Nutrient-Driven Mechanisms of Soil Acidification in Mountain Tea Plantations Along Elevation Gradients: A Case Study of Lushan Tea Plantations in Jiangxi Province, China

Soil acidification and nutrient imbalances threaten mountain tea plantation sustainability, while nutrient-driven acidification mechanisms along elevation gradients in deep soil profiles remain poorly understood. This study examined three tea plantations at different elevations (107, 400, and 711 m) on Mount Lu, Jiangxi Province, sampling soils at five depths to 100 cm. Objectives were to quantify vertical differentiation of acidity, base ions, and nutrients, reveal altitudinal conversion patterns of acidification drivers, and guide fertilizer management. Soils exhibited strongly acidic conditions (pH 3.86–5.46), with the most severe acidification at the mountain foot and surface layer (0–20 cm). Potential acid accumulation in deep soils exceeded 75% (exchangeable aluminum/exchangeable acidity: 80.61%, 77.42%, 80.10% at mountain foot, mountainside, and mountaintop, respectively), and acidification spread was most pronounced at the mountain foot. The mountaintop and surface layer showed strongest acid–base resistance. Cation exchange capacity and base cations decreased with depth; base saturation in surface soil was 47.12% higher than in deeper layers. Organic matter, nitrogen, phosphorus, and available potassium were surface-enriched, whereas total potassium accumulated in deeper layers. Correlation analysis showed that pH was significantly negatively correlated with exchangeable acidity, exchangeable aluminum, organic matter, total nitrogen, and alkaline-hydrolyzable nitrogen, and significantly positively correlated with exchangeable calcium, exchangeable magnesium, and total potassium. Structural equation modeling revealed that nitrogen accumulation was the primary acidification driver, with total effect sizes ranking as total nitrogen (0.549) > organic matter (0.319) > total potassium (0.247) > alkaline-hydrolyzable nitrogen (0.156). Nitrogen drove soil acidification via direct and indirect pathways by increasing exchangeable acidity and depleting base cations dominated by Ca2+. Organic matter exerted bidirectional regulation on soil pH, while total potassium indirectly alleviated soil acidification. These findings underscored the need to adjust the N:K2O ratio from 2:1 to approximately 1.5:1; Organic fertilizers and soil amendments should be applied as base fertilizers, with the mountainside (400 m) being the most suitable elevation for tea cultivation.