International Journal of Geosciences, 2010, 1, 139-148
doi:10.4236/ijg.2010.13018 Published Online November 2010 (
Copyright © 2010 SciRes. IJG
Pedoecological Regularities of Organic Carbon Retention
in Estonian Mineral Soils
Raimo Kõlli1, Tiina Köster2, Karin Kauer1, Illar Lemetti1
1Institute of Agricultural and Environmental Sciences, Estonian University of Life Sciences, Kreutzwaldi, Tartu, Estonia
2Agricultural Research Centre, Tartu, Estonia
Received July 27, 2010; revised August 24, 2010; accepted September 1, 2010
Soil organic carbon (SOC) retaining capacities of epipedon (EP), subsoil (SS) and soil cover (SC) as a whole,
are soil type specific. Depending on individual and sites characteristics, the generalized humus status indices
of soil types (EP and SC thickness and SOC stocks) may vary. Land use and land use change primarily in-
fluence the properties and fabric of the EP, but the humus status (SOC concentration and stock, fabric of ho-
rizons) of the SS remains practically unchangeable. The mean mineral soils SOC stocks, EP quality and SOC
distribution in soil profiles depend mainly on the water regime, mineral composition (texture, calcareous-
ness), development of eluvial processes and the land use peculiarities of soils. The mean area weighted SC
SOC stock of Estonian mineral soils is 99.9 Mg ha–1, thereby the mean hydromorphic soils SOC retention
capacity considerably exceeds the SOC retention capacity of automorphic soils (means are accordingly 127.5
and 78.9 Mg ha–1). The sustainable management of SOC is based on adequate information about actual SOC
stocks and theoretically established or optimal humus status levels of soil types. The aggregate of SOC re-
tained in the mineral soils of Estonia (3,235,100 ha) amounts to 323 ± 46 Tg (1 Tg = 1012 g). Approximately
42% of this is sequestered into stabilized humus, 40% into instable raw-humous material and 18% into forest
(grassland) floor and shallow peat layers.
Keywords: Carbon Retention Capacity, Land Use, Mineral Soils, Pedoecological Regularities
1. Introduction
Soil organic carbon (SOC) is retained in the soil profile
and its horizons in different forms and states and, largely,
determines soil functioning and development [1-3]. A
prerequisite for the pedoecological study of SOC cycling
and dynamics is the quantification of SOC storage by
soil types, especially in those soil layers in which most
soil organic matter (SOM) is accumulated [4,5]. Every
soil type has specific SOC flow throughout the soil cover
(SC) depending on ecological conditions [6,7]. The fea-
tures characteristic of certain soil types may differ in
SOC stocks and cycling intensity, in soil edaphon activ-
ity, and in vertical distribution of the humus profile [4,8,
9]. Differences also exist in, for example, input composi-
tion (biochemical, ash content), in input dynamics, in
characteristics of transformation, and in the residence
time of SOM in soil [5,8]. Several studies [10-12] have
clarified that SOC-retaining capacity depends on the soil
moisture regime, clay and carbonate content in fine earth,
and on the character of soil management. In comparative
analyses and evaluation of local soils SOC-retaining po-
tential, the generalized soil types SOC densities and total
stocks of SC received in various ecological regions [13-
16] were necessary for our work.
In our previous work, the soil humus status (or func-
tioning of soil in relation to SOC stocks and cycling) was
treated separately in arable [17], forest [18] and grass-
land [19] soils. In these works, the basic characteristics
of soil humus status were the thickness and fabric of the
epipedon (EP) layer and SC, stocks and concentrations of
SOC (and SOM) in different soil horizons and layers,
and humus quality, determined by EP types. The main
tasks of the actual study were: 1) to generalize the data
about SOC stocks by soil types, 2) to analyse the influ-
ence of land use change on SOC retention in soil, 3) to
elucidate the generalized pedoecological regularities of
SOC retention in soil and management, and 4) to identify
the share of mineral soils in total Estonian SOC storage.
2. Materials and Methods
2.1. Materials
The sources of the quantitative characteristics of soils
and their organic carbon contents were created by using
the databases PEDON (characterizing soil profiles by
genetic horizons) and CATENA (transects established
for research of soil humus status). PEDON was compiled
initially in 1967-1985 and was updated twice, in
1986-1995 and 1999-2002. CATENA was developed
during field studies from 1987-1992. The bulk density
samples used in our study equate to approximately
one-tenth of PEDON’s and CATENA’s soil profiles.
Data on Estonian soils’ humus status (thicknesses of lay-
ers and SOC stocks) in different soil and land use types
[17-19], as well as soil productivity and EP types, are
taken from our previous papers [20].
