The carborundum tubes were employed to conduct the in situ field experiment. In 2021, four treatments were selected from the experimental station, including: (1) no fertilizer treatment (CK); (2) single application of chemical fertilizer treatment (NPK); (3) single application of organic fertilizer (M); (4) combined application of organic fertilizer and chemical fertilizer (MNPK). The basic soil properties and fertilizer application rates for each treatment are detailed in Tables 1 and2. In the spring of 2021, undisturbed soil samples from the 0~20 cm soil layer under different treatments were collected. The crops for the season were maize. After removing plant roots and other impurities, the samples were air-dried until the moisture content reached the plastic limit of the soil (approximately 22% to 25% moisture content). The large soil clumps were gently broken along the fragile zone, allowing them to pass through a 2 mm sieve, and then further air-dried at room temperature. A portion of the air-dried soil was retained as a control without any straw addition (0 g of straw added). The airdried soil (120 g) from each treatment was taken and mixed with maize straw (0.5~1 cm, 1.5 g), equivalent to actual crop straw returning to the field. The mixture was then filled into carborundum tubes (38 mm of the inner diameter, 55 mm of the outer diameter, 155 mm of the height, 8.5 mm tube wall thickness, and 140 µm by 70 µm pore size where only water molecules, organic C, and air could penetrate the wall while mycorrhizae and plant roots could not pass) and buried in the corresponding 0~20 cm soil layer of the field for in situ field incubation on 20 May 2021. Each treatment has three repetitions. The distance between each tube was 5 cm. Soil samples were collected at different time points, specifically, at 60 days (20 July 2021), 150 days (20 October 2021), and 360 days (20 May 2022) after incubation. During sampling, the carborundum tubes were inverted and gently shaken to release all the soil samples. A four-point method was used to collect 40 g of soil sample from each tube. The remaining soil was carefully placed back into the carborundum tubes, resealed, and buried back into the corresponding soil layers. After being air-dried, the soil samples were pulverized and sieved for the determination of soil nutrients and other indicators. The basic properties of the 13 C-labeled maize straw are as follows: δ 13 C value of 247‰, carbon content of the maize straw used is 356 g•kg -1 , nitrogen content is 10.2 g•kg -1 , and the carbon-to-nitrogen ratio is 34.90.