Litter-derived dissolved organic matter promotes the accumulation and transformation of unstable organic matter in mangrove sediments

Plant litter input is a fundamental driver of carbon cycling in mangrove sediments. However, the mechanisms by which litter decomposition simultaneously regulates soil organic carbon (SOC) accumulation and destabilization remain poorly understood, limiting our ability to predict the stability of blue carbon ecosystems. Herein, we conducted litter addition experiments combined with three-dimensional excitation-emission matrix fluorescence spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry and proton nuclear magnetic resonance, to assess dissolved organic carbon transformation and stability during litter decomposition. Our results demonstrate that litter decomposition alters mangrove sediment properties, with a higher decomposition rate observed in summer compared to winter. During summer, litter-driven carbon accumulation increased significantly (from 20 to 40 g kg⁻¹), characterized by high microbial activity and decarboxylation reactions, leading to substantial accumulation of labile components (e.g., CCCH, XCCH) and a net increase in SOC lability. In contrast, winter decomposition favored preservation pathways, such as dealkylation and oxygen addition, resulting in the enrichment of recalcitrant, carboxyl-rich alicyclic molecules. This mechanistic insight resolves the apparent paradox of simultaneous stability and instability, demonstrating that litter input is a critical determinant of blue carbon fate. Our findings underscore the necessity of incorporating these seasonal dynamics and molecular-level pathways into models for accurate carbon accounting and sustainable management of mangrove ecosystems. • Litter decay boosts mangrove carbon storage (20–40 g kg⁻¹) during summer. • Labile carbon (sugars, amino acids) dominates early decomposition during summer. • Winter leaf decay stores more large-molecule organics in sediment than summer. • Litter decomposition accumulates aromatic component in summer and increases aliphatic component in winter.