## 1 Introduction  
Nitrogen (N) is one of the most important nutrients for securing crop production, which maintains the sustainability and economic viability of the world's agricultural systems [1]. The application of synthetic nitrogen fertilizer has become a major source of nitrogen for crop absorption, and nearly 50% of the world's population is dependent on synthetic fertilizers [2]. To meet the growing demand for food, China has applied more synthetic nitrogen fertilizer to their soils, accounting for more than a quarter of the total use amount globally [1]. However, the overuse of synthetic nitrogen fertilizer may cause nutrient runoff, biodiversity losses, land degradation, and increases in greenhouse gases emissions [3]. Thus, applying more efficient fertilization is essential for reducing synthetic nitrogen fertilizer consumption and optimizing nitrogen use efficiency. Organic materials (e.g., compost manure, kitchen waste, and crop residue) could be used as substitutes for synthetic nitrogen fertilizers with relatively lower costs for sustainable agriculture [4; 5]. The application of organic materials provides the necessary nutrients for crops and also improves soil water-holding capacity, soil aeration, organic matter content, and biological activity [6; 7]. Unlike synthetic fertilizers, organic fertilization helps mitigate nitrogen loss by reducing the risk of nitrogen leaching into the groundwateror volatilizing into the atmosphere as ammonia [8]. Furthermore, organic matter in these fertilizers enhances soil structure and microbial activity, promoting nitrogen retention through microbial immobilization and organic matter decomposition. This not only minimizes nitrogen runoff but also improves soil health and fertility over time, contributing to long-term sustainability in agricultural practices [9]. The nutrients in some organic materials could also be released more slowly, ensuring a long-term supply of nutrients to crops, and the application of different types of organic materials may have different effects on soil nutrient accumulation and crops performance [10]. Compared to the soil total nitrogen, the active organic nitrogen in soils is more sensitive to changes in fertilization practice and environmental factors, which could better reflect the quality of the soil nitrogen pools [11]. The soil active organic nitrogen (ATN) mainly includes particulate organic nitrogen (PON), soil microbial biomass nitrogen (MBN), and dissolved organic nitrogen (DON) [12]. Active inorganic nitrogen is also called mineral nitrogen (N\({}_{\text{min}}\)), including ammonium nitrogen (NH\({}_{4}\)\({}^{+}\)-N) and nitrate-nitrogen (NO\({}_{3}\)\({}^{-}\)-N). Ammonium nitrogen is easily absorbed by colloids, while nitrate nitrogen is easily transported with water, which is the main nitrogen form absorbed and utilized by crops. Generally, the soils in semi-arid regions have thin soil layers with an insufficient water supply and low nutrient contents, which is not conducive to organic matter improvement and nutrient storage [13]. The use of film mulch drip irrigation technology changes the water and heat conditions of soils, which helps preserve soil moisture and improve soil fertility under the application of organic materials. Therefore, this study aimed to explore the changes in the soil nitrogen pools in semi-arid chestnut soils by returning three different organic materials (maize straw, fodder grass, and sheep manure) to the field under drip irrigation with film mulching. The research results could help determine the most appropriate type of organic materials to be returned to the field and also provide a basis for the sustainable use of organic materials in a semi-arid soil.