| Soil is the largest carbon pool in the earth’s surface ecosystem.The soil organic matter carbon pool is 2.5 times that of the atmosphere and twice that of the terrestrial vegetation.It plays an important role in mitigating global climate change.Paddy soil,is a type of soil developed by the rice agriculture,which has a long farming history in China,which represents anthropogenic soil.It has highly organic carbon content and considerable carbon sequestration potential,which is of great significance for maintaining soil health and sustainable agricultural development.Under the influence of long-term hydroponic maturation of rice cultivating,the structure,source,composition,renewal and accumulation dynamics of organic matter in paddy soil reflect the specific soil-crop-microbe interaction,and regulate the carbon cycle process and agricultural productivity of the paddy ecosystem.The accumulation and stabilization process of organic matter in paddy soil and the relative contribution of plants and microorganisms have always been important scientific matters in the study of soil carbon sequestration.The research on the molecular composition of organic matter based on the identification of organic molecules,especially the molecular research of biomarker molecules and humus research in recent years,has promoted the development of soil organic matter molecular research to support novel farming technologies and soil conservation strategies.The historical sequence of rice cultivation in the paddies(chronological sequence of paddy soils)located in Cixi City,Zhejiang Province,has been a very in-depth tool to investigate both domestic and foreign soil formation,organic matter accumulation,and soil microbial flora evolution.Long-term rice cultivation promoted the transformation of soil minerals and the formation of soil aggregates,while soil fertility and ecosystem functions shown positive succession.However,the differences between the molecular composition of soil organic matter and the contribution of microorganisms in paddy soils with different rice cultivation length are still insufficiently understood.In this research,we used the time series soil that evolved into paddy soil under the tidal flat wetland of Cixi,Zhejiang.We collected uncultivated tidal flat topsoil(P0)and rice cultivated of 50 years(P50),100 years(P100)and 700 years(P700)paddy soils topsoil,and used phospholipid fatty acid(PLFA),traditional alkaline extraction(e SOM)and pyrolysis-gas mass analysis technology(Py-GCMS)to study the molecular composition of soil organic matter and biomarkers,to reveal the "molecular contribution" of long-term rice cultivation to the organic matter composition changes of paddy soil,and to provide a scientific basis for further understanding of farmland soil organic matter and its relationship with farmland ecosystem management.The main research results of this thesis were:(1)The amount of soil carbon-loss gradually decreased with the increase of tillage years.The soil p H gradually decreased under different rice cultivation years.The total Ca content of the bulk soils was gradually eluted and leached with the cultivation years increased.The content of elements such as iron and zinc shown an overall increasing trend.The content of SOC and TN in the bulk soils increased sharply at the beginning of tillage,and stabilized with the increase of rice planting years.The carbon and nitrogen contents in alkali-extracted humus increased.The humification process was most obvious in rice fields with prolonged cultivation years.Fe/ OC gradually decreased from 1.69 of P0 to 1.48 of P700.(2)The number of cultivation years significantly changed the molecular abundance of soil organic matter and the relative content of functional groups.P0 had the lowest molecular abundance from all other paddies.With the increase of the cultivation years,the molecular abundance and aromaticity of organic matter were increased,however,there was no significant difference in the molecular abundance of soil organic matter in P100 and P700 paddy fields.The polysaccharide/lignin C-O bonds and aliphatic hydrocarbons were reduced under soil succession,while the absorption peaks of carboxyl and ketones were increased.With the cultivation years increased,the moleuclar groups like fatty acids(FA),sugar esters(SES),phenols(PH),sugars(SU)and phenolic esters(PE),along with other organic compounds accumulated in the paddy soils.(3)The microbial contribution of organic matter in paddy soils varied among years of rice cultivating.Paddy soil reclamation increased soil microbial abundance and the activity of microbial PLFAs in P100 was generally the highest.The chronological soil microbial community was mainly composed of bacteria,followed by fungi and a small amount of actinomycetes.The content of actinomycetes gradually increased from scratch with the extension of farming time.The ratios of bacteria to fungi(B/F)and gram positive to gram negative(G+/G-)were significantly increased compared with P0.Compared with uncultivated soil,the B/F ratio in P50 increased by 67.61%,while the G+/G-ratio in paddies were increased 165-65%.The molecular composition was related to the microbial community.The hydrophobic molecules were selectively protected from microbial degradation,while the organic matter molecules that were effectively utilized by microorganisms gradually decreased with the increase of the microbial PLFAs abundance. |
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