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leached, primary minerals are mostly gone, and only Fe and A1 oxides remain in the soil along with kaolinite (Oxisols).

Plate 3 shows a map of dominant soil orders for the entire world. Although this map necessarily lacks detail due to its scale, the relationship between soils and the biosphere is evident. Different terrestrial ecosystems are correlated with climatic conditions and different soils are correlated with both. For example, Mollisols are common in areas where there are prairies or steppes; a result of grasses as the dominant vegetation and low, seasonal rainfall. Spodosols occur where coniferous forests dominate and the climate is cold and wet. Comparing Fig. 8-5 and Plate 3 carefully will show how strong this correlation is for the entire Earth.

8.3.2.4 Biogeochemical cycling in soils

One of the most important functions of the pedosphere is the cycling of elements that occurs within soils and the transfers that occur between the atmosphere, lithosphere, biosphere, and hydrosphere through soils. Soil is an interface between the atmosphere and lithosphere, between the biosphere and lithosphere, and between roots and soil organisms and the atmosphere. In many ways, soil acts as a "membrane" covering the continents and regulating the flow of elements between these other systems of the Earth.

This soil "membrane" has inputs and outputs, and can transform the elements entering it before these elements leave. Consider the simple cycle of potassium shown in Fig. 8-6. Inputs to the surface of the soil come from atmospheric deposition of particulates, fertilizer applications, and litterfall. Potassium that is released from a crystalline matrix by weathering is also considered an input to soils from rocks even though the rocks are contained within the pedosphere (Zabowski, 1990). Likewise, roots that die and begin to decompose provide inputs of K to the soil - the conversion of organically bound elements to an inorganic form is called mineralization. Within the soil, K can be used to form new clays, attached or released by cation exchange sites, released by dissolution of clays, or taken up by soil organisms (immobilized)

Potassium Cycle

Potassium Cycle

Fig. 8-6 A cycle of potassium in soil.

where eventually they may be mineralized again when these organisms die. Note how the soil solution functions as the transfer mechanism. Potassium may also be removed from the soil by uptake into the biosphere (in which case it may eventually return to the soil through litterfall), erosion of soil, or leaching to groundwater which flows away from the soil to rivers. By knowing inputs and the quantity of an element in the soil, mean residence times (ir) and turnover times (t0) can be calculated. The basis for these calculations is presented in Chapter 4. Biogeochemical cycling in soils is further complicated by the different soil processes occurring in the different soil orders (see Fig. 86 and Table 8-4) which all cycle K and other elements at different rates.

The cycle of potassium is quite simple, as K does not change valence states or have a gaseous phase. In contrast, elements such as carbon and nitrogen both change phase and undergo redox reactions and undergo much more complicated cycling. For example, carbon captured by plants from the atmosphere is reduced to form organic matter, which is then oxidized by either organ isms in the soil, or in roots to provide energy with a release of C02 if the soil is aerobic. Methane may be released if the soil is anaerobic. Other conversions of carbon to humic substances can make it very resistant to further decomposition. The tr of C in various soil fractions can range from a few months to thousands of years (Schlesinger, 1997). Nitrogen cycling is also very intricate, because N can exist as N2, N20, N03 , N02", NH4, and NH3 in soils. Although nitrogen is rarely input to soils by weathering, nitrogen-fixing organisms capture atmospheric N2 and thereby act as a source of the element to soils. The rates at which nitrogen converts from one form to another in soil affect the rate of transfer of N from one "sphere" to another, owing to the fact that soils are at the interface of the atmosphere, biosphere, and hydrosphere, where nitrogen compounds reside. A complicating factor is that nitrogen is the nutrient most commonly limiting to plant growth. Carbon and nitrogen cycling are discussed in more detail in Chapters 11 and 12, respectively.

8.4 Watershed Processes

Watersheds, also known as drainage basins, define a natural context for the study of relationships among soils, geology, terrestrial ecosystems, and the hydrologic system because water and sediment travel downslope under the influence of gravity. This material is a continuation of some of what was presented in Chapter 6.

The concept of a drainage basin has no inherent scale, since watersheds range from small headwater valleys to the catchments of huge rivers that drain continents. The physical and chemical load carried in a river is produced by watershed processes, including weathering and exchange processes in the soil, how the water that runs off of a landscape as streamflow (often simply called runoff) is generated, and the relative importance of different geomorphological processes, climate, and the lithology of the bedrock. Climate and topography are two of the strongest influences on the sediment load of rivers, although sediment routing processes and long-term storage of sediment in flood-plains and structural valleys can create substantial time lags and differences in net sediment delivery to the oceans.

8.4.1 Runoff Processes

The processes through which rainfall is turned into runoff, together with the nature of the material through which water moves, control the chemical characteristics of streamflow. Specific runoff mechanisms operating in a landscape control the flowpaths by which water moves through the landscape. Flowpath-depen-dent differences, such as the total time that water spends in contact with different soil horizons or bedrock (residence time), can strongly influence runoff amounts and timing, the relative contribution of event (new) versus stored (old) water, and runoff chemistry.

8.4.1.1 Runofi mechanisms

The translation of rainfall into runoff occurs by a variety of mechanisms associated with different environments (refer back to Fig. 6-7). Horton overland flow (HOF) occurs primarily in arid or disturbed landscapes where rainfall intensity exceeds the infiltration rate of the ground surface long enough for ponding to occur (Horton, 1933). Observations that most rainfall infiltrates into the soil in humid, soil-mantled landscapes, and therefore that HOF is rare in such environments, led to the recognition of subsurface flow as a major mechanism of storm runoff (Louder-milk, 1934; Hursh, 1936). Subsurface stormflow (SSSF) dominates runoff generation in steep soil-mantled terrain where precipitation infiltrates and flows laterally either through macropores, or over a lower conductivity zone, such as at the base of a root mat or at the soil-bedrock boundary. Saturation overland flow (SOF) occurs in soil-mantled landscapes when an initially shallow water table rises enough to intersect the ground surface over a portion of the catchment (Hewlett and Hibbert, 1967), which then causes runoff by either return flow or direct precipitation onto saturated areas (Dunne and Black, 1970). Topographically driven patterns of soil

Rainfall / Infiltration

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