Strong depth-dependent chemical and structural characteristics of dissolved organic matter (DOM) in organic-rich sediment in a shallow temperate lake

Porewater dissolved organic matter (DOM) in lake sediments is an important contributor to the global carbon cycle. The composition and structures of porewater DOM typically vary with depth, yet the variations in high-resolution depth intervals and the controlling biogeochemical processes are not well understood. Here we performed comprehensive, high-resolution (1 cm interval) characterization of sediment porewater DOM in Baiyangdian, a representative large shallow freshwater lake with abundant biomass deposition in northern China in the temperate zone, by employing chemical analysis, optical techniques, stable isotope analysis, ultra-high resolution mass spectrometry, and 16S rRNA gene sequencing. Anaerolineaceae was the dominant microorganism in the sediment, reflecting fermentation as the key process involved in organic matter diagenesis. Contrasting ¹³C signatures between particulate organic matter (POM) and DOM suggest that selective degradation of organic matter with component-specific isotopic signatures at distinct depths is occurring. Corresponding to the concentration depth profile of POM, the composition and structures of DOM exhibited strong multilayered variations across the depths of 1–4 cm (surface layer), 5–13 cm (middle layer), and 14–19 cm (bottom layer). In the surface layer, intense fermentation of fresh POM rapidly released protein-like fluorescent components whose content decreased with depth, whereas humic-like fluorescent components gradually accumulated. Consistently, the aromaticity and molecular weight of chromophoric DOM analyzed by ultraviolet–visible spectroscopy increased with depth. Due to the exhaustion of labile POM and the decreased fermentation activity, the contents of protein-like and humic-like fluorescent components as well as the aromaticity and molecular weight of chromophoric DOM stayed relatively constant in the middle layer. In the bottom layer, the sharp increase in concentrations of dissolved organic carbon and volatile fatty acids (acetate, propionate, and butyrate) indicated re-enhanced fermentation activity, which was also supported by the prominent decrease in the protein-like and humic-like fluorescents as well as the decrease in aromaticity and molecular weight of chromophoric DOM. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis showed that the surface layer had the highest relative abundance of aliphatic compounds among the three layers, while aromatics and polycyclic aromatics accumulated only in the surface layer and middle layer, indicating their preferential production and consumption during POM deposition. However, lignin-like compounds accumulated gradually and consistently across the three layers, resulting from their refractory nature and continuous degradation of residual POM. Most of the N/S-containing molecules accumulating in the three layers were lignin-like, aromatic, and polycyclic aromatic compounds, which were apparently preserved due to their recalcitrance, probably with the assistance of secondary amination/sulfurization reactions. The δ¹³C values (−30.5 ‰ to –22.7 ‰) of acetate were significantly lower compared to bulk DOM (−20.2 ‰ to −7.8 ‰) and decreased gradually with depth, suggesting an acetogenesis pathway with depth-dependence. Propionate and butyrate were present only in the bottom layer stemming from a re-enhanced fermentation at the depth. Our findings revealed a strong depth-dependent sequential mineralization of organic matter primarily through fermentation, which led to an apparent characteristic depth profile of DOM composition and structures. This offers a model for comprehending how sedimentary organic matter transforms with depth in comparable lacustrine settings.