Abstract: Abstract: Northeast China is one of the most important grain production areas in China and it is the most typical area affected by seasonal freezing and thawing. The freeze-thaw process is a key driving force for soil nitrogen migration and transformation in cold regions, as it changes the physical structure (soil aggregates, hydrothermal conditions) and biological characteristics (microbial community, biological characteristics) of soil and greatly affects the mineral nitrogen availability. However, most studies on the effects of freeze-thaw process on soil nitrogen conversion had just used soil column freezing test using homogeneous soil at the specified freezing and thawing temperature and frequency, or in watersheds with natural underlying surface conditions, such as forest watersheds and arctic alpine regions, thus the nitrogen conversion in the farmland soil affected by agricultural activities could not be well reflected. Therefore, studying the impact of freeze-thaw process on farmland nitrogen conversion and availability is necessary and of great significance for guiding fertilizer management in seasonal freeze-thaw agriculture areas. In order to better understand the effects of freezing-thawing processes on soil mineral nitrogen availability in seasonal freezing and thawing agricultural areas, modified resin core method was used to develop in situ nitrogen cultivation of surface soil in a seasonal freezing agricultural watershed in northeast China. The in situ nitrogen cultivation experiments were carried out in 5 fields with different underling surface during the 2015?2016 freezing-thawing period, and the test devices were taken out in 6 batches before and during the freezing-thawing period for comparative analysis. Results showed that the soil freezing process increased the ammonium nitrogen content of the surface soil by 1.7 times and reduced the nitrate nitrogen content by 19%, which in turn increased the soil mineral nitrogen content and the proportion of ammonium nitrogen. In addition, it also reduced the difference coefficient of soil ammonium nitrogen content among different underlying surfaces by 36%, and increased that of nitrate nitrogen content by 2.5 times. During the melting process of frozen soil, the ammonium nitrogen content of soil did not change significantly, and the nitrate nitrogen content rapidly increased and then tended to be stable. At the beginning of soil melting period, the accumulation of snow and the melting of frozen soil increased the difference coefficient of ammonium nitrogen content by 39% and 30%, and reduced the difference coefficient of nitrate nitrogen content 65% and 40%, respectively. However, the difference coefficient of nitrate nitrogen content was greater than that of ammonium nitrogen in most stages. The melting of snow significantly increased the soil moisture content, which promoted the formation of ammonium nitrogen in the soil and reduced the formation of nitrate nitrogen, but the effect was not significant.