Seasonal soil water status, non-growing season percolation and nitrate leaching under experimentally redistributed precipitation in surface drip-irrigated onion on sandy loam: A three-year lysimeter study

Climate projections indicate a shift in rainfall seasonality towards drier summers and wetter winters, potentially altering crop water demand and the nitrate-leaching risk. This study quantified effects of experimentally imposed precipitation redistribution on water balance, soil water status, percolation, and nitrate leaching in a drip-irrigated onion cropping system on sandy loam. A three-year lysimeter experiment (2011–2013) imposed four precipitation-redistribution treatments by excluding ambient rainfall and reallocating 0, 10, 20, or 40% of May–August precipitation to September–April, while maintaining identical annual totals. Soil matric potential was monitored at 0.1, 0.3, and 0.5 m; percolation, nitrate leaching, yield, and nitrogen uptake were measured across 16 lysimeters. Precipitation redistribution altered cultivation period water input and evapotranspiration despite irrigation buffering. Non-growing season percolation increased significantly with redistribution intensity (+14 to +69 mm; p < 0.001). Nitrate leaching increased only under the strongest treatment (+23 kg N ha⁻¹; p < 0.001), with loads strongly correlated to percolation volume (β = 0.33 kg N ha⁻¹ mm⁻¹). Depth-resolved soil matric potential characterized end-of-season soil hydraulic states, with ψ m at 0.5 m associated with percolation onset timing and ψ m at 0.3 m associated with early non-growing season percolation accumulation, while total non-growing season percolation remained dominated by the imposed precipitation redistribution. Yield declined under stronger redistribution, though the 10% shift showed no consistent yield effect. Year-specific atmospheric demand modulated in-season water use and crop responses, whereas treatment effects on non-growing season percolation were consistent across years. Together, these results indicate that nitrate-leaching risk in irrigated coarse-textured soils emerged from the interaction between precipitation timing, irrigation-mediated crop–soil responses, and redistribution-driven non-growing season percolation rather than from annual precipitation totals alone.