The Sulfur Cycle

Sulfur is one of the macronutrients required by plants and is obtained by them from the soil and from the atmosphere. It is present in proteins and gives a distinctive odor to many substances. It is also a component of the amino acid cysteine and is present in a large number of enzyme systems. Several groups of prokaryotes utilize and release sulfur.

The major reservoirs for sulfur in the global cycle are pyrite and gypsum (an evaporite of seawater) in the lithosphere and in seawater. Very little sulfur is present in living organisms, but within the marine muds and terrestrial bogs where organic matter accumulates under anaerobic conditions considerable amounts are present. The quantities cycled from these sources are small, but the distinctive rotten egg odor of H 2S is often prevalent in the air over such sites. Increasing amounts of atmospheric sulfur compounds are the direct result of human activities and are principal components of air pollution in industrial areas. Most are short‐lived in the air and wash out forming acid rain downwind from the industrial sites. The sulfur cycle is no longer in balance.

The sulfur cycle resembles that of nitrogen in several respects, for example the short‐term movements of both elements is through the atmosphere as a result of the metabolism of bacteria. The gases move rapidly in a closed cycle from the air to the soil and back. There are several subcycles: 1.) a long, deep time cycle of weathering, erosion, deposition, 2.) a predominately atmospheric cycle where bacteria metabolize dead organic matter and release sulfur to the atmosphere where it has a short residence time before being washed back to the soil by precipitation, 3.) a marine cycle where evaporation of sea spray releases sulfur to the atmosphere temporarily and from where it falls back into the sea, and 4.) a soil–plant cycle where organic sulfur in manure or other fertilizer is used to sustain soil microbes and plants.