2.2. Methods
The study used the macro-morphological quantitative
approach (individual soil profiles are characterized on
the basis of soil samples taken by soil genetic horizons)
to measure SOC stocks in soils. The basic characteristics
of soil humus status in our work are the thickness and
fabric of EP and SC, and concentrations and stocks of
SOC in different horizons of soil profiles. For this study
the soil horizon data were generalized in relation to: 1)
the EP (including forest floor), 2) the SC, and 3) the
subsoil (SS), which were arrived at by subtracting data
(X) of EP from SC,
XSS = XSC – XEP (1)
The SOC stocks were calculated on the basis of bulk
density and SOC concentrations (g kg–1) of soil samples
(determined by Tjurin [21]), taking into account the
presence of coarse fractions in soil horizons. In calculat-
ing total stocks by soil types and groups, the soil distri-
bution data received in the course of large scale mapping
(1:10,000) were used [22]. The names of soil groups
correspond to the World Reference Base for Soil Re-
sources (WRB) [23]. Applying program Statistica 7, two-
way Analysis of Variance followed by the Student test of
homogenous groups was used to analyse the data.
2.3. Pedoecological Conditions
The climatic conditions of Estonia are typical of the
temperate-zone of the Atlantic-continental region, with
an annual average air temperature +4 – 6°C and a pre-
cipitation rate of 500 – 700 mm [24]. The dominant pe-
doclimatic conditions are therefore frigid-udic and frigid-
aquic [25]. The principal texture of mineral soils (cover-
ing 76% of the Estonian territory) is loam (28%), sand
(26%), sandy loam (17%) and clay (5%) [22]. Wet
(aquic) soils equate to 36% of Estonia’s mineral soils,
followed by normally moist or fresh (udic) soils (23%),
moist or endogleyic (15%) and dry (aridic) soils (2%).
The study did not include organic soils, which form 24%
of Estonia’s soil cover.
2.4. Areas
As contemporary soil distribution data by land use are
unavailable, the study involved the area of forest, arable
and grasslands for 1993-1998, a period of stable land-use.
At that time, the total area of mineral soils in forest, ar-
able and grasslands measured 2,566,700 ha. The distri-
bution percentage of mineral soils by soil groups and of
soil groups by land use are presented in Table 1. From
Table 1. Studied mineral soil groups and their distribution by land use .*
Forest Arable Grassland
Group No Soil or soil association Soil code by WRB% from
F + A + G area** soils, in % from soil group area
I Rendzic & Skeletic & Gleyic Leptosols LP rz sk gl 1.8 35.4 20.0 44.6
II Mollic & Endogleyic&Calcaric & Endoskeletic CambisolsCM mo gln ca skn18.1 28.3 62.4 9.3
III Cutanic & Endogleyic Luvisols LV ct gln 9.2 20.6 65.0 14.4
IV Glossic & Gleyiglossic Albeluvisols AB gs gsg 12.8 22.3 74.0 3.7
V Haplic & Endogleyic Albeluvisols AB ha gln 6.5 51.9 35.2 12.9
VI Haplic & Endogleyic Podzols PZ ha gln 4.7 100.0 0.0 0.0
VII Mollic & Calcic & Eutric Gleysols GL mo cc eu 15.4 62.0 29.7 8.3
VIII Luvic & Epidystric Gleysols GL lv dyp 9.3 68.2 27.7 4.1
IX Spodic & Umbric & Dystric Gleysols GL sd um dy 7.9 91.6 4.4 4.0
X Saprihistic Gleysols GL his 6.4 65.3 17.3 17.4
XI Fibrihistic Podzols PZ hif 2.5 99.5 0.0 0.5
XII Eroded Cambisols & Regosols RG & CM eroded 1.8 0.0 74.0 26.0
XIII Deluvial Cambisols & Luvisols CM&LV deluvial 1.5 0.0 70.9 29.1
XIV Eutric & Epigleyic & Histic Fluvisols FL eu glp hi 1.5 45.3 6.1 48.6
XV Salic Gleysols & Fluvisols GL & FL sz 0.6 37.7 0.0 62.3
Total (%),
(km2) 100.0%
*Data taken from [22]; **Sum of forest, arable and grassland areas.
Copyright © 2010 SciRes. IJG
Copyright © 2010 SciRes. IJG
the aggregate area of mineral soils the postlithogenic (soil
groups I – XI) and synlithogenic mineral soils (groups
XII – XV) form correspondingly 94.7% and 5.3%, as
divided by Fridland [26]. The share of synlithogenic soils
is different by land use, forming 1.7% from forest, 6.2%
from arable and 19.1% from grassland areas.
