Soils are probably the last great frontier in the quest for knowledge about the major sources and sinks of carbon (C) in the biosphere. The direct effects of deforestation on global patterns of carbon cycles are relatively minor; the effects of changed sink strengths, with deforestation decreasing rates of carbon dioxide (CO2) uptake, may be much larger. Another source of carbon input to the atmosphere has come from the oxidation of soil organic matter during cultivation of native lands such as the Great Plains region of North America and the Eurasian steppes of eastern Russia (Wilson, 1978; Houghton et al., 1983). The standing stocks of soil carbon are twice as large as all of the standing crop biomass of all of the terrestrial biomes combined (Fig. 8.1) (Post et al., 1990; Anderson, 1992). However, the plant and soil systems are strongly coupled, and the rates of inflows and outflows are significantly controlled by rates of above- and belowground herbivory in forests (Pastor and Post, 1988) and in grasslands (Schimel, 1993). The feedback effects of the principal greenhouse gases, namely CO2, methane, and nitrous oxide, are very large (Mosier etal., 1991; Rogers and Whitman, 1991), with the effects of CO2 being some 56% of the total impact (Anderson, 1992). However, the rate of increase of methane is almost twice that of CO2 (Houghton et al., 1987, 1990) and is being closely observed by atmospheric scientists. One of the major concerns of scientists interested in global change is the extent of involvement by soils and soil processes in the evolution of greenhouse gases, and roles of soil biota and organic matter in the global carbon cycle.
We examine next the ways in which soils operate over ecological and geological time spans, and how they may be influenced by, or have an effect on, global change processes. Soil development and change may be viewed as the result of the basic processes of additions, removals, transformations, and translocations (Anderson, 1988). Agiven landscape will experience runon, runoff, transformations, and transfers up and down in the profile, and additions and losses either aerially or pedologically (Fig. 8.2). These processes may be very dynamic for processes such as movement of soluble salts, which vary within seasons, or be measured in thousands of years, for example, for clay weathering processes. The microbial portion of the organic matter cycle will have mean net turnover times of 1-1.5 years, whereas humification processes such as the interactions of clay-humic compounds may be considered intermediate (centuries) in time scale (Stewart et al., 1990) (Table 8.1). These processes can be envisioned readily via the carbon, nitrogen, and phosphorus submodels of the Century model (Fig. 8.3). This model was developed to simulate the additions and losses in agricultural lands and grasslands worldwide (Parton et al., 1987, 1989a), but has now been extended to a wide range of ecosystems including tundra and taiga (Smith et al., 1992) and tropical ones as well (Parton et al., 1989b; Schimel et al., 1994; Smith et al., 1998).
FIGURE 8.1. Pools and fluxes of carbon in major terrestrial ecosystem types: (a) distribution of net primary production, (b) biomass, and (c) soil carbon pools. The total area occupied by each ecosystem type is represented by the horizontal axis with flux or density of the vertical axis; the area is therefore proportional to the global production or storage in each ecosystem type (from Anderson, 1992). Note: The numbers inside the boxed areas are measured in petagrams C (Pg C).
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