Studying the effects of different nitrogen application amount on soil fertility before spring sowing could help determine the suitable nitrogen application amount in Hetao Irrigation Area. In this study

four nitrogen application levels (high-nitrogen N3:325 kg/hm2

medium-nitrogen N2:225 kg/hm2

low-nitrogen N1:125 kg/hm2

and no nitrogen application N0) were set up in the field experiment. The spatial and temporal distribution of water and salt and the changes of total nitrogen

alkali-hydrolysis nitrogen and organic matter contents were studied during freezing-thawing period. Then the soil fertility was evaluated by principal component analysis. The results showed:Firstly

after autumn water application

soil moisture content was significantly higher than before. The surface soil moisture content increased. When it just began to freeze

soil water accumulated to surface layer

and then froze to the deeper layer as the temperature reduced. The irrigation rates were the same in different treatments. The impacts of different nitrogen treatments on the soil moisture content was not significant. The soil moisture could meet the crop water demand. The soil salinities increased with the increasing nitrogen application rates during the growth period. A large amount of salt was leaching when water was applied in autumn. Soil salt was subjected to the combined effects of infiltration and evaporation during the water thawing

resulting in that the surface soil salinities were N0(0.14%) < N1(0.17%) < N2(0.19%) < N3 (0.45%). Secondly

during freeze-thaw period

soil nutrients were mainly concentrated in the surface soil layer of 0-20 cm. During the freeze-thaw period

the contents of total nitrogen

alkali-hydrolysis and organic matter in the soil showed a change pattern of first increasing and then decreasing. The maximum value occurred in the maximum freeze-thaw period

indicating that freezing helped the accumulation of total nitrogen

alkali-hydrolysis and organic matter. The last

soil fertility was comprehensively evaluated before the next spring sowing. Organic matter

organic carbon and total phosphorus on PC1 and PC2 had higher factor loads

contributing to soil fertility. Considering from the composite scores (N3 < N0 < N1 < N2)

high nitrogen treatment led to the low soil fertility

and the highest scores were given in the medium nitrogen treatment

resulting in the highest soil fertility. Therefore

it is

recommended that the nitrogen application rate should be around 225 kg/hm2 for the maize growth

and 20% for the basal

40% for the jointing stage and 40% for the tasseling stage respectively. This fertilization will be beneficial for the improvement of the soil fertility in the following spring sowing and also for the agricultural sustainable development.