Calculation of total SOC storage on the basis of the
recent period of stabilized land use areas (25,667.0 km2)
is justified by the fact that changes in soil properties re-
lated to land use changes take several years to be dis-
cernible, especially in Nordic areas [27]. There has also
been a notable land use changes in the last 15 years [28].
By our estimation, on the basis of different additional
information [29], the area with uncertain management of
mineral soils is ~6,684.0 km2. Therefore the total area of
Estonian mineral soils is 32,351.2 km2, which forms
76.3% of the total SC of Estonia.
2.5. Used Terms
The EP (epipedon or topsoil or humus cover) consists of
the forest floor (or organic horizon) and/or of humus,
raw-humus and peat (histic) horizons. The EP embraces
the most active soil component, which is closely coupled
with plant cover and via which the cycling of the main
part of organic carbon takes place. The term EP therefore
conjoins different classical soil horizons (organic, humus,
raw humus, peat) into one soil layer. The term SC (soil
cover or pedon or solum) encompasses the superficial
earth layer or total (actual) soil resource influenced by
soil forming processes. Therefore the SC consists of EP
and SS (eluvial (E) and illuvial (B) horizons). The thick-
ness of the SC is the depth from the surface to the un-
changed parent material or C horizon or to the boundary
between B and C horizons (in the presence of BC hori-
zon — to the middle of the BC horizon). The SOC re-
taining capacity, given in SOC weight per area in rela-
tion to a certain layer (Mg ha–1), is the amount of SOC
which a soil with certain properties is able to retain or
capture in conditions of equilibrated soil functioning. In
some work the above mentioned parameter is defined as
soil carbon density [16].
3. Results
3.1. Thickness of EP and SC
In order to generalize the soil humus status characteris-
tics, 15 mineral soil groups from forests, arable and
semi-natural grasslands were tabulated (Table 2). The
mean EP thickness of soil groups in the study is mostly
between 21 – 26 cm. Thinner EP occurs in Podzols in
which the humus horizon is absent, and in very young
coastal soils (soil group XV). Relatively thin EP is also
characteristic of strongly podzolized epigleyic soils
(groups IX and XI). The thickest EP is characteristic of
deluvial or colluvial soils formed by sediment accumula-
Table 2. Thickness of epipedon (EP) and soil cover (SC) of mineral soils and their comparison by land use.*
Comparison of EP thicknesses***,
Comparison of SC thicknesses***,
Group No Soil code by WRB n**
Mean EP
(cm) F/A F/G A/G
Mean SC
I LP rz sk gl 7/12/8 < (4) < (3) = 19.0cd (2) (4) = 23.3a
II CM mo gln ca skn 20/46/10 < (7) < (5) > (2) 25.8ef = = = 47.8c
III LV ct gln 11/8/ - (7) 25.6ef = 74.2fg
IV AB gs gsg 18/13/ - < (7) 22.3de = 92.6h
V AB ha gln 26/21/8 < (7) < (3) > (4) 20.9d = = = 74.3f
VI PZ ha gln 27/ - / - 5.0a 64.1e
VII GL mo cc eu 8/6/17 > (4) (2) < (6) 25.9ef = = = 39.8b
VIII GL lv dyp 16/4/3 (1) (6) (7) 25.0ef = > (18) > (17) 55.2cde
IX GL sd um dy 8/2/ - (7) 15.6c (30) 76.0fg
X GL his 5/1/ - (5) 21.6def (18) 46.9bcd
XI PZ hif 13/ - / - 14.8bc 75.8fg
XII RG & CM eroded - /168/ - 24.1ef 54.2d
XIII CM & LV deluvial - /154/ - 43.6g 79.6g
XIV FL eu glp hi - / - /14 26.2ef 37.2b
XV GL & FL sz - / - /8 9.8ab 15.3a
*Mean thicknesses of EP and SC - for forest [18], arable [17] and grassland soils [19]; **Number of studied soil profiles accordingly in forest, arable and
grasslands; ***F - forests, A - arable and G - grasslands; > and < indicate significant (p < 0.05) difference and mutual relationship; > and < non-significant (p >
0.05) difference, and = the absence of significant difference or the means are very similar; ****Letters following the mean indicate significant differences at p <
Copyright © 2010 SciRes. IJG
The thickness of mineral soils’ SC is mostly between
40 – 80 cm, with a standard deviation of 8 – 25 cm (co-
efficient of variability (CV) 18–30%). Thinner soils are
Leptosols (skeletic, rendzic), Fluvisols (salic) and Gley-
sols formed in coastal areas; some poorly developed
Gleysols, and Regosols form in severely eroded arable
areas. The CV of shallow soil thicknesses is 40% in most
cases. The greatest thickness is characteristic of Glossic
Albeluvisols and some deluvial soils’ SCs. The compari-
son of EP and SC thicknesses by land use indicates that
in most cases the arable soils’ EP is significantly thicker
than that of forest soils. In the case of SC thicknesses, no
substantial difference between natural and cultivated
soils was established or, their SC thicknesses are equal
and depend more on soil type peculiarities than on land
3.2. SOC Stocks of EP and SC
EP SOC stocks, in automorphic soils, vary between 16 –
80 Mg ha–1 (Table 3). Significantly smaller (p < 0.05)
SOC stocks accumulate in the EP of automorphic Pod-
zols and arable soils degraded by erosion (soil group XII).
