Mineral-bound lipids 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 that have different structures and biological sources in soils remain unclear, impeding a deeper understanding of the mechanisms underlying OM-mineral interactions. Structurally diverse lipids such as fatty acids, tetraethers, and fatty alcohols are very slowly decomposable biomarkers reflecting plant and microbial origin and therefore, are good indicators of the formation mechanisms 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, while the proportions of bound microbial lipids with diverse structures were at a comparable level, indicating that the importance of OM biological source may exceed the chemical structure in the formation of organo-mineral association. 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 contrast lies in the distinct formation pathways of mineral-bound microbial and plant lipids. Microbes tend to adhere to the mineral surfaces under drier conditions, accelerating the formation of organo-mineral associations, while plant organic matter is adsorbed on the mineral surfaces after initial decomposition. This deciphered a new aspect of microbial contribution to the stability of SOM. Consequently, microbial physiology, particularly hydrotaxis, should be regarded as another biogeochemical agent of SOM stabilization.