Transformation of sulfate and organic S in rice straw in flooded paddy soils and its availability to plants using sulfur-35 labeling

Sulfur transformation and its availability to plants in two flooded paddy soils, derived from Black soil in Heilongjiang (soil 1) and from Red earth in Jiangxi (soil 2), were investigated in a closed incubation system using carrier-free 35S–SO42− and 35S-labeled rice straw. After incubation for 120 days, 51.1% and 60.3% of the 35S added in the Na235SO4 treatment were found in the sulfate–S fraction, 14.0% and 11.1% as sulfide–S, 5.2% and 5.3% as other non-sulfate inorganic S fraction (OI-S), 9.5% and 6.0% as C–O–S, 8.1% and 7.3% as C-bonded S, and 12.1% and 10.0% as non-reducible organic S fraction (NRO-S) for soil 1 and soil 2, respectively. Since soil Eh increased greatly after planting the rice, sulfide–S and OI-S could be transformed into sulfate as plant-available S. Most of the 35S uptake by plants came from sulfate–35S, sulfide–35S and OI–35S, with little contribution from C–O–35S, C-bonded 35S or NRO–35S. In the treatment with 35S-labeled rice straw, 23.3% and 26.4% of the 35S applied was found in the sulfate–S fraction, 25.3% and 26.3% as sulfide–S, 6.0% and 6.1% as OI-S, 16.4% and 14.0% as C–O–S, 10.1% and 8.3% as C-bonded S, and 18.9% and 18.9% as NRO-S at 120th day of incubation in these two soils, respectively. Higher availability of C–O–35S, C-bonded 35S and NRO–35S from 35S-labeled straw was observed in the 35S-labeled straw treatment, compared to the carrier-free Na235SO4 treatment.