EP SOC stocks are slightly larger in soils with higher
carbonate and clay contents. The EP SOC stocks that
accumulate in hydromorphic Gleysols and Fluvisols (110
– 115 Mg ha–1) are significantly greater (p < 0.05) com-
pared with automorphic soils. Exceptions among these
hydromorphic soils are strongly podzolized epigleyic
soils (groups IX and XI) and moderately developed
coastal soils (group XV). Relatively large stocks of SOC
are also accumulated in different kinds of deluvial soils
(group XIII), but the largest stocks are characteristic of
Sapric Gl e ys ols, whose EP is composed of sapric peat.
SOC retention in mineral soils’ SC depends largely on
SS thickness and its capacity to retain SOC. The largest
SOC stocks in the SS (58 – 70 Mg ha–1) are characteristic
of strongly podzolized epigleyic soils (groups IX and XI),
in which the humus-illuvial B horizon (Bh) is formed.
The smallest SS SOC stocks (5 – 8 Mg ha–1) are charac-
teristic of thin Leptosols and of different coastal and
eroded soils. The medium range of SOC stocks occur in
well-aerated automorphic and deluvial soils’ SS (within
limits of 25 ± 3 – 4 Mg ha–1) and in Gleysols (on average
12 – 15 Mg ha–1). The mean of SOC stock densities in
SC are, therefore, smallest in eroded soils and Podzols
(45 Mg ha–1) and largest in saprihistic soils (190 Mg
ha–1). SOC stocks in most automorphic and hydromor-
phic soils are within the limits of 65 – 90 Mg ha–1 and 95
– 130 Mg ha–1, respectively.
3.3. Aggregate SOC Stocks in Estonian Mineral
A total of 323 ± 46 Tg (1012 g) SOC is retained (Table 4)
in mineral soils of Estonia. The largest proportion of
SOC stock (42%) is situated in forest, 28% in arable, 9%
in grassland and 21% in other soils. Aggregate stock of
SOC is bound into the forest floor, stabilised soil humus
(humus, eluvial and illuvial horizons), raw-humus mate-
rial and peat or into the SOM situated in different soil
profile horizons. The stabilized humus, formed in condi-
tions of udic soil moisture regime equates, by our esti-
mation, to ~42% of the mineral soils’ total SOM. Ap-
proximately 40% is sequestered into raw-humus SOM
and the balance, 18%, into forest floor and shallow peat
layers, 75% of the total SOC stock is situated in the EP
or the active layer and 25% in the SS.
Table 3. Stocks of SOC of epipedon (EP) and soil covers (SC) of mineral soils and their comparison by land use.*
Comparison of EP SOC stocks***
(Mg ha–1)
Comparison of SC SOC
stocks*** (Mg ha–1)
Group No Soil code by WRB n**
Mean SOC stocks
in EP****,
(Mg ha–1) F/AF/G A/G
Mean SOC
stocks in SC****,
(Mg ha–1)
I LP rz sk gl 8/12/8 = = = 66.7d > (24) (29) = 74.9bc
II CM mo gln ca skn 22/46/10 = < (32) < (31) 67.8d = (25) (27) 89.8c
III LV ct gln 12/8/ - (13) 69.1de = 92.6cd
IV AB gs gsg 19/13/ - (7) 44.6c (5) 67.1b
V AB ha gln 28/21/8 (5) (4) (9) 45.0c (7) (9) > (16) 70.0b
VI PZ ha gln 31/ - / - 16.3a 44.5a
VII GL mo cc eu 15/6/17 > (39) = (44) 110.0f > (37)= (41) 122.4e
VIII GL lv dyp 16/4/3 = (44) (57) 115.1f = = = 128.8e
IX GL sd um dy 7/2/ - (5) 37.2abc (60) 95.5cd
X GL his 5/1/ - = 155.9g = 191.1f
XI PZ hif 13/ - / - 44.8bc 114.5de
XII RG & CM eroded - /168/ - 29.5b 36.6a
XIII CM & LV deluvial - /154 / - 80.5e 105.3d
XIV FL eu glp hi - / - /14 110.3f 125.1e
XV GL & FL sz - / - /8 51.0cd 56.5ab
*Mean SOC stocks of EP and SC - for forest [18], arable [17] and grassland soils [19]; **Number of studied soil profiles accordingly in forest, arable and
grasslands; ***F - forests, A - arable and G - grasslands; > and < indicate significant (p < 0.05) difference and mutual relationship; > and < non-significant (p >
0.05) difference, and = the absence of significant difference or the means are practically equals; ****Letters following the mean indicate significant differences
at < 0.05.
