Conversion of Natural Ecosystems to Cropland Increases the Soil Net Nitrogen Mineralization and Nitrification in Tibet*1

A comparative study was conducted to determine the NH+4 and NO−3 concentrations in soil profiles and to examine the net nitrogen (N) mineralization and nitrification in adjacent forest, grassland, and cropland soils on the Tibetan Plateau. Cropland soil showed significantly higher inorganic N concentrations in soil profiles compared with forest and grassland soils. NO−3 -N accounted for 70%–90% of inorganic N in cropland soil, while NH+4 4 -N was the main form of inorganic N in forest and grassland soils. The average net N mineralization rate at 0–20 cm depth was approximately twice in cropland soil (1.48 mg kg−1 d−1) as high as in forest (0.83 mg kg−1 d−1) or grassland soil (0.72 mg kg−1 d−1). Cropland showed strong net nitrification, with the net rate almost equal to the total net N mineralization. Urea addition stimulated soil respiration, particularly in forest soil. Most urea-N, however, remained as NH+4 in forest and grassland soils, while NO−3 was the main form of inorganic N to increase in cropland soil. Higher rates of net nitrification in cropland soils suggest that land use change on the Tibetan Plateau may lead to high N losses through nitrate leaching.