Mineralization of organic sulfur in paddy soils under flooded conditions and its availability to plants

In order to identify sulfur supplying potential in paddy soils, an open incubation technique was used to measure sulfur mineralization in four paddy soils under flooded conditions. An exponential equation and a reciprocal plot technique were used to model the sulfur mineralization patterns, and a soil exhaustion experiment with rice (Oryza sativa L.) was conducted to investigate the availability of various organic sulfur pools to plants. For all of the four soils tested, the release of sulfate-S and total inorganic sulfur mineralized was curvilinear with time, while those of other inorganic sulfur forms were linear with time. During 28 weeks of incubation at 30 °C, the amount of total inorganic sulfur mineralized ranged from 16.7 to 36.2 mg S kg−1 soil. Based on both the data of cumulative sulfate-S and total inorganic sulfur mineralized, the reciprocal plot technique appeared to best describe sulfur mineralization. Examination of soils after incubation revealed that the bulk of the mineralized sulfur was mainly derived from the C-bonded S and non-reducible organic S pool, while the majority of mineralized sulfur under soil sulfur exhaustion by rice was derived from the C-O-S pool.