Table 4. Total SOC stocks in Estonian mineral soils (Tg ± SE*).
Characteristics** Automorphic soils Hydromorphic soils Totally
F + A + G soils
All mineral soils
of Estonian SC
Forest land, (103 ha) 489.7 782.1 1271.8 -
Arable land, (103 ha) 813.0 221.6 1034.6 -
Grasslands, (103 ha) 154.3 106.0 260.3 -
Total, (103 ha) 1457.0 1109.7 2566.7 3235.1
Forest soils stock, (Tg) 35.5 ± 4.2 101.9 ± 13.5 137.4 ± 17.7 -
Arable soils stock, (Tg)*** 65.8 ± 5.5 25.4 ± 8.0 91.2 ± 13.5 -
Grasslands soils stock, (Tg) 13.7 ± 2.2 14.2 ± 3.3 27.9 ± 5.5 -
Total SOC stocks in SC, (Tg) 115.0 ± 11.9 141.5 ± 24.8 256.5 ± 36.7 323.2 ± 46.2
Total SOC stocks in EP, (Tg) 81.3 ± 10.7 111.3 ± 24.3 192.6 ± 35.0 242.7 ± 44.2
Total SOC stocks in SS, (Tg) 33.7 30.2 63.9 80.5
Mwa SOC density of SC, (Mg ha–1) 78.9 127.5 99.9 99.9
Mwa SOC density of EP, (Mg ha–1) 55.8 100.3 75.0 75.0
Mwa SOC density of SS, (Mg ha–1) 23.1 27.2 24.9 24.9
Mwa SOC density of SC of forest soils, (Mg ha–1) 72.5 130.3 108.0 -
Mwa SOC density of SC of arable soils, (Mg ha–1) 80.9 114.6 88.2 -
Mwa SOC density of SC of grassland soils, (Mg ha–1) 88.8 134.0 107.2 -
*Sum of Standard Error; **Abbreviations: Mwa – weighted (by area) mean; SC – soil cover; EP – epipedon, and SS – subsoil; ***The SOC stocks in soil
groups XII-XIV supplemented the preliminary data [17] used in calculating the aggregate SOC stocks in arable soils.
The share of automorphic and hydromorphic (situated
mostly on lowland) soils in the aggregate SOC storage of
mineral soils is 45 and 55% respectively. This is inverse
to the share of these soils’ areas, which are accordingly
57 and 43%. The data indicate that the SS of hydromor-
phic soils is relatively poorer in SOC compared to auto-
morphic soils. The share of postlithogenic mineral SC in
the sequestration of SOC stocks (94%) is almost total
compared to the role of synlithogenic mineral SC (6%).
The area weighted average EP SOC density of hy-
dromorphic soils exceeds the rate of automorphic soils
by 1.8 times (Table 4). Although the EP thickness of
forest soils is significantly smaller than in arable soils,
another important influencing factor — the SOC concen-
trations of forest soils — is generally higher, and conse-
quently, the SOC stocks in EP and SC may be approxi-
mately similar in forest and arable soils [17,18].
3.4. Comparative Analysis of Soils’ Humus
Although some aspects of soil humus status peculiarities
were explained on the basis of data given in Tables 2
and 3, more comprehensive humus data are presented in
Table 5. For characterization of soils’ humus status dif-
ferences in connection with land use, the adequate sets
were formed separately from forest and arable soils for
auto- and hydromorphic soils. The same schema was
used for comparative analysis in the humus status of cal-
careous and non-calcareous soils. The humus status dif-
ferences between auto- and hydromorphic soils are given
in the last section of Table 5. Using formula (1) also
allows easy determination of the average (weighted by
profile number) SS humus status parameters (thickness
and SOC stock). The mean data in Table 5 are weighted
by the number of profiles, which are slightly different
from the area weighted means (Table 4). Only the area
weighted means were used for modelling and as bench-
marks. But by profile number weighted means are more
convenient in explaining influences of land use change,
moisture conditions and soil calcareousness on SOC-
retaining capacity.
4. Discussion
4.1. Pedo-Ecological Regularities of SOC
The generalized area weighted SOC stock density’
means of SC, EP and SS may be taken as benchmarks
(standards, model contents) in comparative analysis of
the SOC retention capacities of different land covers.
