<p>Frozen regions sequester dissolved organic carbon (DOC) in ice, yet its transformation, especially coupled with other elements, remains unexplored. We found that freezing accelerated DOC transformation when coupled with nitrite, and the transformation pathway varies with carbon source. At pH 3.0, dissolved black carbon (DBC) underwent oxidative degradation with decreasing molecular weight and aromaticity, whereas leached DOC (LDOC) exhibited humification, increasing the humification index from 0.38 to 0.65. The difference arises from nitrite protonation to HNO₂ and NO⁺ under the strongly acidic conditions induced by the freeze concentration effect. HNO₂ amplified the oxidative capacity of H₂O₂ for DBC decomposition, where H₂O₂ concentration in the DBC system was 6.9-fold higher than LDOC. However, NO⁺ converts electron-rich phenols within LDOC (2.65-fold higher than DBC) into phenyl radicals, triggering polymerization to humus. These findings reveal different nitrite-induced DOC turnover during freezing, providing valuable insights into carbon source-sink dynamics during carbon-nitrogen coupling in ice.</p><p></p>