Integrated Analysis Of Molecular Composition Changes Of Soil Organic Matter And Its Response To Tillage Practices And Planting Patterns

In recent years,the application of biomarkers to study the molecular composition of soil organic matter（SOM）has become a new direction of SOM research,which can be used to analyze the source and degradation of SOM and reveal the stabilization mechanism of soil organic carbon（SOC）at the molecular level.Therefore,clarifying changes in the molecular composition of SOM is important for improving our understanding of soil carbon turnover.Many studies have focused on the changes in the molecular composition of SOM in forests and grasslands,while little is known about the changes in the molecular composition of SOM under different agricultural management practices.In addition,there have been some studies related to the modifications in SOM molecular composition under different land uses,but it is difficult to conclude the overall change of global scale.To further clarify the effects of different land uses on the SOM molecular composition and reveal the changes in SOM molecular composition under different agricultural management practices,this study could be the first to clarified the differences in SOM molecular composition（free lipids,cutin,suberin,and lignin）under three land uses of farmland,grassland and forest on global scale by Integrated analysis;meanwhile,a long-term positioning field experiment in Weibei dryland（started from 2007）was conducted to evaluate the effects of different tillage practices（N:no tillage,C:conventional tillage）and planting patterns（W:continuous wheat system,M:continuous maize system）on the molecular composition of SOM（including free lipids,cutin-,suberin-and lignin-derived compounds）in soil profiles（0-20,20-40 and 40-60 cm layers）.The solvent extraction,base hydrolysis,Cu O oxidation methods and gas chromatography-mass spectrometry（GC-MS）were used to analyze and quantify lipids,cutin-,suberin-and lignin-derived compounds.The main results were as follows:1.Integrated analysis of SOM molecular composition changes under different land usesThere were significant differences in the molecular composition of SOM under different land uses.The contents of free lipids（n-alkanes,n-alkanols,n-alkanoic acids,and cyclic lipids）,cutin,and lignin phenols in forest soil were significantly higher than in grassland and farmland.The contents of suberin in grassland and forest soil were significantly higher than that in farmland.The ratio of suberin to cutin for grassland was the highest（2.96）and for farmland and forest were 1.68 and 2.21,respectively.The sources of lignin in farmland and grassland soil were mainly angiosperm leaves and herbs,while the sources of lignin in forest soil were not specified.The ratio of syringic acid to syringaldehyde（Ad/Al）_S and the ratio of vanillic acid to vanillin（Ad/Al）_V of farmland soil were 1.25 and 1.58,respectively,which were significantly higher than those of grassland（0.46 and 0.69）and forest（0.78 and 0.7）.2.Effects of different tillage practices and planting patterns on soil lipid biomarkersIn the 0-20 cm soil layer,under continuous wheat system,no-tillage increased SOC,readily oxidizable carbon（ROC）,dissolved organic carbon（DOC）,microbial biomass carbon（MBC）and soil total nitrogen（TN）contents compared with conventional tillage,while decreased MBC under continuous maize system.The effects of different tillage practices on free lipids were found mainly in the 0-20 cm soil layer.Under continuous wheat conditions,compared with conventional tillage,no-tillage increased the contents of n-alkanes,n-alkanols,n-alkanoic acids,total free lipids and plant-derived SOM by 96.6%,40.8%,48.1%,61.6%and 92.7%,respectively.Compared with conventional tillage,the average chain length of n-alkanes(ACL_al)and n-alkanoic acids(ACL_ac)of no-tillage were higher,and the ratio of acyclic aliphatic lipids to cyclic lipids（Acyclic/Cyclic）was lower.Tillage practices had no significant effect on suberin and cutin,but no-tillage increased the ratio of C₁₆ω-hydroxy-alkanoic acids to all hydrolysable C₁₆ fatty acids(ω-C₁₆/ΣC₁₆).For different planting patterns,in the 0-20 cm soil layer,continuous wheat system increased SOC content compared with continuous maize system under no-tillage conditions,while decreased SOC under conventional tillage;in the 40-60 cm soil layer,continuous maize system increased SOC content by 36%-51%.Changes in soil free lipids contents were similar as for SOC.In the 40-60 cm soil layer,the contents of all free lipids under continuous maize system were higher than that of continuous wheat system,and continuous maize system treatment increased total free lipids contents by 56.2%-98.1%.In the 0-20 cm soil layer,continuous wheat system increased the contents of suberin and cutin,and in the deeper soil,continuous maize system increased the content of suberin by 36.7%-55.9%.Overall,continuous maize system increased ACL_al and ACL_ac,but decreased the ratio of short-chain to long-chain n-alkanoic acids（RSL）.With the increase of soil depth,SOC,labile SOC fractions,TN,n-alkanes,n-alkanols,n-alkanoic acids,total free lipids,plant-and microbial-derived SOM contents decreased.3.Effects of different tillage practices and planting patterns on soil ligninThe effects of tillage practices and planting patterns on lignin content were mainly found in depth of 0-20 cm soil layer.Compared with conventional tillage,no-tillage increased lignin content by 22.6%-36.2%,and continuous maize system increased lignin content by 18.2%-31.2%as compared with continuous wheat system.The ratio of3,5-dihydroxy benzoic acids to vanillyls（3,5-DHBAs/V）of conventional tillage was significantly higher than no-tillage in 0-20 cm soil layer,without effecting the deep soil.The（Ad/Al）_S,（Ad/Al）_V and 3,5-DHBAs/V of continuous wheat system were higher than continuous maize system.In conclusion,the contents of SOC and SOM molecular components in forest soil were high,and the degradation of lignin was accelerated by tillage activities in farmland.The long-term experiment study showed that no-tillage and continuous maize system can increase total free lipids and lignin contents,while inhibit the degradation of lignin and SOM,and proved to be the excellent management practices for carbon sequestration in Weibei dryland.