The SOC-retaining capacity of soil depends on soil type
characteristics (EP thickness, moisture regime, texture
and carbonate content) and soil management. Smoothed
and interpolated by soil types and land use on the back-
ground of Estonian postlithogenic mineral soil matrices,
the isolines of SOC densities and concentrations may be
taken as preliminary humus status parameter standards
for different soil types [17,18].
Another aspect in analyses of SOC density levels is
treating them according to their primary determining
SOC densities properties. For example, a high role in
determination of SC SOC stocks belongs to the EP
thickness (Figure 1(a)). A good correlation (r = –0.63, n
= 283, p < 0.001) exists between SOC stocks and cation
exchange capacity (CEC, kmol ha–1) of the EP (Figure
1(c)). SOC stocks of SC depend not so much on SC
depth, as on SS texture, which is expressed by the index
of specific surface area (Figures 1(b) and 1(d)).
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Table 5. Comparative analysis of soils humus status by land use, soil moisture conditions and calcare o usne ss.
Automorphic soils Hydromorphic soils
Thickness (cm) SOC stocks
(Mg ha–1) Thickness (cm) SOC stocks
(Mg ha–1)
Soil group, characteristic Indice*
Forest soils Mwp 79 20.0 72.4 52.3 76.4 3922.0 63.2 103.9 134.0
SE 0.77 2.70 4.0 4.2 1.01 2.94 9.6 9.1
Arable soils Mwp 88 26.8 63.2 61.0 84.6 1323.1 52.2 93.1 104.6
SE 0.73 2.56 3.8 4.0 1.76 5.09 16.7 15.8
Difference d 6.8 9.1 8.6 8.1 1.1 11.1 10.9 29.4
Significance of difference p <0.0010.015 0.118 0.167 0.597 0.063 0.574 0.112
Calcareous soils Mwp 121 24.3 46.7 69.8 89.4 5025.1 43.4 120.8 133.6
SE 1.01 1.99 2.9 3.2 0.89 2.75 7.8 7.8
Non-calcareous soils Mwp 120 17.3 77.1 38.2 63.1 4819.5 66.8 72.7 111.4
SE 1.76 2.00 3.0 3.3 0.91 2.81 7.9 8.0
Difference d 7.06 30.4 31.6 26.3 5.6 23.4 48.1 22.1
Significance of difference p <0.001<0.001<0.001 <0.001 <0.001 <0.001 <0.0010.050
Automorphic soils Mwp 241 24.3 61.9 54.1 76.3 - - - - -
SE 0.55 1.64 2.84 2.83 - - - -
Hydromorphic soils Mwp - - - - 9822.3 54.9 97.2 122.7
SE - - - - 0.87 2.58 4.45 4.44
Difference d 1.5 7.0 43.1 46.4 1.5 7.0 43.1 46.4
Significance of difference p 0.132 0.023 <0.001 <0.001 0.132 0.023 <0.001<0.001
*Mwp - weighted (by profile number) mean, SE – standard error, d – difference between Mwp-s, n – number of studied profiles and p – significance.
(a) (b)
(c) (d)
Figure 1. The SOC stocks (Mg ha1) in EP and SC of mineral soils in relation to depth and selected pedoec ological propertie s.
(a) SOC stocks in EP in relation to EP depth (cm); (b) SOC stocks in SC in relation to SC depth (cm); (c) SOC stocks in EP in
relation to EP CEC (kmol ha1); (d) SOC stocks in SC in relation to SC Index of Specific Surface Are a (105).
Copyright © 2010 SciRes. IJG
The regular changes (dependent upon soil properties)
may be observed in mean SOC stocks of mineral soils
(Table 3). In both postlithogenic calcareous soils (Table
5), automorphic (soil groups I – III) and hydromorphic
(groups VII and VIII), the SOC stocks in EP and SC ex-
ceed SOC stocks of non-calcareous automorphic (groups
IV – VI) and hydromorphic soils (group IX). Therefore
in non-calcareous soils’ SS, the SOC stocks are abso-
lutely and relatively higher (in automorphic 24.9 Mg ha–1,
hydromorphic 38.7 Mg ha–1) than in calcareous soils
(19.6 and 12.8 Mg ha–1, respectively).
Soil calcareousness is connected with soil profile de-
velopment (i.e., forming of illuvial and eluvial horizons).
