Moisture Impact On The Bound Microbial Lipids In Geological Archives: From Simulation To Geological Records

Carbon and water cycles are two critical issues for the Earth’s surface system processes.Because of close interactions,the dynamic linkages between the C and water cycles remain largely unknown,greatly preventing the understanding of C sequestration under the changing hydroclimatic conditions.Understanding the impacts of hydrological conditions on soil and aquatic carbon storage is of great importance to the study of climate change and carbon neutralization.The hydrological impact on the interaction of microbial and plant-derived lipids remains unclear mainly due to the lack of proper methods to quantify the mineral-bound organic matter.Lipid biomarkers are useful tools to study the preservation of microbial necromass bound to minerals from the molecular perspective.Normally,the lipid biomarkers were extracted directly with organic solvents,which were“free”lipids,while the lipid biomarkers can also interact with minerals by forming covalent bonds,which need further processing to extract and were referred to as the“bound”lipids.The interactions with minerals were the key to the long-term preservation of organic matters.The offline hydrous pyrolysis was used to release the microbial and plant lipids bound in clay minerals.The bound proportions described the interactions of organic matters and minerals.The degradations of lipids at high temperatures also enable evaluations of potential bias caused by differential degradations.For the first time,we present the impact of water content on the lipid bounds in soils and sediments.Soils from the shore of Lake Erhai and the Northeast China Transect,peatland,lake,and marine sediments were used to study the water impact on organo-mineral interaction.The temporal variations of the bound proportion of isoprenoid glycerol dialkyl glycerol tetraethers（iso GDGTs）under changing hydrological conditions were investigated by analyzing the free and bound iso GDGTs in the loess-paleosol sequences in western China.The GDGTs in soil profiles were analyzed to investigate the oxygen level impact on the distribution of GDGTs since the oxygen levels were frequently impacted by hydrological conditions in surface soils.Based on the above work,the main conclusions are reached as follows:1)By heating the soil and sediments at increasing temperatures in an anoxic atmosphere（150,200,250,300,350,400,and 450℃,respectively）,the extractable lipid content progressively increased,reaching the maximum at 200℃or 250℃and followed by a progressive decrease thereafter,indicating the release and degradation of bound lipids.Both the portions of microbial and plant-derived organic matter bound on clay minerals were high and the bound proportions of microbial lipids were higher than the plant-derived lipids.Among the microbial lipids,the ethers were more prone to be bound than fatty acids.2)The proportions of bound lipids in aquatic environments were much lower than in the soils,and the extractable iso GDGTs keep decreasing during the whole pyrolysis in aquatic environments.The soil samples with a water content gradient surrounding the lake Erhai showed that the portion of biomarkers（tetraethers,monoalkyl glycerol ether,fatty acid,and fatty alcohol）bound on clay minerals decreased with increased soil water content,reflecting the water impact biomarkers bound on clay minerals.This relationship was confirmed in soils from the Northeast China Transect,where the bound proportion of iso GDGTs decreased with increased mean annual precipitation（MAP）from 160 to 814 mm,and the average proportion of bound iso GDGTs with the MAP<500mm（96±1.8%）was higher than that with MAP>500mm（87±5.7%）,revealing the moisture impact on the bound microbial lipids in geological archives.The decrease of bound lipid portion with increasing water content is related to the lower attachment of microbial cells to minerals.The microbes tend to attach to the mineral phase e.g.by extracellular polymeric substances（EPS）in drier conditions to avoid dehydration due to the thin water film on the surface of minerals.Microorganisms secret more EPS in drier conditions,which reinforces the microbe-mineral interactions and thus the bound membrane lipids.The increased microbe-mineral closeness and bonding under drier conditions raise the preservation of the microbial necromass in the（semi）arid regions,which is critical for carbon sequestration in（semi）arid regions.3)The values of proxies based on the free and bound GDGTs differ but the temporal trends were comparable.The proxies revealed a decreasing trend of temperature since 11.8 ka.During the early Holocene,the climate is relatively dry and the dry-wetting climate was frequent,while a constant wet climate was found during the mid-Holocene.The bound proportions of iso GDGTs,affected by the diagenetic process,were exceptionally low at the dry-rewetting transitions.The average values of bound proportions were lower under the frequent dry-rewetting processes than in the constant wet stages.The frequent dry-rewetting processes destructed soil structure and released the bound lipids to free pools,and thus the lipids underwent more severe degradations,which lead to soil carbon loss.4)Organic matters degraded at high temperatures and showed four main stages for the diagenetic degradation of lipids,including the desorption of absorbed lipids,the release of mineral-bound lipids,the destruction of macromolecules,and the destruction of released lipids.These processes influenced the applications of lipid-based proxies.For example,the carbon preference index and average chain length of alkanes decreased during the diagenetic process,which could make reconstructed temperatures and precipitations towards lower values.The methylation index and cyclization index of br GDGTs were less affected by degradation while the aridity index based on the ratio of iso GDGTs and br GDGTs(R_i/b)increased first and then decreased along with the release and degradation of bound GDGTs.5)Besides the diagenetic impact,other environmental factors such as changes in oxygen level and pH should also be paid attention to when applied in the ancient archives.By investigating the br GDGTs in the soil profiles,the oxygen levels were shown to be the direct control of hydrological impact on the br GDGTs since hydrological conditions could change the oxygen levels in surface soils.Therefore,the main control of lipids should be considered when using these lipid-based proxies,especially when variations of other factors like hydrological condition and pH were larger than the temperature.The approach of free and bound lipids in this study could be used to trace the sequestration and release of microbial necromass and has great potential for carbon sequestration research in a variety of geological archives.The findings in this study,including the higher stability of microbial necromass in semiarid conditions and the decrease of mineral protection during the frequent dry-rewetting processes,have broad implications for the dynamic linkage of carbon and water cycles.