Study On The Distribution Characteristics Of Typical Functional Groups/Groups In Soil Based On Fourier Infrared Spectroscopy

Soil organic carbon is an important component of terrestrial carbon stocks,and changes in its content have important implications for climate change.Functional groups determine the chemical properties of soil organic compounds and influence the sequestration and transfor-mation of soil organic carbon.Exploring the changes in the types and contents of functional groups in soils has positive implications for understanding the changes in soil organic carbon content.To this end,in this study,the typical vegetation（forest,shrub and grassland）soils in the Ziwuling Mountain of the Loess Plateau were used as the research object,and based on the comparison of the performance of different infrared spectroscopy techniques,a high-per-formance soil spectroscopy technique was selected to investigate the distribution characteris-tics of functional groups/groups in different vegetation soils and different grain-level agglom-erates,and to explore the main soil physicochemical properties that mainly affect the changes of functional groups/groups.The main findings of the study are as follows.（1）Attenuated total reflection（ATR-FTIR）spectroscopy technique is only applicable to the analysis of higher purity substances and is not applicable to soil samples with complex composition;transmission spectroscopy（T-FTIR）technique can be used for higher purity sub-stances and soil samples with complex composition,but dilution with KBr is required and the preparation process is more complicated;diffuse reflection spectroscopy（DR-FTIR）spec-troscopy technique can be directly used for the determination of organic matter and soil sam-ples with complex composition without dilution by KBr and sample preparation,which is simple and easy to operate.In the quantitative analysis of mixed specimens,DR-FTIR is su-perior to T-FTIR spectroscopy,and is more advantageous for the quantitative analysis of a large number of soil samples.（2）The diffuse reflection infrared spectra of the surface（0-20 cm）soils of eight vegeta-tion types,namely Quercus wutaishansea（QW）,Platycladus orientalis（PO）,Pinus tabuliformis（PT）,Robinia pseudoacaci a（RP）,Sophora viccifoli a（SV）,Rosa xan-thine（RX）,Hippophae rhamnoi des（HR）and Stipa Bungeana（SB）,showed similar spectral patterns,and their functional groups/groups were mainly free OH,intermolecularly joined OH,methyl CH₃,methylene CH₂,benzene ring C=C,and ether bond C-O-C.By ana-lyzing the variation of typical functional groups/groups in different vegetation types,the mag-nitude of the characteristic absorption peak area ratio of functional groups/groups and the slope of the fitted line K of the target soil mass fraction could be compared.Using QW soil as the reference sample,a good linear fit was found between the peak area ratios of functional groups/groups in the bands 3000-2800 cm^-1 and 1760-1550 cm^-1of PT,HR and SB soils with methyl CH₃ and benzene ring C=C characteristic absorption peaks,respectively,and the mass fractions of the samples.The absolute values of their slopes K all showed K_PT<K_HR<K_SB,indicating that the magnitudes of the contents of functional groups methyl CH₃ and benzene ring C=C in different vegetation types of soils were PT>HR>SB.For the comparison of soil organic carbon（SOC）,total carbon（TC）,total nitrogen（TN）,huminic acid（HA）,fulvic acid（FA）and huminin（HM）with typical functional groups/groups in different vegetation types of soils In the RDA analysis of methyl CH₃ and benzene ring C=C,a significant positive corre-lation was found between these soil chemical properties and typical functional groups/groups.Among them,SOC,HA,FA,HM and TN,explained the content of functional groups/groups methyl CH₃ and benzene ring C=C at a significant level（p<0.05）.（3）The diffuse reflection IR spectra of Robinia pseudoacacia（RP）,Hippophae rhamnoides（HR）and Stipa Bungeana（SB）vegetation subsurface（20-40 cm）soils with different grain-level water-stable agglomerates>5 mm,5-2 mm,2-1 mm,1-0.5 mm,0.5-0.25mm and<0.25 mm showed similar spectral patterns,with the types of functional groups/groups being mainly free OH,The variation of typical functional groups/groups in dif-ferent particle size agglomerates can be analyzed to compare the magnitude of the character-istic absorption peak area ratio of functional groups/groups with the slope K of the fitted line of the mass fraction of the target agglomerate soil.Using SB<0.25 mm grain size soil as the reference sample,it was found that the peak area ratios of functional groups/groups in the bands 3000-2800 cm^-1 and 1760-1550 cm^-1 in RP,HR and SB soils with>5 mm,2-1 mm and<0.25 mm grain size soil agglomerates,respectively,had a better linear fit with the mass frac-tion of the samples.A good linear fit was obtained.The absolute values of the slope K of the functional group/group methyl CH₃ in the soil agglomerates of different grain levels in RP were K_>5mm>K_2-1mm>K_<0.25mm,indicating that the magnitude of the functional group/group methyl CH₃ content in the soil agglomerates of different grain levels in RP was>5 mm>2-1mm><0.25 mm.The slope of the functional group/group benzene ring C=C in the soil ag-glomerates of different grain levels in RP and HR The absolute values of the slope K of soil agglomerates at different grain levels in the SB showed K_>5mm>K_2-1mm>K_<0.25mm,indicating that the magnitude of the functional group benzene/group benzene ring C=C content in soil agglomerates at different grain levels in the RP and HR was>5 mm>2-1 mm><0.25mm,and the absolute values of the slope K of soil agglomerates at different grain levels in the SB showed K_>5mm>K_2-1mm>K_0.5-0.25mm,indicating that the magnitude of the functional group/group benzene ring C=C content in different grain level soil agglomerates in SB is>5mm>2-1 mm><0.5-0.25 mm.In the RDA analysis of soil different grain level agglomerates MWD,GMD,Clay,Silt,Sand and SOC with typical functional group/group methyl CH₃and benzene ring C=C,SOC and GMD were positively correlated with methyl CH₃ and benzene ring C=C,and MWD,Clay,Silt and Sand were negatively correlated with methyl CH₃and benzene ring C=C.Among them,MWD,GMD,Clay,Silt and Sand,did not reach a significant level of explanation for the content of functional groups/motifs methyl CH₃ and benzene ring C=C（p>0.05）.In summary,the contents of functional groups/groups of methyl CH₃ and benzene ring C=C in different vegetation soils were found to be forest>shrub>grass,and the contents of functional groups/groups of methyl CH₃and benzene ring C=C in different grain size agglom-erate soils were found to be>5 mm>2-1 mm><0.25 mm.The diffuse reflectance infrared spectroscopy technique has excellent performance and can characterize the differences in the stability of different vegetation and agglomerates on It is suitable for the qualitative and quan-titative analysis of soil functional groups/groups in loess plateau.