High nutrients surplus led to deep soil nitrate accumulation and acidification after cropland conversion to apple orchards on the Loess Plateau, China

Nitrate accumulation in the deep vadose zone poses direct or potential threat of environment and agriculture due to the long hydraulic retention time, especially in areas with limited rainfall. This study explored the primary causes and associated environmental problems of deep soil nitrate accumulation in apple orchards on the Loess Plateau by investigating nutrient (N, P, and K) surplus, deep soil nitrate accumulation and acidification, fertilization habits, and farmer awareness, through a combination of questionnaire and experiment. The results revealed much larger nutrient inputs than outputs, with N, P, and K surpluses 1.8–4.0-fold higher than required for apple tree growth. The huge N surplus caused nitrate accumulation and acidification in deep soil under apple orchards (up to 6 m). Soil nitrate storage in 25-year-old apple orchards within 6 m depth ranged from 5039–14,693 kg ha–1, 4.7–11.7-fold higher than under cropland. In addition, soil pH in 25-year-old apple orchards was 0.31–0.40 lower at 2–4 m depth than that of cropland. Overfertilization was the main contributor to the high cost of apple production, with most farmers unaware that they had overfertilized; the farmers also lacked an environmental conscious. We recommend multiple fertilizations, reasonable ratios of chemical and organic fertilizer, and scientific fertilization guidance to balance the economic benefits and environmental protection of apple orchards. In addition, farmers should be educated on environmental protection to sustainably develop the apple industry on the Loess Plateau.