Profile Development

The abiotic and biotic factors noted above lead to certain chemical changes down through the top few decimeters of soil [Fig. 1.6(a), 1.6(b)]. In many soils, particularly in more mesic or moist regions of the world, there is leaching and redeposition of minerals and nutrients, often accompanied by a distinct color change (profile development). Thus as one descends through the profile from the air-litter surface, one passes through the litter (L), fermentation (F), and humification (H) zones (O;, Oe, and Oa, respectively), then reaching the mineral soil surface, which contains the preponderant amount of organic matter (A horizon). The upper portion of the A horizon is termed the topsoil, and under conditions of cultivation, the upper 12-25 centimeters (cm) is called the plow layer or furrow slice. This is followed by the horizon of maximum leaching, or eluviation, of silicate clays, Fe, and Al oxides, known as the E horizon. The B horizon is next, with deeper-dwelling organisms and somewhat weathered material. This is followed by the C horizon, the unconsolidated mineral material above bedrock. The solum includes the A, E, and B horizons plus some of the cemented layers of the C horizon. All these horizons are part of the regolith, the material that overlies bedrock. More details on soil classification and profile formation are given in soil textbooks, such as Russell (1973) and Brady and Weil (2000).

The work of the soil ecologist is made somewhat easier by the fact that the top 10-15cm of the A horizon, and the L, F, and H horizons (O;, Oe, and Oa) of forested soils contain the majority of plant roots, microbes, and fauna (Coleman et al., 1983; Paul and Clark, 1996). Hence a majority of the biological and chemical activities occur in this layer. Indeed, a majority of microbial and algal-feeding fauna, such as protozoa (Elliott and Coleman, 1977; Kuikman et al., 1990) and rotifers and tardigrades (Leetham et al., 1982), are within 1 or 2 cm of the surface. Microarthro-pods are most abundant usually in the top 5 cm of forest soils (Schenker, 1984) or grassland soils (Seastedt, 1984a), but are occasionally more abundant at 20-25 cm and even 40-45 cm at certain times of the year in tallgrass prairie (O'Lear and Blair, 1999). This region may be "primed," in a sense, by the continual input of leaf, twig, and root materials, as well as algal and cyanobacterial production and turnover in some ecosystems, while soil mesofauna such as nematodes and microarthropods may be concentrated in the top 5 cm. Significant numbers of nematodes may be found at several meters' depth in xeric sites such as deserts in the American Southwest (Freckman and Virginia, 1989).

Litter, of

Dark brown partially decomposed organic matter, O2 Black, well-decomposed amorphous organic matter, abundant roots, O3 Dark gray mixture of organic and mineral material, abundant roots, Ah, ochric A horizon Gray and leached, few roots, E, albic E horizon

Dark brown accumulation of sesquioxides and humus, few roots, Bs, spodic B horizon

Relatively unaltered acid material with high content of quartz, C, As.

FIGURE 1.6. (a) Diagram of a Podzol (spodosol in North American soil taxonomy) profile with minerals accumulating in subsurface horizons. This is the characteristic soil of coniferous forests (from FitzPatrick, 1984). (b) Diagram of a Cambisol profile, with the organic matter well mixed in the A horizon; due to faunal mixing there is no mineral accumulation in subsurface horizons. This is the characteristic soil of the temperate deciduous forests (from FitzPatrick, 1984).

FIGURE 1.6. Continued.
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