Characteristics of Soils Accumulated with Calcium Carbonates in Mongolia

概要

論文概要
[Abstract of Thesis]
論文題目[Thesis Title] * Characteristics of Soils Accumulated with Calcium Carbonates in
Mongolia
（モンゴル国における炭酸カルシウムの集積した土壌の特徴）
所

属 [Affiliation] 生命環境科学研究科
専攻
Doctoral Program in Biosphere Resource Science and Technology
Graduate School of Life and Environmental Sciences

氏

名 [Name]

NARANGEREL SARUUL

学籍番号 [Student ID Number]

201830251

論文概要 [Abstract of Thesis]

The distribution of pedogenic carbonate not only reveals the types of soil-forming processes
that exist in a certain area, but it also serves as a large carbon sink in the terrestrial ecosystem,
which can have an impact on delaying the rise of atmospheric CO2. Different responses to changes
in the environment should be expected from the property of pedogenic calcium carbonate in soil.
This requires illustrating the characteristic of pedogenic carbonate production in a variety of
environmental conditions. However, the distribution of humus carbon and organically bound
calcium in soil stabilizes the soil organic carbon and leads to its accumulation in soils. Many studies
were held mostly on acidic soil. Although the distribution of humus content in soil of arid climatic
condition is very low, it has a significantly important role in the soil formation process. There is
very little research done on its characteristics in soils with high carbonate content. Because the
secondary calcium carbonate accumulation in soil has been recognized as one of the most relevant
pedological processes in very specific natural and climatic condition of Mongolia, it is valuable to
study and distinguish the characteristics of soil with pedogenic calcium carbonate accumulations
depending on the different environment. Therefore, the goal of this work was firstly, to incorporate

the results of earlier studies on the morphological and physicochemical properties of soil with
calcium carbonate accumulation from different environment, secondly to study the feature of humus
carbon and organically bound calcium in the soil, and finally, to

find the impact of climatic factors

on soil properties with calcium carbonate accumulation in Mongolia.
In the field survey observation, the soil in high mountain, mountain steppe and eastern
steppe were dark and dominated by silty clay texture in surface soil. And the soil was gradually
changed to light colored with sandy clay loam texture dominant in surface at desert steppe and
desert. The depth of calcium carbonate accumulation (CaCO3) was shallow in soil of mountain
region and became deep in east and northeastern steppe. And it gradually appeared shallower to the
soil surface in desert steppe and desert. But it's also possible that in the desert steppe, the carbonate
accumulation layer became shallower because the top layer was lost by wind erosion and grazing
impact.
According to physicochemical analysis, the sand content in surface soil were increased from
high mountain to desert which also led the coarse soil texture. Due to the low decomposition rate,
the soil profiles in high mountain steppe had highest accumulation of OC and TN, whereas
mountain steppe, steppe, desert steppe, and desert regions had lower concentrations. The CaCO3
content determined by chemical analysis varied depending on the parent material and the process of
soil formation in each environmental state. However, the soil pH (H2O) was mostly moderately to
extremely alkaline in studied soils. The correlation between pH (H2O) and the ratio of CaCO3 and
total carbon (TC) were different depending on the environmental condition. In terms of the
correlation coefficient, it was noticed that the regression was weak (r = 0.24) at desert steppe zone
while it was very strongly correlated in high mountain (r = 0.91) and mountain steppe (r = 0.92),

and strongly (r = 0.72) correlated in steppe area. This indicates that the soil pH (H2O) is less
dependent on carbonate accumulation comparing with the other natural state. Because of the arid
climatic condition leads to higher content of exchangeable bases in soil than humid condition, the
content of exchangeable bases was generally increased from high mountain to desert steppe. The
extractable Ca2+ which is the most dominant cation among exchangeable bases was moderately to
strongly correlated with CaCO3 content and shows different regression depending on parent
material and natural zone. The part of Ca2+ in the carbonate accumulated horizons could be derived
from the dissolution of CaCO3 with CH3COONH4 (pH 7.0). The cation exchange capacity (CEC) in
A horizon was decreased from high mountain to desert steppe area, except the study sites of S9, S10
and S11. Due to higher clay contents in S9, S10 and S11 the soil profiles showed highest CEC
among steppe area. Based on the environmental condition, the effective factor of CEC was OC in
high mountain (r = 0.98) and mountain steppe (r = 0.73) while the clay has impact on CEC in soil of
desert steppe (r = 0.83).
Based on the field survey and physicochemical characteristics, the studied soils were
classified as Regosols and Leptosols at high mountain steppe. At mountain steppe Leptosols,
Calcisols and Chernozems were determined. In eastern and northeastern steppe area, Kastanozems
were distributed while in central and western steppe zone soils were classified as Chernozems,
Kastanozems and Calcisols. For the southern part of Mongolia in desert steppe the soils were
related to Cambisols and Calcisols however in western desert steppe zone Luvisols, Calcisols and
Regosols were found. In the most southeastern part at the desert zone the soils were classified into
Regosols and Gypsisols.
It was demonstrated that Cp and Cap contents influence the buildup of OC in non-calcaric A

horizon. From the result it was shown that the Ca2+ ions have important role in cationic bridging
effect between organic and inorganic components in non-calcaric horizon.
Among the climatic factors of studied sites, the aridity index has impact on the depth of
CaCO3 accumulation in soils. It indicates when the index is over 0.2 the accumulation depth
became deep. Furthermore, the aridity index is the significant climatic determinant for OC (r =
0.82), TN (r = 0.87) concentration and CEC (r = 0.74) in the surface horizon. The pH (H2O) in
surface soil was affected by mean annual precipitation (r = -0.71). It was demonstrated that mean
annual rainfall/potential evapotranspiration ratio had the strongest impact on Cp (r = 0.57) and Cap
(r = 0.82) followed by mean annual temperature for the non-calcaric horizon. During the vegetative
season, when biological and soil processes are most active, Cp and Cap are more impacted by
increasing temperatures and precipitation. Based on the result it was confirmed that although there
some general differences were found depending on natural regions, the climatic factors played a
major role in the characteristics of soils with calcium carbonate accumulation in Mongolia. ...