Background <p>Ice cover and thawing processes play a critical role in regulating biogeochemical cycling at the sediment–water interface (SWI) in seasonally frozen lakes. However, a comprehensive understanding of the spatiotemporal distribution and transformation mechanisms of nitrogen at the SWI remains limited. In this study, we employed high-resolution diffusive gradients in thin films technology to investigate in situ distribution patterns, diffusion fluxes, and source–sink dynamics of dissolved nitrogen at the SWI across the ice-covered and ice-thawing periods in Xingkai Lake, located at the border between China and Russia.</p> Results <p>Under the ice-covered conditions, NH₄⁺-N concentrations at the SWI ranged from 0.08 to 2.70&#xa0;μg L⁻¹, while NO₃⁻-N concentrations varied between 0.08 and 2.07&#xa0;μg L⁻¹. During the ice-thawing period, NH₄⁺-N concentrations ranged from 0.08 to 2.22&#xa0;μg L⁻¹, and NO₃⁻-N concentrations from 0 to 1.89&#xa0;μg L⁻¹. Diffusion flux analyses indicated that both NH₄⁺-N and NO₃⁻-N were released from sediments into the overlying water from the ice-covered period to the ice-thawing period, suggesting that sediments functioned primarily as a nitrogen source. Hypoxic and reducing conditions under ice cover inhibited nitrification, promoting NH₄⁺-N accumulation, while warming and reoxygenation during the ice-thawing period enhanced the mineralization of organic nitrogen and stimulated nitrification. These processes increased nitrogen availability and intensified exchange dynamics at the SWI.</p> Conclusions <p>Our findings highlight the profound impact of seasonal shift in source–sink behavior on internal nitrogen dynamics, and deepen the understanding of biogeochemical cycling under ice cover in seasonally frozen lakes. Future studies could aim to clarify how ice formation and melting impact regional nitrogen cycling, with a particular focus on the biogeochemical processes that regulate bioavailable nitrogen in lake ecosystems.</p>