【Objective】 The use of the diffusion method coupled with mass spectrometry to determine δ¹⁵N of inorganic nitrogen in soil and water samples in researches in the environmental, ecological and agricultural fields. Compared with the distillation method, the diffusion method consumes less labor and material resources, and can be used in massive operation. The diffusion method not only works when the sample is low in nitrogen content, but also avoids the risk of cross contamination and the need for fractionation. However, for application of the diffusion method, it is essential for the researchers in China to have a sound diffusion-incubation system which is able to perform rapid and accurate determination of δ¹⁵N of inorganic N in the soil. 【Methods】The diffusion method proceeds as follows: put a set amount of soil extract in a small airtight grass container; add some alkaline reagent to convert NH₄⁺-N into NH₃, which is adsorbed by acid-spiked filter paper; for determination of NO₃^--N, titrate some alkaline reagent to remove NH₄⁺-N in the sample; and add some Devarda’s alloy to reduce NO₃^--N into NH₄⁺-N. In the light of characteristics of the variation of inorganic nitrogen in soil extracts, diffusion conditions, such as incubation temperature, incubation time, type and rate of reagents were tested and optimized. 【Results】Results show that for soil extracts, >2 mg L^-1 in inorganic N concentration, only 20 ml soil extract is needed. put it into a 250 ml flask; hang 2 pieces of filter paper spiked with 10 μl 1 mol L^-1 H₂C₂O₄ each in the flask; add 0.1 g MgO, and then incubate the sample for 24 h at 25 ℃ on a rotator running at 140 r min^-1 to complete the processes of diffusion and recovery of NH₄⁺-N; and then replace the used filter paper with two new ones also spiked with H₂C₂O₄; incubate it on a rotator running at 140 r min^-1 for 48 h to remove remaining NH₄⁺-N; and again replace the used filter paper with two new acid-spiked ones, add 0.1 g Devarda’s alloy, and incubate it for 24 h to complete the processes of diffusion and recovery of NO₃^--N. For soil extracts < 2 mg L^-1 in inorganic nitrogen concentration, 50 ml is needed to ensure accuracy of the determination once the same incubation procedure is followed. The experiment also reveals that nitrogen impurities that may affect accuracy of the determination, come mainly from highly purified water, filter paper, acid absorbent, MgO and Devarda’s alloy. To avoid the effects of the nitrogen impurities, samples should be incubated at 25 ℃ for 24 h, and filter paper dried as far as possible in NH₃-free environment. In addition, results of the determination should be calibrated against that of the control.【Conclusions】 It can be concluded that this method greatly shortens the incubation cycle of the general incubation method and is capable of accomplishing diffusion and recovery of both NH₄⁺-N and NO₃^--N simultaneously, decreasing the amount required of a sample and reducing the risk of contamination by nitrogen impurities by optimizing the rate of MgO and Devarda’s alloy. However, the diffusion method discussed here is oriented towards determination of ¹⁵N-labeled soil extracts, 2~8 mg L^-1 in inorganic N concentration, but not applicable to determination of soils samples natural in ¹⁵ N abundance or low in nitrogen concentration. Consequently, in future studies, efforts should be devoted to such issues as how to remove nitrogen impurities and quickly, how to rule out the interference of soluble organic nitrogen in the soil.