D Transformations within the deep soil profile

The soil can be divided into saturated and unsaturated zones (fig. 3-6). The boundary between these zones varies seasonally and from year to year. In some locations the saturated zone extends to the surface of the soil in early spring; at other times and locations, it may be hundreds of feet below the surface.

The unsaturated zone includes the root zone and an unsaturated area below the root zone. The root zone is characterized by an abundance of macropores, created in part by decaying roots and wormholes. The macro-

pores allow rapid downward movement of substances carried by percolating water.

The root zone is also characterized by an abundance of carbon created by the decaying roots. Because micro-organisms require carbon, biological transformations occur rapidly within the root zone, especially when the soil temperature is warm and adequate moisture is available.

Microbial activity is drastically reduced below the root zone. As a result nitrate, which is available for a variety of other transformations within the root zone, can remain in the nitrate form for years below this zone of microbial activity.

Within the saturated zone or in the ground water, contaminants can remain unchanged for long periods because of the absence of micro-organisms. However, in soils that have a seasonal high water table, the root zone can become saturated and anaerobic. In this environment anaerobic bacteria can thrive, creating ideal conditions for denitrification (the conversion of nitrates to gaseous forms of nitrogen).

Figure 3-6 Transformations on or in the soil

Solar radiation

Soil surface

Unsaturated / zone

NHg Volatilization

Figure 3-6 Transformations on or in the soil

Solar radiation

NHg Volatilization

Macropores

Soil surface

Unsaturated / zone

Water table V Fluctuates seasonally

Ground water or saturated zone

Organic matter decomposition

Macropores

Water table V Fluctuates seasonally

Ground water or saturated zone

Organic matter decomposition

Rapid biological transformation

Slow y biological transformation

Very slow biological transformation

N 2 Very rapid transformation

Rapid biological transformation

Slow y biological transformation

Very slow biological transformation

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