From Leptosols to Podzols (Table 3) and from Eutric
Gleysols to Dystric Gleysols the SS’ SOC density in-
creases accordingly from 8.2 – 28.2 and from 12.4 – 58.3
Mg ha–1. If the highest EP’ SOC stocks (156 Mg ha–1)
are characteristic of Saprihistic Gleysols, then the highest
SS’ SOC stocks characterize Fibrihistic Podzols (~70
Mg ha–1). Among synlithogenic soils the modest SS’
SOC stocks (5 – 7 Mg ha–1) are characteristic of eroded
and coastal soils (groups XII and XV), but relatively
higher (15 – 25 Mg ha–1) of deluvial soils and Fluvisols,
which may be classified as buried soils.
Clearly visible is the influence of soil moisture condi-
tions on SOC stocks. The reported data, as well as our
previous data prove the increase of SOC stocks in the
following sequence of soil moisture conditions: dry <
normally moist < gleyed or endogleyic < gley- or epig-
leyic < histic gleysoils.
ANOVA results show that SOC stocks in soils with
udic moisture conditions are usually relatively stable, as
their stocks in SC (Mg ha–1) vary in different sites by an
average of 25–45%. At the same time, SOC densities
vary to a larger extent in hydromorphic soils (CV
43–57%). This demonstrates the instability of Gleysols’
(histic, epigleyic) humus status.
Mean area weighted SC SOC stocks of post- and
synlithogenic soils are accordingly 102.5 and 78.3 Mg
ha-1. The SOC retaining capacity of postlithogenic auto-
and hydromorphic soils exceeds that of synlithogenic
auto- and hydromorphic soils accordingly by 2.1 and 1.3
times. Therefore, the normally developed SC’ SOC re-
tention capacity exceeds that of synlithogenic soils’ SC,
which is periodically influenced by different geological
4.2. Influence of Land use on Soil Humus Status
Comparative analysis of the EPs of arable and natural
soils (on the basis of soil groups I – V or automorphic
soils) shows that the EP of arable soils is on average 6.8
cm (p < 0.001) thicker (Tables 2 and 5). But in the case
of hydromorphic soils, the land use change have not
caused substantial increases of EP thickness, which may
be explained by transformation of raw-humous SOM into
more stabilized form, with increased soil bulk density.
The clearest differences between natural and arable soils
are observed in the fabric of EP, caused by the presence
of forest (grassland) floor on natural areas.
Land use change from forest to arable land causes a
decrease in exogenic SOC stocks and homogenization of
SOC concentration. As a result, the equalization of
stocks occurs in EP. Consequently, the diversity of SC
on arable land either decreases or is lost. Land use
change does not cause substantial changes in SS fabric
and humus status, while the thickness of SC and the level
of SOC stocks in the SC remain at approximately the
same level (Table 5). EP (or topsoil) is always more
sensitive to external influences compared with SS. The
SOC of SS does not participate actively in soil function-
ing and may be considered as a buried resource.
The mean area weighted SOC retaining capacities of
natural (and semi-natural) areas are higher compared
with arable ones (Table 4). Some differences in means
of SOC retaining capacities (taken by land use) are
caused by differences in the soil type and textural com-
position. For example, the main SOC accumulators into
mineral SC on arable lands are Cambisols, Gleysols, Alb
-eluvisols and Luvisols, on forest lands Gleysols, Podzols,
Cambisols and Albe luvisols and on grasslands Cambisols,
Gleysols, Fluvisols and Albeluvisols [17-19]. The pre-
dominant mineral soils (> 70%) in Estonian arable land
are the more fertile automorphic mineral soil types with
loamy textures, whereas the predominant mineral soils
(~40%) in forest lands are hydromorphic sandy soils.
4.3. Management of SOC
The goal of sustainable SOC management should be the
attaining of SOC stock density optimal to soil type. After
determining theoretical SOC density and actual status it
is possible to evaluate the existing situation: is there a
deficit, excess or optimal SOC stock density in the soil?
For this purpose the mean weighted SOC-retaining ca-
pacities estimated according to soil types may be used.
Additionally, suitable technology and digitized large-
scale soil maps with soil distribution patterns should be
The accumulation of stable SOC is a slow process.
According to Kolchugina and Vinson [30], the imple-
mentation of ecologically sound soil management prac-
tice results in an increase of forest soil SOC stocks by
0.5% and arable soils by 0.1% per year. With directed
soil management the annual SOC storage increase may
be in the limits of 0.1 – 0.7 Mg C ha-1 [5]. The results of
Romanovskaja [31] show that the average loss of SOC
from the abandoned arable land reached 0.46 Mg C ha–1
yr–1, but the increase of SOC storages after seven years
was expected. The great SOC stocks losses in the first
years after a change from forest to arable land use are
also reported by other researchers [5,32]. One possible
reason may be the greater share of potentially mineraliz-
able SOM in forest soils. According to Semenov et al.
