Mineralization-immobilization of soil organic S and oxidation of elemental S in subtropical soils under flooded and nonflooded conditions

Abstract

Information on the influence of soil moisture on elemental sulphur (S0) oxidation and transformation into organic S in semi-arid subtropical soils is scarce. We studied the impact of three moisture regimes on the mineralization of soil organic S, and the oxidation and immobilization of S0 in acidic (pH 4.9), neutral (pH 7.1) and alkaline (pH 10.2) subtropical soils. Repacked soil cores were incubated under aerobic (40% and 60% water-filled pore space, WFPS) and flooded soil conditions (120% WFPS) for 0, 14, 28 and 42 days with and without incorporated S0 (500 µg g–1 soil). Soil moisture had profound effects on these processes and the mineralization of native soil organic S, oxidation of applied S0 and transformation of S0 into soil organic S proceeded most rapidly at 60% WFPS, irrespective of soil pH. Mineralization of native soil organic S resulted in the accumulation of 34, 49 and 44 g SO42–-S g–1 soil in acidic, neutral and alkaline soil in a 42-day period at 60% WFPS. The oxidation rate of added S0 during the initial 14-day period at 60% WFPS was highest in alkaline soil (428 µg S cm–2 day–1), followed by neutral soil (326 µg S cm–2 day–1), and lowest in acidic soil (235 µg S cm–2 day–1). These rates are several folds higher than those reported in earlier studies because now we computed the oxidation rates by including the amount of S0 that was immobilized to organic S. Of the applied S0 at 40% and 60% WFPS, 2.6% and 6.0%, 3.4% and 10.0%, and 9.4% and 14.4% oxidized to SO42–, and 15.0% and 17.6%, 17.6% and 19.6%, and 17.6% and 23.6% transformed into organic S in the 42-day period in acidic, neutral and alkaline soil, respectively. These results suggest that in order to synchronize the availability of S with plant need, S0 may be applied well before the seeding of crops especially in acidic soils and in rainfed regions where soil moisture remains at less than 60% WFPS. Apparently no oxidation of S0 and significant reduction of SO42–-S (7, 53 and 78 µg SO42–-S g–1 in acidic, neutral and alkaline soil, respectively) under flooded conditions suggest that S0 is least effective for correcting S deficiency in flooded soil systems such as rice fields.