Archaeal lipids in soils and sediments: Water impact and consequences for microbial carbon sequestration

The mineral protection of microbial necromass is critical for soil carbon stabilization. The water impact on the interactions between minerals and microbial necromass, however, remains unclear due to a lack of proper methods to quantify the mineral-protected microbial necromass. Here, we used offline hydrous pyrolysis to release the archaeal necromass—isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs) bound to clay minerals by heating samples at increasing temperatures (150, 200, 250, 300, and 350 °C) under argon atmosphere. The content of extractable isoGDGTs in soils reached the maximum at 200–250 °C, indicating the release of bound isoGDGTs. By investigating the free and bound isoGDGTs along a soil transect with increasing soil water content (SWC), we found that the portion of bound isoGDGTs reached up to 97% of total isoGDGTs in dry soils, and decreased with increased SWC, suggesting that SWC controls isoGDGT stabilization on clay minerals. This relationship was confirmed in the soils from the Northeast China Transect, where the proportion of bound isoGDGTs decreased with increased mean annual precipitation from 160 to 814 mm. Such a connection was further supported by the water-saturated samples of the lake and marine sediments, which have a much lower proportion of bound lipids than the soils. The decrease of bound lipid portion with increasing water content is related to the lower attachment of microbial cells to minerals in wetter soils. Microbial cells are more strongly attached to the mineral phase under drier conditions due to the thin water film on the mineral surfaces. In addition, microorganisms secrete more extracellular polymeric substances under drier conditions, which reinforces the interactions between microbial-derived organic matter and minerals. The increased microbe-mineral interactions under drier conditions offer more stable microbial legacies and stabilize organic matter in the (semi)arid regions. • Mineral-bound archaeal lipids were released using offline pyrolysis. • Bound archaeal lipids could account for 97% of total archaeal lipids in soils. • The proportion of bound archaeal lipids negatively correlated with soil water content. • Aridity favors the preservation of archaeal necromass in soils.