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The Inorganic Carbon Cycle
György VÁRALLYAY
Research Institute for Soil Science and Agricultural
Chemistry of the Hungarian Academy of Sciences
Budapest, Hungary
IV. Alps-Adria Scientific Workshop
28 February–5 March, 2005, Portorož, Slovenia
Sphere interrelationships
atmosphere
biosphere
hydrosphere
soil
lithosphere
Várallyay nyomán
The assessment of soil carbon pools and
fluxes includes both
soil organic carbon (SOC) and
soil inorganic carbon (SIC) pools
• their dynamics
• their interactions with aquatic and biotic
(primarily vegetational) regimes
• their C-sequestration „activities”
Major C reservoirs in the Earth System
Sphere
SIC
SOC
1015 g
Total C
760
–
760
–
560
560
Pedosphere
1 700
1 500
3 200
Hydrosphere
38 000
1 000
39 000
48 000 000
17 000 000
65 000 000
Atmosphere
Biosphere
Lithosphere
(Drees et al., 2001)
Soil C pool of world soils
Soil order
Alfisols
Andisols
Aridisols
Entisols
Gelisols
Histosols
Inceptisols
Mollisols
Oxisols
Rocky land
Shifting sand
Spodosols
Ultisols
Vertisols
Total
Area
(Mha)
1262
91
1570
2114
1126
153
1286
901
981
1308
532
335
1105
316
13,083
SOC
Density
Pool
SIC
Density
Pool
(tons/ha)
(tons/ha)
(bill. tons)
34
0
290
124
6
0
26
96
0
0
9
0
0
50
43
0
456
263
7
0
34
116
0
0
5
0
0
21
945
125
220
38
42
281
1170
148
134
128
17
4
191
124
133
(bill. tons)
158
20
59
90
316
179
4190
121
126
22
2
64
137
42
1526
(adapted from Eswaran et al., 2000)
A simplified representation of the global carbon cycle.
(in Pg=1015g)
Balance: 218.5 Pg/year enter the atmosphere 215 Pg/year is removed from
the atmosphere.
increasing CO2 concentration
The long-term geochemical cycle of carbon at the
surface of the Earth
Soils and near-surface geological formations – as a
biogeochemical interface between the spheres of the
Earth system – play a strategic role in the global C
balance.
The SIC pool is considerably higher, but more stable
and less reactive than the SOC pool.
CaCO3
MgCO3
Na2CO3
The importance of SIC in the global C balance is often
ignored, in spite of the fact that pedogenic processes,
as
carbonate leaching
are important factors
silicate-mineral weathering
of carbon sequestration
Soil Inorganic Carbon – SIC
• primary or lithogenic carbonates
(originating from the parent rock material)
dissolution
translocation
transport

by
water
(organic) acids
CO2 (soil atmosphere)
+ soil Ca2+ , Mg2+, Na+
• secondary or pedogenic carbonates
CaCO3
MgCO3
Na2CO3
- accumulation horizon
- lime coatings (pseudomycelium)
- concretions
- lime pans
CO2 + H2O
?
H2CO3 + Ca2+
- climatic
- hydrologic
- vegetation
- soil
CaCO3
zones
Acid volatiles + igneous rocks  sedimentary
rocks + salty oceans/seas
( > 0.018 + 0.13 · 1015 g C/year emission from volcanic
activities)
Idealized soil C cycle for humid conditions
(ppt. > Evtr)
Atm CO2
Soil CO2
HCO3-
SIC
Plant C
SOC
groundwater
(loss)
HCO3-
(Drees et al., 2001)
Idealized soil C cycle for subhumid to semi-arid
conditions (ppt.  Evtr)
Atm CO2
Soil CO2
Plant C
HCO3-
SIC
?
(Steady
state)
Groundwater
(Drees et al., 2001)
Idealized soil C cycle for semi-arid to arid
conditions (ppt. < Evtr)
eolian
Atm CO2
Soil CO2
Plant C
HCO3-
SIC
(Long-term
storage)
SOC
(Drees et al., 2001)
Pathways, reasons and consequences
of the inorganic carbon cycle
During weathering and soil genesis considerable
changes take place in the SOC and SIC cycles:
- physical, chemical and biological weathering;
- dissolution – precipitation;
- leaching – accumulation
depending on soil reaction, carbonate status, texture,
structure, moisture regime, biological activities, etc.
The processes are strongly influenced by climate (and
climate changes), surface and subsurface hydrology,
vegetation and land use pattern and various human
activities.
In the Alpok-Adria region a huge amount of
sedimentary rocks, mainly CaCO3, was formed
during the various geological periods.
In some places these sediments are the „parent
material” of the soil formation processes, but in
extended areas there are only non- or slightly
weathered rocks on the surface, sometimes
with characteristic „karst” symptoms, and
peculiar carbonate regimes.
In the Carpathian Basin the main carbonate
resources are the
 calcareous Quaternary (Pleistocene) loess
deposited to drylands or into water and
waterlogged territories;
 calcareous Holocene aeolian sand;
 calcareous alluvial deposits of rivers coming
from limestone watersheds;
 calcareous colluvial materials transported by
lateral erosion from carbonatic surroundings.
Surface and subsurface waters play an important,
often decisive role in their state, horizontal and
vertical distribution and have significance in the
carbon cycle and carbon sequestration.
Development of calcium carbonate
accumulation layers in the Danube Plain
H2O (rainfall)
H2O with CO2 content (soil solution)
SOC
CO2-loss  CaCO3 precipitation
SIC
leaching
CaCO3 accumulation layer
level of groundwater effect
evaporation
concentration  weakly soluble CaCO3
(and MgCO3 precipitation)
SIC
groundwater level
In Hungary – due to various reasons (acid rain, improper
fertilizer application etc.) – a quite serious CaCO3-loss
was measured:
 part of the dissolved carbonates was „destroyed”
completely
CaCO3 + 2H+  H2CO3  H2O + CO2
and contributed to the increase in CO2 concentration
of the surrounding atmosphere
 another part was leached by downward filtration.
Leaching has world-wide significance in the SIC
cycle.
According to comprehensive C balance studies the icefree land area of the Earth surface for potential leaching
is 45×1012 m², consequently, if we assume a
8 g C/m²/year flux, then the sequestration rate is
estimated as 0.36×1015 g C/year.
This
inorganic carbon sequestration
„potential” (capacity”)
is a new soil function;
consequently
it should be evaluated in a modern,
function-specific
„soil quality” assessment system.
A Gaunt view
Thank you very much
for your attention !