Dissolved organic carbon decomposed or polymerized in ice depending on nitrate-mediated carbon-nitrogen coupling

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. Pyrogenic dissolved black carbon in ice degraded oxidatively during carbon-nitrogen coupling, causing carbon loss, while compost-derived dissolved organic carbon humified and stabilized, as shown in freezing experiments with straw biomass derivatives.