Study On The Changes Of Organic Carbon Characteristics In Peat Profiles Of Peatlands In Changbai Mountainous

Peatlands account for 3%of the world’s land area,store 547 Pg of organic carbon,accounting for about 30%of global soil carbon stocks,and are important global carbon"sinks",so the peatland carbon cycle plays an important role in the regional and even global carbon cycle.Studies have found that during the formation and accumulation of aerobic and anaerobic layers of peat,organic carbon is affected by various chemical、physical or biological transformations,and complex organic geochemical processes occur.Therefore,in order to understand the carbon cycle in peatland,it is necessary to understand the organic geochemical process of peat formation,that is,the change law of the organic carbon characteristics of the profile during the formation of peat.Therefore,three peatlands with different vegetation types（Moon Bay Peatland,Anbei Peatland and Jinchuan Peatland）in Changbai Mountain were selected,and peat profiles were collected separately（2 peat profiles were collected in Moon Bay and Anbei,and 1 profile was collected in Jinchuan）,and the profiles were divided into 2cm or 5cm to determine the total organic carbon（TOC）,total nitrogen（TN）,stable isotopes of carbon and nitrogen(δ¹³C、δ¹⁵N),oxidizable organic carbon（KOC）,soluble organic carbon（DOC）,and each layer of peat were divided into 2cm or 5cm samples to determine the stable isotopes of carbon and nitrogen(δ¹³C、δ¹⁵N),easily oxidized organic carbon（KOC）,soluble organic carbon（DOC）,Fourier transform infrared spectroscopy（FTIR）and solid-state 13C nuclear magnetic resonance spectroscopy(¹³C NMR)were used to analyze and characterize the functional groups and molecular composition of organic carbon profiles in the profile,and the significance of organic carbon characteristic changes in peat profiles on peat organic geochemical processes was analyzed.The main conclusions are as follows:（1）The contents of peaty TOC and TN in the profiles of the three peatlands in Changbai Mountain increased with the increase of depth（p<0.01）.However,there was no consistent trend of TN content in deep peat profiles with depth,and TN content in deep peat profiles in Moon Bay decreased with the increase of depth.In general,the changes of TOC and TN content of profile peat were mainly affected by organic geochemical processes,which could reflect the carbon-rich process of organic matter during the accumulation of peat formation.The surface peat C/N of the three peatlands in Changbai Mountain decreased with the increase of depth,while the deep peat C/N of the profile gradually increased or remained unchanged with depth.The C/N change of profile peat can reflect the aerobic mineralization and decomposition process of surface peat organic matter and the anaerobic transformation process of deep peat organic matter.（2）The surface peatδ¹³C of the three peatland profiles in Changbai Mountain increased slightly with the increase of depth,but the increase was less than 2‰.With the further increase of profile depth,the mean value of peatδ¹³C in the sections of Moon Bay、Anbei and Jinchuan decreased to-27.00‰（150cm）、-27.20‰（100cm）and-27.11‰（150cm）of the bottom layer,respectively.The isotope kinetic fractionation ofδ¹³C in the surface layer of the profile is weak,mainly affected by the Souss effect,while the change ofδ¹³C in the deep layer of the profile reflects the preferential utilization mechanism of substrate.In the sections of Moon Bay and Jinchuan peatland,the mean value of peatδ¹⁵N increased with the increase of depth（p<0.01）,while the deep peatδ¹⁵N in the Anbei profile decreased gradually with depth（p<0.01）,indicating that the change of peat ¹⁵N in the profile may be affected by mycorrhizal migration and TN content,which could not indicate the organic geochemical process of peat.（3）The surface peat DOC of the three peatland profiles in Changbai Mountain decreased with depth（p<0.01）.The mean DOC values of peat in the sections of Moon Bay、Anbei and Jinchuan decreased from 3.84g/kg（2.5cm）、8.30g/kg（2.5cm）and 2.71g/kg（8cm）to 3.10g/kg（100cm）、1.99g/kg（60cm）and 2.03g/kg（61cm）,respectively.However,the variation trend of deep peat DOC in the profile is different.Therefore,the DOC change of profile peat can only reflect the aerobic rapid decomposition process of the surface peat of the profile,and cannot indicate the organic geochemical process of peat in an anaerobic environment.The distribution characteristics of peat KOC in the three peatland profiles in Changbai Mountain area vary with depth,which may be affected by climate or sedimentary environment,and cannot reflect the organic geochemical process of peat formation.（4）According to the compound types identified by FTIR,the contents of peat polysaccharides in the sections of Moon Bay、Anbei and Jinchuan decreased from 59.19%（2.5cm）、62.28%（2.5cm）and 56.96%（8cm）on the surface to 49.09%、47.04%and 48.52%at the bottom layer,respectively.The content of aromatic hydrocarbons in peat increased from26.08%（2.5cm）、21.46%（2.5cm）and 25.63%（8cm）in the surface layer to 30.91%,、23.82%and 32.71%at the bottom layer,respectively.The content of peat aliphatic compounds increased from 7.60%（2.5cm）、9.97%（2.5cm）and 11.68%（8cm）in the surface layer to 12.84%、18.45%and 12.16%at the bottom layer,respectively.Therefore,the profile changes of peatine polysaccharides,aromatic hydrocarbons and aliphatic compounds identified by FTIR can reflect the organic geochemical processes of peat active organic matter decomposition and inert organic matter accumulation.（5）The ¹³C NMR spectral signal showed that the proportions of peat O-Alkyl C and O₂-Alkyl C components in the Moon Bay and Anbei profiles decreased with depth.The proportion of Alkyl C components increases with depth.However,the peat profile trend of Jinchuan was different,and the proportion of peat O₂-Alkyl C increased with depth and then decreased,from8.18%（14-16cm）to 9.79%（36-38cm）,and then decreased to 9.13%（148-150cm）.The proportion of Alkyl C components decreased with increasing depth,from 24.42%（14-16cm）to17.79%（148-150cm）.The ¹³C NMR molecular hybrid model(¹³C NMR_MMM)showed that the carbohydrate content of peat in the Moon Bay and Anbei profiles decreased from 47.75%（25-30 cm）and 37.81%（20-25 cm）to 28.68%（138-140 cm）and 18.56%（98-100 cm）,respectively,consistent with the FTIR results.The carbohydrate content in the Jinchuan profile first increased from 31.03%（14-16cm）to 34.67%（72-74cm）,and then decreased to 28.99%（148-150cm）,and the same results as FIIR were not found.This suggests that NMR spectroscopy can reflect carbon-rich deoxidation processes during peat formation,but should be applied with plant type in mind.