Document Type : Research Paper
Authors
1
Soil and Water Research Department, Kermanshah Agricultural and Natural Resources Research and Education Center, AREEO, Iran.
2
Soil and Water Research Institute, Agricultural Research, Education and Extension Organization, Karaj, Iran.
3
Soil and Water Research Department, East Azerbaijan Agricultural and Natural Resources Research and Education Center, AREEO, Iran.
Abstract
Background and Objectives: Sulfur is the fourth most essential and commonly required nutrient for all living organisms, including plants, animals, humans, and microorganisms. The effectiveness of elemental sulfur depends on its degree of oxidation, a process driven by both chemical and biological mechanisms. Various soil variables, such as pH, texture, temperature, moisture content, and salinity can influence sulfur oxidation in soils. Globally, and particularly in Iran, soil salinity poses a significant threat to agricultural productivity and food security. In Iran, more than 50% of soils are affected by salinity constraints. Soil moisture is also a critical variable for the biological oxidation of sulfur. However, the interactive effects of soil moisture and salinity on sulfur-oxidizing microbial communities and the oxidation process remain unclear. Therefore, the aim of this study was to investigate the effect of sulfur application under different moisture conditions on sulfur oxidation and on selected chemical properties of six soil series with varying characteristics and salinity levels.
Materials and Methods: In this study, numerous soil samples were collected from across the country (Iran) at a depth of 0–30 cm. Following chemical analysis, six soil samples with different salinity levels were selected. The effects of two moisture contents (40% and 60% of saturation) and three sulfur application rates (0, 500, and 1000 kg/ha), sourced from the Research Institute of Petroleum Industry, along with the bacterium Thiobacillus, were evaluated. The experiment was conducted in a completely randomized block design with three replicates for each treatment, and incubated for one year at a constant temperature of 25°C in the incubation chamber of the Soil and Water Research Institute laboratory.
Results: The results indicated that in non-saline and slightly saline soils (electrical conductivity less than 4 dS/m), sulfur application significantly decreased soil pH and increased electrical conductivity, as well as the availability of phosphorus, iron, zinc, and sulfate. The most pronounced reduction in pH and enhancement of nutrient availability in these soils were observed in the treatment with 1000 kg S/ha under 40% moisture saturation. In contrast, in highly saline soils (electrical conductivity greater than 8 dS/m), the effect of sulfur on these properties was markedly limited and, in most cases, non-significant. Furthermore, the highest rate of sulfur oxidation across all soil types was recorded in the 500 kg S/ha treatment under 40% moisture. Increasing the moisture level to 60% led to a reduction in the oxidation rate across all salinity levels, attributable to decreased oxygen availability and suppressed microbial activity. The highest sulfur oxidation rate (195.5 µg S cm⁻² day⁻¹) was measured in the saline Lenjan soil, while the lowest rate (2.4 µg S cm⁻² day⁻¹) was recorded in the highly saline Jazoushi soil on the 25th day of incubation.
Conclusion: These results highlight sulfur’s potential as an effective amendment for improving chemical properties in non-saline, and mildly saline soils. For saline soils, pre-treatment methods such as leaching are strongly recommended to enhance sulfur’s effectiveness.
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