【Objective】The 15N isotope ratio of NH4+-N plays a crucial role in nitrogen transformation research and is widely applied to soil, water, and other samples with sufficient inorganic nitrogen content. While the diffusion method is the most effective technique for isolating and transforming soil NH4+-N, its application remains limited for low-concentration and natural abundance NH4+-N. 【Method】In this study, it is aimed to evaluate the feasibility of using the diffusion method alone for transforming and measuring low-concentration (≤1 mg·L-1) labeled and natural abundance NH4+-15N. To establish a diffusion system for rapid and accurate determination, this study improved the acid trap material and production method, incubation time, reaction volume, and incubation temperature. 【Result】The results demonstrated that: (1) The envelope diffusion packet acid trap exhibited higher nitrogen recovery?(95.0%) and better precision (standard deviation <0.010 atom%) compared to suspended acid traps. Envelope packets fabricated from domestic high-purity polytetrafluoroethylene (PTFE) could replace imported Merck Millipore PTFE, reducing costs by >90%.?The improved diffusion packet not only reduced the cost, simplified the operation and also increased N recovery to 99.8%. (2) The diffusion method accurately and sensitively measured the labeled NH4+-15N concentrations as low as 0.2 mg·L-1 with an error <0.006 atom%, and the difference between the corrected and the expected value was <0.020 atom%. In addition, diffusion method further exhibited robust performance in natural abundance NH4+-15N (≥1 mg·L-1), with an error ≤0.30‰ and a deviation from the expected less than 0.38‰. (3) Expanding the sample volume to 100 mL for low-concentration labeled samples and employing envelope packet acid traps for 8-day diffusion at 25 ℃ not only significantly enhanced recovery, measurement accuracy and precision, but also enabled accurate quantification of low-concentration NH4+-N (≥0.2 mg·L-1) with minimal isotopic deviation (Δ15N<0.020 atom%) after correction with standards. For even lower concentrations of labeled or natural abundance NH4+-N, it is advisable to combine diffusion method with additional methods. While heating can effectively shorten the incubation time and improve recovery, it also amplified interference from exogenous nitrogen impurities.?And heating may lead to increase the interference of dissolved organic nitrogen (DON) in natural soil or water samples, necessitating cautious application during practical measurements. 【Conclusion】Overall, diffusion remains a powerful method for transforming and measuring NH4+-15N in soil. This study enhances the concentration threshold and accuracy of labeled sample diffusion methods while significantly reducing experimental duration. The improved diffusion protocol can be used for measuring NH4+-15N at different concentrations and abundances, providing a theoretical foundation for accurate soil inorganic nitrogen analysis.