THE EFFECT OF TEMPERATURE AND WATER POTENTIAL ON SULFUR OXIDATION IN SOILS

Abstract

The effectiveness of elemental sulfur fertilizers depends on the rate of their oxidation to the plant-available sulfate form. Although soil temperature and water potential are known to exert strong influences on sulfur oxidation, these effects have not been quantified. A laboratory incubation study, including a complete factorial of four temperature treatments (3, 15, 23, and 30†;C) and six water potential treatments (ranging from approximately −0.007–10 MPa), was conducted to define the influence of these parameters on the rate of oxidation in six soils. Oxidation rates and maximum rates were generally parabolically related to water potential at all temperatures. The maximum oxidation rate was observed at water potentials near field capacity, except in two soils with extreme moisture retention capacities: a sand (<-0.01 MPa) and a clay (–0.27 MPa). These differences were attributed to different air-filled porosities at high water potentials. Oxidation rates were exponentially related to temperature, and mean Q10 values ranged from 3.2–4.3, indicating that sulfur oxidation is much more responsive to changes in soil temperature than most other biologically mediated reactions. This sensitivity to temperature probably accounts for the limited effectiveness of most elemental sulfur fertilizers in Western Canada, where soil temperatures remain below 15†C for most of the year. Defining oxidation rate as a function of temperature and water potential will facilitate prediction of plant-available sulfur release from elemental sulfur and lead to the development of improved fertilizers and fertilization practices.