Short-term soil profile responses to controlled ozonation of irrigation groundwater in an arid irrigated agroecosystem of Kyzylorda, Kazakhstan

Soil salinization and alkalinity constrain irrigated agriculture in arid regions, particularly where shallow groundwater and limited drainage promote salt accumulation. Ozonation is increasingly applied for water treatment as a strong oxidant and disinfectant, yet its implications for soil chemical dynamics under field irrigation remain insufficiently documented. This pilot field study assessed short-term soil-profile responses to irrigation with ozonated groundwater under an arid production system in Tasboget, Kyzylorda (Kazakhstan). Groundwater (12–18 m boreholes) was conveyed through the same hydraulic line for all treatments, with ozone injection controlled at set-points of 0 (control), 0.5, 1.0 and 2.0 mg O₃ L⁻¹. Water was treated in a contact tank with an average hydraulic contact time of ~7 min. Baseline soil sampling was conducted in spring 2024 before fertilization, and post-treatment sampling was performed 21 days after initiation of water treatments (four irrigation events). Composite soil samples (five subsamples per depth) were collected from five depth intervals (0–20, 20–40, 40–60, 60–80 and 80–100 cm). Soil pH, salinity indicators (including ECe), humus, macronutrients and saturation extract ions were determined using relevant GOST/standard protocols. Across the 0–100 cm profile, soil pH decreased with increasing ozone set-point. In the 0–20 cm layer, pH declined from 8.30 under the control to 7.81 at 2.0 mg O₃ L⁻¹, while at 80–100 cm pH decreased from 7.52 to 7.31. Salinity indicators tended to improve under ozonated irrigation; for example, ECe at 80–100 cm decreased from 3.210 dS m⁻¹ (control) to 3.020 dS m⁻¹ at 2.0 mg O₃ L⁻¹. Mineral N increased under the highest ozone set-point (e.g., 28.6 to 39.7 mg kg⁻¹ at 0–20 cm), whereas available P₂O₅ and K₂O showed modest topsoil decreases at the highest dose. These findings suggest that controlled ozonation may influence soil chemical status indirectly (e.g., via redox/carbonate equilibria and leaching dynamics) rather than by changing conservative salt loads in irrigation water. Given the short duration and aggregated treatment means, longer fully replicated seasonal trials with residual ozone/ORP monitoring are required to confirm agronomic relevance.