| Humic substanceas a stable component of soil organic carbon has been the research focus of scientists. Its formation and transformation are a slow and long-term. A lot of researchs are only limited to the short time scale dynamic changes in the past, and that have a certain time limitations in the stability of the study of soil humic substances. Therefore, in the humification process, it is necessary to strengthen the long-scale study.In order to compensate for the short long term comings in the previous studies, we chosen the Baoji loess profile for the study. First, the contents of total organic carbon (TOC) and its major components (such as stable organic carbon (StOC), mineral-protected organic carbon (MOC), chemical resistant organic carbon (COC))were analyzed for both loess and paleosols layers in the Baoji profile; the stable organic carbon accounted for the proportion of TOC changes as assessed over time. Then humic substances were extracted quantitatively, and then the fractionation and spectroscopic analysis were applied to study the structural characteristics of different humic. Finally, the relationships the clay particle contents, clay mineral, iron oxides contents with TOC contents, humic substances were correlated. The main findings were as follows:(1) The results of TOC and its major components-stable organic carbon were showed, that as burial time increases, TOC. StOC, MOC contents decreased, and that TOC contents were higher in paleosols (except S7) than those in Baoji loess in the paleosol and loess profile. For StOC constitute. MOC was dominated in StOC accounted for an average of75.51%.(2) Humic acid (HA), fulvic acid (FA) quantitative results were shown, that humic acid component contents show FA> HA in the Baoji loess-paleosol protie; compared with loess, HA both and FA contents were higher in paleosol. In general, with the burial time increasely, humic acids components reduced gradually in both loess and paleosol.(3)The clay particle contents, clay minerals contents and iron oxides were well correlated with TOC contents and its fractions. The clay particle contents with TOC, StOC were positively correlation, with r=0.73and0.50. respectively.The coefficients were obvious between kaolinite contents and TOC, StOC, MOC, COC, with r=0.92,0.81,0.63and0.64, respectively; there was a certain correlation between the oxalate iron and TOC, StOC, with r=0.67and0.53, respectively.(4) The clay particle contents, clay minerals content, iron oxides, soil organic carbon and its fractions with humic acid and other fractions were also positively correlated. The clay particle contents with HA, FA were well correlated, with r=0.73and0.70. In contract to illite, there were obvious correlations between Kaolinite contents and HA, FA, with r=0.87and0.91. The oxalate iron with HA, FA was also positively correlated, with r=0.64and0.50. There were good correlation of TOC and HA, FA, with r=0.98and0.94.(5) Different types of humic acid were identified by elemental analysis, FTIR, NMR analysis. The results show that humic acid, fulvic acid composition and structural characteristics were basically the same in different burial times, with some differences with under different burial conditions. Carboxyl carbon contents of FA were greater in loess than those in paleosol, but the aromatic carbon contents were less than paleosol; carboxyl carbon contents of HA in loess were similar to paleosol, but aromatic carbon contents in loess were higher than paleosol. Compared with HA, FA had less hydrocarbon content of the alkyl chain, higher the carboxylic/carbony content, while HA contains more unsaturated aromatic substances, and contained a certain amount of fatty groups.(6) For TOC and clay minerals as one of the objects of study, two profiles were comparative analysised of loess/paleosol samples collected from Luochuan and Baojiprofile. The contents of these indicators and trends found in good agreement with specific performance in both sites, as follows:â‘ TOC contents of in paleosols were higher than loess in two sections, meanwhile soil organic carbon showd a trend to decrease with increasing burial time;â‘¡Humic substances in two sections of the trend were basically the same,and they were gradually reduced with the increase of burial time;â‘¢There were the same type of clay minerals in two profiles, mainly illite and kaolinite.In addition to the above in common, there were some differences between two sections, mainly in the following aspects:â‘ TOC maximum and the average value were much larger in Baoji section than Luochuan section, from the point of view of StOC constitute, stable organic carbon were MOC-based in two profiles, but MOC accounted StOC average proportion of (75.51%) in Baoji section was higher than Luochuan (33.85%). Meanwhile, the relationships between TOC and clay particle contenst, kaolinite contents were not closer as in Luochuan than Baoji profile. The results seem to imply that the contribution of the minerals in the soil organic carbon stability in Baoji section was greater than Luochuan profile;â‘¡Compared with Baoji section, HA/FA, PQ values were lightly higher in Luochuan paleosols, the law showed the contrary in loess. In addition, there were better correlations in Baoji section between humic substances and clay particle contents, kaolinite contents than Luochuan section. According to the results of these analyzes, we speculated that the stability of soil organic carbon mechanism may be the main mineral protection in Baoji section, while there may be many stabilization mechanisms in Luochuan profile. |
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