## 4 Discussion
The higher the stable organic matter content, the greater the nitrogen supply potential of the soil. The content of PON was significantly higher than the other nitrogen fractions (MBN and DON), indicating that PON was more sensitive to fertilization practice and could be used to evaluate its impact on changes in soil active organic matter and soil quality [30]. Our study also found a higher PON content under the application of organic materials, which is in line with the results reported by [PERSON] et al. (2011) [31]. A larger part of the nitrogen in the soil is organic nitrogen, accounting for about 92\(\rightarrow\)98% of the total soil nitrogen. However, organic nitrogen cannot be directly absorbed by crops and must be converted into inorganic nitrogen through mineralization for crops utilization [32]. The content of inorganic nitrogen affects the crop growth, and its dynamic changes are inseparable from the crop development [33]. The results of this study revealed that the application of organic materials significantly increased soil NO\({}_{3}\)\({}^{-}\)-N content, while the soil NH\({}_{4}\)\({}^{+}\)-N content was very low. There are three reasonable reasons for explaining this: firstly, the organic material contains a certain amount of soluble organic matter, which provide exogenous nutrients for the multiplication of aerobic nitrifying bacteria and also effectively improve soil aeration for nitrification, thus significantly increasing the NO\({}_{3}\)\({}^{-}\)-N content [34; 35; 36]. Secondly, organic materials could increase soil ion exchange sites and enhance ion adsorption capacity, which accumulate more NO\({}_{3}\)\({}^{-}\)-N in soils [37; 38]. Thirdly, the main inorganic form of nitrogen in dry farming soil is NO\({}_{3}\)\({}^{-}\)-N [38]. From 2018 to 2019, the accumulation of both NH\({}_{4}\)\({}^{+}\)-N and NO\({}_{3}\)\({}^{-}\)-N showed a similar trend that was firstly increased and then decreased, which was influenced by the microbial community involved in nitrification. The number of nitrifying microorganisms is generally inhibited by the alternation of freezing and thawing, but it does not completely cause fatal damage to them, and some bacteria are still highly active which promote nitrification [39]. Meanwhile, the litter and snow cover create an isolation layer between the soil surface (with low temperature) and the soils under the cover layer. This layer has a warm and anaerobic environment which also promotes nitrification [40; 41]. Moreover, the number of denitrifying enzymes during the freezing period of the soil are decreased, resulting in a rapid increase in ammonia nitrogen after soil ablation, which promotes the denitrification process for reducing the ammonia nitrogen content in the soil. Correlation analysis showed that soil TN had a significantly positive correlation with ATN but not with N\({}_{\text{min}}\), indicating that the changes in organic nitrogen fractions were more obvious than those in inorganic nitrogen fractions after the application of organic materials. A significant correlation was found between PON and TN, suggesting that the change in PON content could be a good indicator for the variation in soil nitrogen pools [42].