Overfertilization is linked to soil acidification, nutrient surplus, and salinization in 13-m profiles of flood-irrigated apple orchards

Deep-soil chemical properties remain poorly documented in irrigated apple production systems compared with rainfed systems, although flood irrigation can accelerate the downward movement of fertilizer-derived solutes. Here, we hypothesized that flood irrigation amplifies the fertilization-induced changes in soil chemical properties in deeper soil layers, and sampled 22 independent 13-m soil profiles from permanent cropland and flood-irrigated apple orchards (FIAOs) of different ages on China’s Loess Plateau to examine depth-dependent changes in soil pH, soil organic carbon (SOC), NO 3 − -N, NH 4 + -N, Olsen-P, available K, exchangeable Ca (Ex-Ca), and electrical conductivity (EC). Results indicated that a 13-m profile was sufficient to capture the stabilized depth of SOC, NO 3 − -N, Olsen-P, and available K, but not pH and Ex-Ca. Compared with cropland, 30-year FIAOs showed significant changes in 13-m depth-averaged values, including a 0.27-unit decrease in pH and increases of 0.50 g kg −1 in SOC, 107.4 mg kg −1 in NO 3 − -N, 36.5 mg kg −1 in available K, and 374 μS cm −1 in EC. Partial least squares path modeling (PLS-PM) and regression analyses revealed depth-dependent relationships linking fertilization and stand age to soil acidification, nutrient accumulation, and salinization risk. Across the profiles, NO 3 − -N, Olsen-P, available K, and Ex-Ca were negatively associated with pH, while NO 3 − -N and available K were the main factors positively mediating EC. Overall, our results highlight early warning signs of deep-soil degradation in FIAOs, including acidification, salinization, and nutrient surplus. These findings suggest a need to reassess fertilizer and irrigation inputs to prevent further soil degradation and improve the sustainability of irrigated apple production systems.