Soil Genesis and Classification, Sixth Edition. S. W. Buol, R. J. Southard, R. C. Graham and P. A. McDaniel. 2011 John Wiley & Sons, Inc. Published 2011 by John Wiley & Sons, Inc.
Mollisols: Grassland Soils of Steppes and Prairies
Mollisols (from Latin mollis, soft) are characterized by having a deep, dark, friable and relatively fertile surface horizon (or horizons) known as a mollic epipedon (Figure15.1; Figure 15.2). The vast majority of Mollisols are formed under grassland vegetation, where the annual proliferation of fine roots contributes to a relatively high organic carbon content. Other Mollisols may include soils of poorly drained lowland hardwood forests and some well-drained forested soils, often with significant understory vegetation. In addition to a mollic epipedon, Mollisols are characterized by relatively high base status to a considerable depth. Accordingly, these soils possess a high level of native fertility that has been widely exploited for agricultural production, often with minimal inputs of lime and fertilizers. There is also considerable biological activity associated with Mollisols, with earthworms, rodents, and various insects typically playing an important role in the formation of these soils.
SettingMollisols occupy approximately 7% or slightly more than 9,128,000 km2 of the ice-free global land to area (as shown in Table 20.3) and occur most commonly in the temperate grasslands of the middle latitude. These ecosystems occupy as much as 15,100,000 km2 of the global land area (Schlesinger 1997) and are extensive in North America, South America, Asia, and Europe. Ecologically and climatically, midlatitude grasslands represent the broad expanses between drier desert and moister forest communities, and include both the short-grass steppe and the tall-grass prairie. Although Mollisols are the dominant soils of these ecosystems, extensive areas of Aridisols and Entisols can be found in the drier steppe. Alfisols are common in the moister regions where the tall-grass prairie gives way to forest.
The short-grass grassland, or steppe, often resembles a pastured meadow extending monotonously to the horizon where grasses typically stand 15- to 30-cm high. Only in unusually wet years do patches of taller grasses develop enough to give the vegetative cover an uneven appearance. Sagebrush (Artemisia spp.) is a major compo-nent of the drier steppe regions of the western United States and, where dominant, gives the landscape a shrubby appearance (Fosberg 1965). Blue grama grass (Bouteloua gracillis) is common on drier Ustolls (Thorp 1948), and small soapweed (Yucca glauca) is prominent in places. Its roots spread as much as 2 m vertically and
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Figure 15.1. Profile photo of a Mollisol (Cryoll) from Montana showing black soil colors in the top 20 cm. Average organic matter content in the top 20cm is approximately 6.8%. Scale is in decimeters (C) and feet(F).
Figure 15.2. Pachic Argicryoll from Lemhi County, Idaho. Soil has formed in glacial drift and has a thick (pachic) mollic epipedon. Mean annual precipitation is 430 mm; native grasses and sagebrush are the dominant vegetation. The upper right-hand side of the profile has been extensively mixed by badgers. For color detail, please see color plate section.
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10 m laterally. Western wheatgrass (Pascopyrum smithii) and buffalo grass (Bouteloua dactyloides) are found on Ustolls. Buffalo-grass sod was used by pioneers for building houses. Stipa is the main genus in the western Russian steppes. The mean annual temperatures and precipitation of 2C and 200 mm at Urga, Mongolia, 14C and 250 mm at Quetta, Pakistan, and 4C and 360 mm at Williston, North Dakota, represent climatic conditions on middle-latitude steppes, respectively (Finch et al. 1957). In the Great Plains of North America, mean annual precipitation may be as low as 250 mm at the steppe-desert boundary along the western margin (Anderson 1987).
The tall-grass prairie is grassland of relatively luxurious growth of vegetation that stands 1- to 3-m high at maturity. The natural stands of the Argentine Pampas were so tall that a person riding on horseback could disappear from sight. Big and little bluestem grasses (Andropogon gerardii and Schizachyrium scoparium) are among the tall grasses found on Udolls of the Great Plains. Tall-grass prairies develop under relatively moist conditions: 8C and 810 mm in eastern Iowa and 16C and 760 mm in central Oklahoma. On the eastern Great Plains, the tall grass prairie is replaced by forest where mean annual precipitation exceeds 7501,000 mm (Anderson 1987).
Borcherts analysis (1950) of the climate of the prairie triangle or peninsula of the Great Plains of North America lists the essential ingredients of the climates of middle latitude grasslands. These features occur in different proportions over these lands: (1) severe, dry winters with much wind and relatively slight accumulations of snow; (2) relatively moist springs in most years; and (3) droughty summers with some thunderstorms and tornadoes. It is also important to recognize that some Mollisols exist in tropical areas of the world, although on a global basis, only accounting for approximately 4% of all Mollisols. Many tropical Mollisols have formed on calcareous parent materials most commonly under udic and ustic soil moisture regimes, and have organic carbon contents that rival or exceed those of their temperate counterparts.
The boundaries between desert, steppe, tall-grass prairie, and forest are often irregular and have not remained stationary over time. Complexities in the geographical distribution of parent materials, topography, climate change, and fire history are among the reasons for this. On the borders of drier zones, coarse-textured soils allow greater infiltration and deeper penetration of sporadic rains, thereby favoring extension of grass into drier regions. Similarly, on the borders of more humid zones, coarse-textured soils favor forest growth in the prairie lands, as in the case of the cross-timbers of Texas (Thorp 1948).
