Organic matter composition modulates its degradation under the combined effects of iron (oxyhydr)oxides and light

The extent of organic matter (OM) degradation in sunlit surface soils and shallow marine sediments is critical to global carbon and nitrogen cycling. Association with iron (oxyhydr)oxides can reduce degradation, but the role of light in iron–OM interaction is underexplored. Here we show that OM composition determines contrasting responses to iron and light. Carboxyl- and/or hydroxyl-containing OM undergoes oxidation and decarboxylation, releasing carbon dioxide, whereas protein-like carboxyl–amine OM forms cross-linked products through condensation and polymerization, with only limited degradation. Iron and light promote oxidation–decarboxylation by enhancing electron transfer and visible-light absorption, while iron promotes protein unfolding and light may provide energy for polymerization. Under high-irradiance conditions equivalent to one-month cumulative daylight exposure, a conditional assessment of representative sunlight-exposed topsoil OM indicates potential carbon losses of ~8% for carboxyl- and/or hydroxyl-type OM versus ~1% for carboxyl–amine-type OM. Photochemically promoted amide formation may limit OM degradation and carbon loss, informing carbon management and process-based carbon cycle models. This study shows that iron minerals and sunlight drive different fates for organic matter in soils and sediments depending on its chemistry: some forms are broken down and lose carbon, while others form cross-linked products that resist degradation.