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 soils with increasing water content as well as aquatic sediments. The proportions of mineral-bound microbial lipids were higher than the plant-derived lipids, with microbial-derived lipids of diverse structures showing similar proportion levels. 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 the water content. This discrepancy stems from the different pathways through which microbial and plant lipids become mineral-bound. 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, as a crucial biogeochemical factor in SOM stabilization.
 

microbial lipids, offline pyrolysis, mineral-associated organic matter, soil water content