## 4 Discussion
Studies have shown that a single application of chemical fertilizers can increase soil nitrogen content to a certain extent, but the effect is not as good as the application of organic materials [20]. In this study, the application of organic materials significantly increased both the content of active nitrogen and TN, which was attributed to the decomposition of nutrients in organic materials over time [7]. The release of these nutrients can promote the growth of crop roots and promote the transformation of the active nitrogen pools. On the other hand, organic materials contain activity nitrogen-like components that can directly increase soil active nitrogen components after being applied to the soil [12; 21]. From 2018 to 2019, the changes in TN content under the four treatments were insignificant, indicating that the TN is more stable than the active nitrogen pools under the changes in fertilization practice in a short term. Generally, when the TN content of the soil is less than 2 g kg\({}^{-1}\), the soil is considered to be nitrogen-deficient [22]. In this experiment, the TN content under different treatments at different times was between 1.15 to 1.40 g kg\({}^{-1}\), indicating that the soils at the test site were not high in nitrogen. DON represents an unstable nitrogen source for soil microorganisms. In our study, the content of DON and the proportion of DON in TN under the organic material treatments were higher than that of CK, indicating that the application of organic materials is an efficient method to improve soil DON. Besides, the plant-based organic materials (MS and FG) were better at increasing the DON content than the animal-based organic material (SM). Many factors simultaneously affect the degradation and migration of DON content, including soil water content, temperature, litter and plant residues, soil physiochemical properties, and farming methods [18]. Studies have shown that during repeated freezing and thawing, the microorganisms in the soil gradually adapt to temperature changes, reducing the mortality rate, as the soil aggregates also tend to be stable. Therefore, the low content of soluble nitrogen in the soil during the freezing period (October 2018 to April 2019) could be attributed to the fact that most of the microorganisms did not die as soon as the freezing period began. Hence, they continued to consume soluble organic nitrogen in the soil [23; 24]. During the thawing period, utilization of plastic mulching increased soil moisture and temperature, which promoted the release of soluble organic matter and might change the biological activities in the soil, thereby increasing the concentration of DON. MBN is considered as an important indicator of soil quality, which can quickly respond to changes in soil management practice [2]. Although MBN only accounts for 2-6% of TN, it has a vital influence on the rate of nutrient cycling in agricultural ecosystems [25; 26]. Due to the rapid generation of soil microbial biomass, MBN is considered to be the most unstable soil active nitrogen component, which represents the source of mineral nitrogen and the absorption of mineral nitrogen [27]. Our study found that the MBN content was increased from October 2018 to April 2019, which might a result of the dead microorganisms that were decomposed and released nitrogen to the soil, along with the destruction of soil aggregates and increased enzyme activities during the freeze-thaw cycle [28]. Moreover, the content of MBN and MBN/TN ratios under organic material treatments (MS, SM, and FG) were higher than that of the control treatment, indicating that returning organic materials to the field could efficiently increase soil microbial activity. The film-mulched drip irrigation technology created better water and heat conditions, which also helped optimize soil conditions for enhancing microbial activities. Previous studies showed that returning organic materials to the field increased the content of stable organic matter in the soil, which was stored in the form of particulate organic matter [29].