Mineral-bound lipid formation in soils and sediments: the importance of microbial pathways

Mineral protection is the most important mechanism for the long-term preservation of soil organic matter (SOM). However, the proportions of mineral-bound organic compounds, varying in chemical structures and biological origins, remain unclear, impeding a deeper understanding of the mechanisms underlying SOM-mineral interactions. Structurally diverse lipids such as fatty acids, tetraethers, and fatty alcohols are slowly decomposable biomarkers reflecting plant and microbial origin and, therefore, are good indicators for exploring the formation of organo-mineral associations. Here we used offline pyrolysis to quantify the mineral-bound lipids in aquatic sediments (i.e., lake and marine sediments) and soils with varying water content. Diverse microbial lipid types, such as monoalkyl glycerol ethers (MAGEs) and branched fatty acids, exhibited comparable mineral-binding proportions despite their structural differences. These microbial lipids generally showed higher proportions in mineral-bound forms compared to plant-derived lipids. This suggests that the biological origin of SOM may play a more significant role than chemical structure in the formation of organo-mineral associations. In addition, the proportion of bound microbial lipids was higher under drier conditions, whereas the proportion of bound plant-derived lipids was not affected by water content. We attributed this discrepancy to the different pathways through which microbial and plant lipids become mineral-bound, as microbes are more likely to attach to mineral surfaces under drier conditions, facilitating the formation of organo-mineral associations, while plant organic matter is adsorbed on the mineral surfaces after initial decomposition. This sheds new light on the microbial contribution to SOM stability, highlighting microbial physiology, especially hydrotaxis (water-directed movement), as a crucial biogeochemical factor in SOM stabilization.