Effects of Zinc Oxide Nanoparticle Spiking on Physicochemical Parameters and Substrate-Induced Respiration in Ultisols

This study investigates the concentration-dependent effects of zinc oxide nanoparticles (ZnO-NPs) on soil physicochemical properties and substrate-induced respiration in ultisols. The ZnO-NPs were synthesized via the chemical precipitation method using zinc sulfate and potassium hydroxide precursors. Characterization using X-ray Diffraction confirmed a highly crystalline hexagonal wurtzite structure, while Scanning Electron Microscopy and Transmission Electron Microscopy revealed predominantly spherical morphology with an average particle size of 20 nm. Soil samples were collected from a depth of 0–20 cm and treated with varying concentrations of ZnO-NPs (0, 100, 500, 1000, 2000, and 4000 mg kg⁻¹). Soil pH was determined potentiometrically in a 1:2.5 soil-water suspension, and moisture content was measured gravimetrically after oven-drying at 105°C. Exchangeable cations (Ca and Mg) were extracted using ammonium acetate, while anions (SO₄²⁻, NO₃⁻, and PO₄³⁻) and Total Organic Carbon were analyzed using standard spectrophotometric and Walkley-Black wet oxidation methods, respectively. Microbial activity was quantified via the alkali absorption method, measuring the soil respiration rate as µg CO₂ g⁻¹ soil h⁻¹ during a 7 day incubation period. The results demonstrated a clear biphasic (hormetic) response. At low to moderate doses (100–500 mg kg⁻¹), ZnO-NPs acted as a biostimulant, peaking at 500 mg kg⁻¹ with significant increases in pH (5.42), moisture content (20.1%), NO₃⁻ (22.1 mg kg⁻¹), and respiration rate (52.3 µg CO₂ g⁻¹ soil h⁻¹). However, concentrations exceeding 1000 mg kg⁻¹ induced severe inhibitory effects. At 4000 mg kg⁻¹, the soil experienced significant acidification (pH 4.60) and reduced moisture-holding capacity (14.8%). Statistical analysis confirmed that differences between treatments were significant at p <0.05. These findings suggest that while ZnO-NPs can improve the physicochemical and biological status of acidic ultisols at controlled doses, high accumulation triggers nanotoxicity and soil degradation.