Dynamic response of soil nitrogen to freeze-thaw processes in different cenotypes in the forests of the Tianshan Mountains

Seasonal freeze-thaw regimes are the main factors influencing nitrogen（N） transformations in the temperate forests of northern China and are one of the key processes in the soil N cycle. However,the response to freeze-thaw cycles in different cenotypes is unclear. In this study,we conducted experiments to understand the effect of soil freeze-thawing on N transformations in soils（0-15 cm） from different cenotypes（woodland,grassland,and shrubs） in the forests of the Tianshan Mountains by simulating extreme soil freeze-thawing in different natural snow covers formed by canopy shading.The results showed that the content of ammonium nitrogen,NH4+-N,and microbial biomass nitrogen（MBN） in the soil of the different cenotypes was positively correlated with soil temperature（at a depth of 5 cm）,whereas the content of NH4+-N in forest soil was lower than that in the other communities in the deep-freezing period. The content of nitrate nitrogen,NO3--N,was higher in forest soil than in the other community types. NO3--N was the major form of soil mineral N during the seasonal freeze-thaw period in the forests of the Tianshan Mountains; it accounted for 78. 4% of soil mineral N. NO3--Ncontent decreased by 64.6% from the early-thawing to late-thawing period in shrub soil,indicating a high risk of soil NO3--N loss. The N mineralization rate was significantly affected by the freeze-thaw period,and the cenotypes significantly influenced the ammonification rate. The main process involving soil N in the forests of the Tianshan Mountains during the freezing period was N immobilization. Investigating the response of soil N to freeze-thaw patterns in different communities would help us understand the soil N cycle in the northern forest regions.