The Compositions And Structural Characteristics Of Humic Substances And Their Evolution With Time In Luochuan Loess-paleosol, Shaanxi

Soil humic substance (HS) is a kind of dark macromolecule organic compound which is amorphous and complex. It’s the main part of soil organic matter and plays an extremely important role in soil. HS can not only provide various nutrients for plants, but also improve crop quality, and at the same time it gives great contribution to soil structure. HS is also helpful in reducing environmental pollution. Researches found that organic pollutants and harmful heavy metal ions can be absorbed well on some parts of HS. So research about the stability and transformation on HS is absolutely useful and important.It’s well known that clay particles and clay minerals can protect organic matter by some well researched way. Result is that the dominant factor and main way have not been fully understood, and there is less systematic research of HS evolution characteristics on a long time span. We studied the quantitative contants and structure properties of HS in17loess-paleosol profiles in Luochuan Shaanxi province by some physical, chemical and spectroscopy methods. The main results are as follows:(1) There are higher TOC in paleosols than in underlying loess. The average contents of TOC in S0-S8and L1-L8are2.12g/kg and1.54g/kg respectively. The contents of MOC are higher in paleosol than in loess, while StOC and COC are at the same level.(2) The unstable part of soil organic carbon is decomposed and the remaining is StOC in long buried soil. It takes140～250ka and330～410ka to reach a stable state in loess and paleosol respectively, because of the indifferent mineral composition, organic compositions and chemical structure.(3) The correlations between caly and TOC, COC are positive (r=0.46and0.54respectively), whereas these are poor between fine sand and TOC, COC.The correlations between Kaolinite and TOC, COC are positive (r=0.74and0.61respectively), while the contents of illite and montmorillonite are not related to TOC and COC. (4) Approximate order of the contents of HS component in paleosols is HM>HA>FA, whereas in loess it’s FA>HM>HA. And these HS components are decreasing both in loess and paleosols with the increasing of burial time.(5) Analysis between HS components and TOC components shows that the relationship between HA, HA+FA, HA+FA+HM and TOC are positive, r=0.89,0.94and0.96respectively. The HS that can be extracted takes93%of TOC on average.(6) HA, HA+FA and HA+FA+HM are related with kaolinite (r=0.62,0.57and0.65respectively). Complex HS components are relatived with Fep. These means the stability of soil organic matter is due to kaolinite and Fep.(7) There are more aliphatic structures and humification trend in HS in paleosols than in loess. HAs have more aromatic constituents and less carbonyl and aliphatic structures than FAs.