The Soil Sulfur Cycle

The terrestrial S cycle involves significant interactions between the pedosphere, the hydrosphere, the biosphere, and the atmosphere. The S cycle (Fig. 15.5) is similar to the soil N cycle, and unlike P, these elements undergo chemical and microbially mediated transformations leading to volatilization. Not only is the biosphere a repository for highly mobile forms of S, but several key reactions of the cycle are accelerated by, and sometimes completely controlled by, microbiological activity. The soil microbial biomass acts as the driving force behind mineralization-immobilization and oxidation-reduction transformations. The primary input of S to soil occurs during the weathering of soils, which releases sulfate into the available pool. Other inputs include plant residue inputs, S fertilizer, pesticides, and irrigation water. Atmospheric deposition can be a significant input of S in areas affected by atmospheric pollution due to combustion of fossil fuels. Losses of S from the soil system include plant uptake and harvesting of residues, long-term fixation into minerals, leaching of soluble sulfate, and gaseous losses of volatile forms of S.

FeS FeS2

FIGURE 1 5.5 Sulfur (S) transformations in nature. Elemental sulfur is shown as a storage product, and the possibility of sulfate sorption in certain soils is included.

FeS FeS2

SO4-Lakes Ground water

FIGURE 1 5.5 Sulfur (S) transformations in nature. Elemental sulfur is shown as a storage product, and the possibility of sulfate sorption in certain soils is included.

TABLE 15.4 Major Reservoirs of Sulfur in the Earth"

Reservoir

Total sulfur (X1015g)

Atmosphere

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