Impacts of fertilizer practices on environmental risk of nitrate in semiarid farmlands in the Loess Plateau of China

The lack of understanding of nitrate dynamics in soil profiles of semiarid regions hampers the assessment of the environmental risks associated with nitrate. A long-term field experiment established in the Loess Plateau of Northwest China in 1984 was used to investigate the seasonal dynamics of water and nitrate contents in the soil profile (0–300 cm) under bare fallow and continuous winter wheat (Triticum aestivum L.) with various fertilizer treatments. For treatments without mineral N input (i.e., no fertilizer, farmyard manure alone, and with P fertilizer), the amount of nitrate accumulated in the soil profile (52–120 kg N ha^?1, the average for June, August, February and April) was significantly lower than that (292 kg N ha^?1) accumulated in the bare fallow treatment. A large amount of nitrate (1,065 kg N ha^?1) was found accumulated in the soil profile with the treatment applied with mineral N at a rate of 120 kg N ha^?1 year^?1 for 17 years (1984–2001) and this nitrate moved downward during the wet season (from August to February). Clearly, the amount of nitrate accumulated in the soil profiles, and its tendency of downward movement, appears to potentially be an environmental risk as it may reach groundwater. Fertilization as mineral N fertilizers coupled with FYM or P resulted in 50–70% less nitrate accumulation in the soil profiles than that using mineral N fertilizer alone, and therefore the environmental risk was reduced. It is proposed that a “break point” of nitrate distribution existed in the soil profiles, providing an indication of soil depth to which nitrate can transfer.