Oxidation of elemental sulfur in paddy soils as influenced by flooded condition and plant growth in pot experiment

A pot experiment was conducted to study the effects of rice cropping on elemental sulfur (S⁰) oxidation in three paddy soils under flooded conditions. Unaltered wet samples of paddy soils derived from cinnamon soil, black soil and red soil, respectively, were used. Repacked soils were incubated for 14, 28 and 42 days with or without addition of 2,000 mg kg^-1 S⁰ (particle size of 0.075 cm) and with or without cropping. Substantial amounts of oxidized S⁰ were found to be immobilized into organic S or taken up by plants. The apparent mass of oxidized S was calculated as the sum of the net increase of the sulfate-S and organic-S pools, and S taken up by plants. Under flooded conditions and in the absence of rice, the oxidation rates in all three soils were the greatest during the initial 14 days and then declined with progressive incubation. Incubation for a 42-day period resulted in oxidation of 5.58-10.15% of applied S⁰. Meanwhile 3.23-5.60% of the applied S⁰ was converted into organic S. Cropping with rice could significantly increase S oxidation and conversion of S⁰ into organic S in soils. Incubation for 42 days resulted in the oxidation of 10.28-14.87% of the applied S⁰, and conversion of 5.74-8.06% of applied S⁰ into organic S in the three soils. The results suggest that S⁰ fertilizers should be applied to the soil horizon rich with rice roots (i.e. the rhizosphere) in flooded paddy soils in order to increase the efficiency of S⁰ oxidation and plant availability.