Coupled effects of rainfall intensity and soil texture on nitrate behavior in unsaturated soil columns

The vadose zone (VZ) mediates contaminant migration to aquifers, yet its attenuation efficiency under transient conditions remains poorly understood. Most studies have employed batch experiments at a fixed saturation degree, which could not capture the coupled effects of water and oxygen content under transient conditions. This study evaluated nitrate behavior using unsaturated soil columns packed with sandy loam (SL) and loamy sand (LS) subjected to infiltration rates of 6.25-75 mm/h for simulating the VZ environment. Water and oxygen contents were continuously monitored, and pore-water samples were collected at multiple depths to characterize nitrate transport and fate. The presence and abundance of denitrifiers were analyzed to support apparent denitrification within the column. The modular 71.5-cm column packed with SL had a higher water content (∼0.3 v/v) and a capillary fringe of ∼22 cm, and retained 38.8-41.4% of introduced nitrate, predominantly at mid-depths (∼38.5 cm). Under a moderate infiltration rate (6.25 mm/h), 40.8% apparent denitrification occurred in SL. In contrast, coarser-textured LS with a shallower capillary fringe (∼10 cm) showed stronger vertical nitrate transport, with most nitrate remaining in the deeper ∼60.5 cm layer (25.5-47.7%). Although LS still showed 15.8% apparent denitrification at 6.25 mm/h, no apparent denitrification was detected under very heavy rainfall (75 mm/h) due to enhanced drainage, which inhibited stable microbial niche development. These results show that infiltration rate and soil texture control nitrate partitioning between apparent denitrification and leaching by regulating water residence time. Groundwater vulnerability to nitrate contamination may therefore vary with rainfall conditions.