Abstract: Law of changes in nitrate under different soil moisture and supply of different forms and quantities of N fertilizer in Yinchuan alluvial plain were studied by means of field experiments and indoor cultivation.The results showed that nitrification process is directly related with soil moisture content and forms of N fertilizer.Nitrate obviously appeared 9 days after the application of N fertilizer.Nitrification process was restrained under dry or saturated soil water conditions.Granulation of urea can slow down nitrification process and form less nitrate than other forms of N fertilizer.Therefore,applying large-sized urea granules can reduce nitrate loss and increase N availability to plants.Nitrate increases in soil profile with the increasing of application rate of N fertilizer.Nitrate in the cultivated layer increases due to application of more N fertilizer,especially large-sized urea granules.However,granulated urea only provides the lowest amount of nitrate in the whole soil profile compared with the other forms of N fertilizer applied.The Nitrate content would not get up to a harmful level if N fertilizer is applied reasonably,but nitrate is accumulated in the 60 cm-deep soil profile under traditional N fertilizer application,easily causing nitrate leaching due to high aeration of Guanyu soil.

Key words: Guanyu soil, large-sized granulated urea, nitrate, dynamic