Abstract:Humic substance (HS) can be divided into humic acid (HA),fulvic acid (FA) and humin (HM) according to solubility in alkali or acid solution.HM is a fraction tightly combined with soil minerals and hardly extracted.In recent years,researchers have conducted a series of research on HM and advanced some means of separating,fractionating and purifying the substance.Among them the Pallo method is quite complicated to operate,but capable of fractionating HM into ironlinked HM (HMi),claycombined HM (HMc),inherited HM(IH) and highly developed HM (DH).As the fraction of inherited HM is very limited in quantity,it was ruled into the fraction of highly developed HM,which was then called insoluble humin residues (HMr) in this paper.In the experiment,samples of cultivated black soil and meadow soil were collected from the Jilin Agricultural University Campus (N43°48',E125°23'),and samples of Aeolian sandy soil from the Yaojingzi Grassland Research Station (N44°45',E123°45'),Northeast Normal University,Jilin Province,China.The above samples were all taken from 0～20 cm topsoil layers,except the samples of black subsoil,which was collected from the soil layer,70～100 cm in depth in the tableland.The black subsoil (2 mm mesh) was mixed with powdered corn stalks (0.42 mm mesh) and some bacteria of normal soil bacterial groups.Then the mixture was put under incubation at 25℃ for 90 days.Results show that HMr is the dominant fraction of humin,accounting for 30%～60% of the total organic carbon(TOC) in quantity,and followed by HMi and HMc,accounting for 3.1%～5.5% and 1.5%～4.1%,respectively.The content of HMi is a little bit higher than that of HMc in all the soils,except in the incubated soil.The E4/E6 and ΔLogK of HMi is higher than that of HMc in all the soils.The IR of HMi is almost the same as that of HMc regardless of difference in soil,but the new-formed HMi in the incubated soil showed stronger aliphatic CH vibration (2 920 cm-1).In terms of 2 920/1 620 ratio of both HMi and HMc,a decreasing order of incubated soil> Aeolian Sandy soil> black soil> meadow soil is found.From the above results,it can be concluded that HMi is simpler and more aliphatic than HMc in molecular structure,and HMi and HMc are simpler and more aliphatic in the incubated soil than in the normal soils.