[32] the share of easily mineralizable SOM in
sod-podzolic forest and arable soils forms, respectively,
6.0% and 3.2% of total SOM.
The turnover period of EP organic carbon is much
shorter than in SS and is controllable with soil manage-
ment, primarily on arable lands. For ecologically-based
soil management the identification of soil EP type is es-
sential. The best EP of Estonian arable soils belongs to
the neutral mild type. The main constraints of arable EPs
may be high acidity, low humus content, low biological
activity, unsuitable mineral composition, the raw-hu-
mous fabric and unfavourable moisture conditions. These
factors, limiting SOC turnover and the level of produc-
tivity constraints, may be regulated by improving SOC
management. The means for controlling or conversion of
EPs into good productive status are soil drainage, liming,
equilibrated fertilization and periodic input of new or-
ganic matter. Our previous research indicates that with
the transformation of forest soils with good productive
EP properties (fresh-moist-wet mull and moder) into
arable ones, the neutral mild (from mull) and eluvic
low-humous (from moder) EP types are formed [17,20].
One possibility of embedding additional carbon (or
atmospheric CO2) into the soil is to increase soil produc-
tivity, which subsequently causes SOC stock increases in
soil horizons [33,34]. Another opportunity is deep
ploughing, which displaces the rich SOC layer mechani-
cally into SS (less active layers) protecting SOC from
decomposition and ensuring its prolonged persistence.
The optimization of soil humus status should be soil
type-specific and arranged in a step-by-step approach, to
increase both soil productivity and the annual inflow of
new organic matter into the soil.
Though SOC densities in hydromorphic soils are quite
high, their quality is low, due to low humus quality. The
humus of hydromorphic soils is unstable, chemically
unsaturated and weakly condensed [20,35]. Therefore
Gleysols should be managed very carefully, especially
during preparation for cultivation. There is a risk of los-
ing a large part of SC’ SOC, which is weakly bound to
soil mineral particles. The reversion of low-fertility ar-
able lands to grasslands and their latest afforestation may
lead to additional sequestration of atmospheric CO2 [36].
4.4. Comparing SOC Stocks Densities of
Estonian Soils with Soils of other Regions
Comparison of SOC stocks of Estonian SC with other
regions of the world [13,37] reveals the characteristics of
the Nordic area: SC is relatively thin and poor in humus.
Robert [11] assigns a value of 98 – 102 Mg ha–1 for the
mean SOC stocks of 0.3 m soil layer in a boreal area,
which corresponds with our mineral soils weighted av-
erage of EP (Table 4). The average SOC density of the
forest soils 0.3 m layer in Russia equals 81 and for the 1
m layer 114 Mg ha–1; the same parameters for the entire
country are accordingly 101 and 180 Mg ha–1 [16]. Bat-
jes [38] reported for Central and Eastern Europe Pod-
zoluvisols, Cambisols and Gleysols 0.3 m layer mean
SOC densities, respectively, of 49, 69 and 114 Mg ha–1.
Lal et al. [8] provided 116 Mg ha–1 as the mean SOC
stock for grassland ecosystems, which is very similar to
the EP density of our lowland mineral soils. Soils with
aquic water conditions in the north-western USA tend,
according to Kern et al. [14], to be similar to the SOC
stocks of Estonian Gleysols (varying from 90 – 190 Mg
ha–1). Puzachenko et al. [15] estimated the global pool of
SOC to be 1,347 Pg, from which it has been concluded
that the mean SOC global density equals 109.5 Mg ha–1.
5. Conclusions
1) SOC retaining capacities of EP and SC as a whole are
soil type-specific. The average humus status indices of
soil types (EP and SC thickness and SOC stocks) may be
used as benchmarks in sustainable SOC management.
2) Land use and land use changes primarily influence
the properties and fabric of the EP. In the humus status
(SOC concentration and stock, fabric of horizons) of SS,
the differences between native soil and cultivated soil of
the same type are practically absent.
3) The mean mineral soils SOC stocks, EP quality and
SOC distribution in soil profile depend mainly on water
regime, mineral composition (texture and calcareous-
ness), development of eluvial processes and peculiarities
of land use.
4) The aggregate of SOC retained in the mineral soils
of Estonia (3,235,100 ha) amounts to 323 ± 46 Tg. Ap-
proximately 42% of this is sequestered into stabilized
humus, 40% into instable raw-humous material and 18%
into forest (grassland) floor and shallow peat layers.
Some 75% of the total SOC stock is situated in biologi-
cally active EP and 25% in SS, characterized by long
SOC turnover periods.
6. Acknowledgements
Funding for the research was provided by the Estonian
Ministry of Education and Research (Project No.
Copyright © 2010 SciRes. IJG
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