There is considerable evidence that grassland boundaries have migrated back and forth over time. As a result, it is likely that many older Mollisols have developed under more than one climatic regime and plant community. Dry-to-moist climate changes during the Holocene have resulted in forest encroachment into prairie (Fenton 1983). Periods of warmer and drier conditions such as the altithermal period have also allowed prairie expansion (Ashworth and Brophy 1972). Curtis (1959) refers to the altithermal period as the great period of prairie expansion. Significant portions of the Lake Michigan basin were occupied by soils, with the lake (called Lake Chippewa) being small and standing at 75 m (230 ft) above sea level.
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Fire, both natural and anthropogenic, is also an important agent in many grassland-forest ecotones. At the edges of the grasslands, such as with the boundary of deciduous forest in Wisconsin, extensions of prairies by fire have formed preferentially on topography over which fire moves easily, namely ridge tops and some windward slopes. Advance of aspen forest into prairie regions has been observed in Canada following a reduction in prairie fires that coincided with settlement in the early 1900s (Bird 1961, cited in Anderson 1987).
Mollisols occur on deposits and landscapes with a wide range of ages. Many areformed in Holocene-age deposits associated with glaciation. Others, especially those that have argillic horizons, occupy older deposits and landscapes (probably late Pleistocene) that have experienced variation in climate and vegetation. These Mollisols are clearly polygenetic and probably were under forest vegetation during glacial periods (Fenton 1983). Development of polygenetic Mollisols in the Palouse prairie region of the Pacific Northwest United States may span a period of approximately 40,000 years (McDaniel and Hipple 2010). Some Russian soil scientists suggest that the post-glacial-age Mollisols evolved during a changing climate. A poorly drained condition gave way to better-drained conditions as the climate became warmer and drier. The soils became alkaline, and then dealkalized (solodized) and were left with the present carbonate-rich condition.
Pedogenic ProcessesMelanization, the process of darkening of the soil by addition and decomposition of organic matter, is the dominant process in Mollisols. It is the process by which the mollic epipedon forms and dark soil colors extend down into the profile. Soil color, structure, and organic carbon data presented in Table 15.1 reflect the strong influence of melanization in the A horizons of two Mollisols formed under grassland vegeta-tion. Melanization is actually a bundle of several more-specific processes including extension of roots of prairie vegetation into the soil profile; microbial decomposition of organic materials in the soil, producing some relatively stable, dark compounds (humification); and reworking of the soil and organic materials by earthworms, ants, cicada nymphs, and rodents (bioturbation) (Hole and Nielsen 1970). In Mollisols, melanization is driven primarily by the incorporation of organic matter directly into the mineral soil.
It may be somewhat surprising that melanization and the accompanying accumulation of organic carbon is such a dominant process in temperate grassland soils, given the relatively low net primary production of these ecosystems (Schlesinger 1994). However, despite the relatively low net primary production, the net annual addition of carbon to Mollisols typically exceeds that for soils of tropical and temperate forests (Bolin et al. 1979). This has been attributed to the high proportion of material derived from roots coupled with relatively low rates of decomposition (Oades 1989). Roots are as much as 80% of the total biomass in many grasslands (Lauenroth and Whitman 1977; Fenton 1983). Thorp (1948) estimated that annual
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additions of raw organic matter to Ustoll soil profiles ranged from 590 to 1030 kg dry weight ha1 (520 to 900 lb acre1), mostly by in situ root death, and as much as 1250 kg ha1 organic matter may be added annually to Udolls of the tall-grass prairies. Both the depth of rooting and quantity of roots have been strongly correlated with the mollic epipedon thickness (Cannon and Nielsen 1984).
Numerous studies have documented the rapid accumulation of organic carbon in Mollisols. Schafer et al. (1980) found that the amount of organic carbon accumulation in the 0-to-10-cm depth of 50-year-old soils formed in mine spoils was similar to that of nearby reference soils (Ustolls). Ulery et al. (1995) showed that organic carbon content more than doubled in developing Mollisols after just 41 years.
Organic materials undergo significant change once added to the soil. The stable humus formed during melanization is a combination of the less palatable parts of the original organic matter, plus complex organic compounds synthesized by soil micro-organisms (Oades 1989). Many of these resistant organic compounds are polymers of phenolic and aromatic functional groups (Martin and Haider 1971). The association between clays and aromatic humic substances in the Ca-rich environment (Table15.1) afforded by Mollisols produces aggregates that are resistant to physical disintegration and further biological change. This stability is reflected in the average age of organic carbon in Mollisols, as assessed by radiocarbon dating techniques. Soil organic carbon of Mollisols (and Histosols) is older than that of other soil types (Oades 1989). Average ages of stable organic carbon in Mollisols from the Great Plains range from several 100s of years up to 3,000 years (Hseih 1992).
Wet conditions lead to increased production of plant biomass, decreased turnover, and subsequently greater accumulation of soil organic matter. Lowering of soil redox status curtails aerobic decomposition, thereby reducing the efficiency and rate of decomposition. In addition, the higher heat capacity of wet soils results in lower maximum temperatures, which also reduces decomposition rates. These conditions are responsible for the occurrence of Mollisols in the most poorly drained positions of Alfisol-dominated landscapes of the Midwest United States (Brown and Thorp 1942). In Mollisols of Iow...