| Soil samples of 0~120 cm depths and plant samples of wheat were taken from 30 farmlands with different fertility levels in both irrigated and dryland regions in Ningxia.The NH4-N contents,NO3-N contents and nitrogen uptake amount of plant were determined in order to investigate the spatial variability of soil mineral nitrogen and the relations between distribution of mineral nitrogen in soil profiles and nitrogen accumulated rates in aboveground parts of wheat.The results showed that the contents of NH4-N,NO3-N and mineral nitrogen at the depths of 0~120 cm in irrigated regions were significantly higher than that in dryland regions.There were very significant differences between different soil samples,and the variation coefficients were quite high.However,there were no significant differences between different soil layers either in irrigated regions or in dryland regions,and their distributions in all profiles were relatively even.There were obvious positive correlations between the contents of mineral nitrogen at different depths.There were evident differences of accumulated amounts of mineral nitrogen in 0120 cm among different soil types,such as irrigating warped soil,dark loessial soil,cultivated loess soil and sierozem,the amounts of which were 370.0,155.1,117.5 and 61.0 kg/hm2,respectively.The accumulated amounts of mineral nitrogen in soil profiles were positive correlated with the yield,biomass and nitrogen uptake of wheat.Thus it can be considered that the mineral nitrogen in deeper soil layers makes an important contribution to nitrogen uptake of plants. |