijaitsevan happaman sulfaattimaan
ja vieressä sijainneen lajitekoostumukseltaan samanlai-
sen tavanomaisen maan hiili- ja typpivarojen määrää ja
mineraalitypen (ammonium- ja nitraattityppi) esiinty-
mistä. Molemmilla pelloilla oli viljelty pitkään perunaa,
ja satojen typen otto oli poikkeuksetta runsaampaa kuin
typpilannoitus. Maaprofiilit tutkittiin kerroksittain 240
cm:n syvyyteen saakka. Sulfaattimaalla 20–240 cm:n
välisissä maakerroksissa oli 292 tonnia hiiltä ja 25 ton-
nia typpeä hehtaaria kohti, kun tavanomaisessa maassa
vastaavalla syvyydellä oli hiiltä 152 tonnia ja typpeä 11
tonnia hehtaaria kohti. Syksyllä sulfaattimaassa oli mi-

neraalityppeä 435 kg/ha ja tavanomaisessa maassa 137
kg/ha. Typen arveltiin mineraloituneen pääasiassa maan
orgaanisesta aineksesta. Valtaosa sulfaattimaan mine-
raalitypestä oli ammoniummuodossa, kun taas tavan-
omaisessa maassa vapautunut typpi oli nitrifioitunut
nitraattitypeksi. Ammoniumtypen runsauden sulfaatti-
maassa arvellaan olevan seurausta ainakin maan suu-
remmasta vesipitoisuudesta ja heikommasta ilmanvaih-
dosta, alhaisemmasta pH:sta ja runsaammasta liukoisten
metallien pitoisuudesta. Tutkimuksen päätulos oli ha-
vainto siitä, että etenkin sulfaattimaan syvemmissä ker-
roksissa, ja myös tavanomaisella maalla, voi esiintyä
suuria mineraalityppimääriä, jotka voivat kuormittaa
pinta- ja pohjavesiä tai haihtua kaasumaisina typpiyh-
disteinä. Nämä mineraalitypen varat ovat tähän asti jää-
neet huomaamatta, koska maanäytteet typpimäärityksiä
varten otetaan yleensä korkeintaan metrin syvyyteen
saakka.

SELOSTUS
Happaman sulfaattimaan typpi- ja hiilivarat

Maija Paasonen-Kivekäs ja Markku Yli-Halla
Teknillinen korkeakoulu ja Maa- ja elintarviketalouden tutkimuskeskus (MTT)

A comparison of nitrogen and carbon reserves in acid sulphate and non acid sulphate soils in western Finland
Introduction
Material and method
Results
Discussion
Conclusions
References
SELOSTUS