Abstract: Riparian zone is a hot spot region of nitrogen (N) biogeochemical cycle in terrestrial-aquatic ecotone, while the freezing-thawing cycle resulted from the temperature change during spring snow-melt is one of the important factors affecting soil N transformation process and N loss. In this paper, a laboratory simulation experiment was conducted to study the responses of soil dissoluble N and net N mineralization rate to different freezing temperature and freezing-thawing frequency in a riparian zone under the vegetation types of Sorbaria sorbifolia, Larix gmelinii, and cropland. The results showed that freezing-hawing frequency had significant effects on the soil dissoluble N concentration, and there was a similar variation trend of the dissoluble N concentration under the three vegetation types, i.e., peaked after the first freezing-thawing cycle and became stable after the tenth freezing-thawing cycle. The soil inorganic N concentration under the three vegetation types increased significantly under freezing-thawing conditions (P<0.05). Freezing temperature had significant effects on the soil net N mineralization rate (P<0.05),  with the maximum after the first freezing-thawing cycle and decreased gradually with increasing freezing-thawing cycle. As affected by freezing-thawing cycle, the soils under the three vegetation types all had the risk of N loss. Due to the higher background value of soil inorganic N and the lack of plant N absorption in spring, cropland had a higher risk of soil N loss with snowmelt runoff, as compared to the other two ecosystems.

Key words: spring maize, soil carbon storage, soil moisture., soil rotation tillage, soil nitrogen storage