Mollic Epipedon - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 38

5.5 Factors Influencing the Formation of Mollic Epipedons

The key factor influencing the distribution of soils with a mollic epipedon is
grassland or steppe vegetation. Our analysis showed that grassland was the native
vegetation of 93 % of the representative Mollisols and 74 % of the Vertisols with
mollic epipedons (Table 5.1 ). Savanna was most common on Alfisols (58 %) and
Andisols (53 %), and mixed or broad-leaved forest was the dominant vegetation for
the formation of Inceptisols (86 %) with mollic epipedons. Whereas SOC is
contributed in forest ecosystems primarily as litterfall, in grassland most of the
SOC is added to soils by fine-root turnover (Nielsen and Hole 1963 ) (Table 5.4 ).
From 70 to 76 % of the roots in grassland ecosystems are associated with the mollic
epipedon (Douglas et al. 1967 ). According to the SoilGen2 model, bioturbation and
fine-root turnover are two of the four properties that can be used to predict the
development of mollic epipedons in loess-derived soils (Finke 2012 ). Plant lignin
content is particularly important to SOC levels in Mollisols (Parton et al. 1987 ).
The climate (rainfall, temperature) is the second soil-forming factor that is
important for the formation and distribution of soils with mollic epipedons or in
mollic subgroups (Table 5.4 ). Bravo et al. ( 2007 ) segregated Mollisols and
Aridisols along a 350-km transect in Argentina at the 400-mm isopleth for mean
annual precipitation (MAP). With the MAP > 400 mm, a mollic epipedon was
dominant; between 400 and 300 mm, a mollic or ochric epipedon was present; and
in areas with MAP

300 mm, an ochric epipedon was predominant due to its low
SOC content. According to Liu et al. ( 2012 ), soils with mollic epipedons are most
common in areas with 500-1,500 mm/yr MAP and a mean annual air temperature
ranging from 5 to 20 C. In the USA the prairie-forest (pedocal-pedalfer) boundary
normally is recognized at the 750-mm MAP isopleth (Marbut 1935 ). The SoilGen2
model recognizes soil temperature and moisture as two key properties influencing
the formation of a mollic epipedon in soils from loess (Finke 2012 ). These findings
imply that the climatic influence is dominantly related to the amount and type of
SOC produced by the vegetation.
Relief or topography has not been well researched in relation to the formation of
Mollisols and mollic epipedons. Within the temperate deciduous biome, restricted
drainage and decomposition may lead to the development of a mollic epipedon
(Abtahi and Khormali 2001 ). This may explain why all 30 of the Inceptisols within
the mollic subgroup had an aquic soil-moisture regime (Table 5.3 ) and that broad-
leaved forest was present on 33 % of the Alfisols, Entisols, and Inceptisols in mollic
subgroups (Table 5.1 ). Topography may influence the development of the mollic
epipedon (Table 5.4 ). In Argixerolls and Mollic Hapludalfs in Iran, the mollic
epipedon forms on the summit and toeslope landscape positions but not on the
more eroded back slope and footslope positions (Khormali and Ajami 2011 ).
The relation between Mollisols and parent material has been reasonably well
studied. The existence of base-rich materials is important to the distribution of soils
with a mollic epipedon. Our analysis shows 45 % of the Mollisols as being derived
from loess and 26 % of the Alfisols in mollic subgroups (Table 5.4 ). Calcareous till
was a common parent material for Mollisols (31 %), Inceptisols (19 %), and

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