Conventional flooding irrigation and over fertilization drives soil pH decrease not only in the top- but also in subsoil layers in solar greenhouse vegetable production systems

Conventional flooding irrigation combined with over-fertilization in intensively used solar greenhouse vegetable production systems are jeopardizing soil productivity and groundwater quality due to soil acidification and nitrogen (N) leaching. While it has been shown that excessive application of N fertilizer in greenhouse production systems significantly reduces topsoil pH values, it remains unknown if also subsoil pH values do change too and if surplus N fertilization and hydrological N losses lead to nutrient imbalances. In this study, soil samples from six soil layers from 0 to 300 cm were taken from 45 greenhouse fields with three representative cultivation years (2, 5, and 10 years). Soil samples from 5 adjacent corn fields served as comparison. Results show that (1) compared to soils from adjacent corn fields, the pH in the 0–30 cm soil layer was slightly elevated two years and five years after greenhouse establishment, but was lowered by 0.52 ± 0.06 units after ten years. Moreover, also the pH in deeper soil layers (30–300 cm) significantly decreased with cultivation years. (2) A significant imbalance of N:P:K ratios in greenhouse top soils was found, as P and K accumulates while N is leached to subsoils. (3) Structural equation modeling indicated that changes in the mineral N concentration in the 0–30 cm soil layer was driving soil pH changes. Our results demonstrate that N accumulation in top soils and N leaching to deep soil caused by excessive irrigation explains observed declines in surface and deep soil pH values of conventional greenhouse vegetable production systems. Consequently, for avoiding groundwater contamination with N and for ensuring sustainable soil productivity, current schemes of irrigation and fertilization management need to be adapted to avoid N leaching, soil acidification, nutrient accumulation and imbalances.