Keywords: Sulfur, Sulfate, Mineralization, Immobilisation, Organic, Leaching.

The amount, chemical forms and distribution of sulfur varies in soils across different management, climatic and geochemical conditions. Both inorganic and organic forms exist in soils, with organic sulfur normally consisting of the larger pool. However, plants take up the sulfate form of sulfur via roots and this is therefore the most important form in soils from a nutritional point of view. Soils receive sulfur from wet and dry deposition, which either exists or is transformed into sulfate, and sulfate or elemental sulfur can be intentionally added to soil in fertilizers. Sulfate is also the form that is prone to being leached out of the rooting zone. Where elemental sulfur fertilizers are used, their oxidation is a biological process, but the rate is often limited by the physical form of elemental sulfur in the soil. Sulfur in soils undergoes microbial immobilisation and mineralization concurrently. The net outcome of these two processes varies with time and net mineralization is difficult to measure and to predict.

The forms of sulfur in soil organic matter are both heterogeneous and complex. It remains unclear how different fractions of organic sulfur are interrelated in the turnover processes. Microbial biomass-sulfur is probably the most active component of soil organic sulfur. Methods which fractionate soil organic sulfur into different pools according to their biological and biochemical lability need to be developed, and these may be more informative. Data on soil sulfur turnover from field studies, which are essential for developing and validating models, are scarce and scattered.

Prof. Steve P McGrath, Dr. Fangjie Zhao and Dr. Mechteld M A Blake-Kalff, Rothamsted Research, Harpenden, Hertfordshire, AL5 2JQ, UK.

27 pages, 4 figures, 4 tables, 105 references.