Dissolved black carbon input reshapes molecular phototransformation and fate of dissolved organic matter at the soil-water interface.

Dissolved black carbon (DBC) constitutes an important marine dissolved organic matter (DOM) pool through both natural and anthropogenic sources. Upon entering the soil, DBC undergoes sorption reactions and substantially alters the molecular composition of DOM in the environment. However, how the altered DOM affects its photoreactivity and molecular composition before being integrated into the marine environment remains poorly understood. Here, we traced the sorption of two distinct DBCs (pyrolyzed at 300 °C and 500 °C) at the soil-water interface and their effects on the photoreactions and molecular variations of DOM in the environment using Fourier transform ion cyclotron resonance mass spectrometry coupled with optical spectral analyses. The DBCs after the sorption decreased their condensed aromatics and N-containing compounds while competitively displacing O-rich lignins and tannins from soil organic matter. The molecular composition changes significantly reshaped their photoreactivity. Specifically, the derived DOMs exhibited up to 3.7-time higher formation rates (r) and apparent quantum yield (Φ) of photochemically produced reactive intermediates (PPRIs), including DOM*, O, and •OH, than pristine DBCs and soil indigenous DOM except for the Φ of DBC500. The O-rich macromolecules from soil organic matter and small aromatics from DBCs controlled the derived DOMs' r and Φ under light irradiation and thus their stability and fate. After photoreactions, both pristine DBC and derived DOM samples significantly reduced their aromaticity, molecular weight, and redox activity. These molecular alterations represented a transition from aromatic-rich terrestrial DOM toward aliphatic-dominated marine DOM characteristics. These findings provide insightful clues for understanding how DBC input affects the molecular transformation and fate of DOM during its traverse through terrestrial to aquatic environments and bridge the molecular evolution gaps of DBC along the critical zones of the global carbon cycle.