Acidification of Surface Soil in Croplands in the Semiarid Middle Tibet Plateau, China

The research of temporal variability of fertility input rates and how this variation affects soil acidity is imperative in the semiarid agricultural areas. Through the investigation of soil pH in cropland in the semiarid middle Tibet Plateau, China, in 2008 and the analogy with the spatial-temporal variation of pH of soils and their causing factors, It was shown that significant changes of soil pH in the area occurred since the Second National Soil Survey in 1980s, With average pH values decreasing from 7.70 in 1980s to 7.22 in 2008. The annual nitrogen rates were significantly different during three periods: 1985-1990, 1990-1995, and 1995-2005, where mean annual N rates increased from 7.7 kg ha^-1 yr^-1 to 94.01 kg ha^-1 yr^-1, followed by a decrease to 49.09 kg ha^-1 yr^-1 and then increased to 169.82 kg ha^-1 yr^-1. Fertilizer input rates were causing nutrient imbalances, contributing to acidification. A significant increase of fertilizer input, particularly ammonium nitrogen fertilizer, most likely caused the severe acidification in Ustic Cambisols and Dar Aquic Cambisols. We recommend that reduce annual rate of ammonium nitrogen fertilizer to avoid acidification. Efforts should be made to readjust soil nutrient inputs to reach an optimal, sustainable level.