| The preservation and stability mechanisms of soil organic carbon (SOC) are the important factors to evaluatethe capacity of soil carbon pool and the potential of sustainable utilization. In order to understand the preservation time and mechanisms of SOC, in the present study, the vertical distributions and partition of total organic carbon (TOC), stable organic carbon (StOC), mineral-protected organic carbon (MOC), and recalcitrant organic carbon (ROC) and their correlations with the contents of clay minerals and different forms of iron oxides were investigated in the Baoji and Luochuan profiles and aggregates (>2 mm,1～2 mm,0.5～1 mm,0.25～0.5 mm, 0.05～0.25 mm and<0.05 mm) in mordern soil (dark loessial soil). The main results were observed as the followings:(1) The ancient and modern soils have the similar inorganic composition, silt and fine silt were mainly composed by quartz, feldspar, and a little mica, clay minerals were composed by illite, kaolinite, vermiculite. According to the quantitative calculation of them, illite was the highest content of clay minerals, followed by kaolinite and vermiculite. Iron oxides primarily existed as free crystalline iron oxide (Fed) and amorphous iron oxide (Feo) forms。(2) A decreased trend of TOC contents was shown with increasing depth in the both loess/paleosol profiles. The TOC contents decreased gradually from 12.12 to 1.02 g/kg (from So to S8) and from 4.90 to 1.25 g/kg (from L1 to L8) in the Baoji profile, Luochuan profile had the same trend with smaller range of variation. The ratios gradually increased from 20% to 80% from bottom to surface. For the Luochuan profile, there was also a similar trend, but not as obvious as in the Baoji profile.(3) A decreased trend of TOC contents was shown with decreasing grain size. For example, the TOC contents decreased gradually from 19.20 (>2 mm) to 7.03 g/kg (<0.05 mm) in the aggregates. The average MOC/StOC value was 63%, it indicated that MOC was the main fractions of StOC, it proved mineral played an important role in the preservation of soil organic carbon.(4) In ancient soils, the TOC contents were well correlated with clay particle contents, with r=0.72 (Baoji) and 0.63 (Luochuan), respectively. The TOC contents and its fractions were also positively correlated with the clay minerals, TOC, StOC, MOC and ROC were positively correlated with the contents of kaolinite, with r values of 0.92,0.72,0.52,0.81 (Baoji) and 0.78,0.58,0.50, 0.49 (Luochuan). The TOC contents were also well correlated with iron oxide contents. In ancient soils, the contents of TOCwere positively correlated with Fep, with r values of 0.90 (Baoji) and 0.82 (Luochuan).(5) In aggregates of modern soil, the TOC contents and its fractions were also positively correlated with the inorganic substances. In 0.25-0.05 mm aggregates, the contents of illite were well correlated with StOC and MOC contents, with r=0.71 and 0.68, respectively. The iron oxide contents were also well correlated with TOC contents. TOC was positively correlated with Fed, with correlation coefficients of r=0.72 in 0.5～0.25 mm,0.25～0.05 mm aggregates, and ROC was also positively correlated with Fed, with correlation coefficients of r=0.68 in 0.25～0.05 mm aggregates.(6) These results indicated that the labile SOC almost was decomposed and the mostly stable SOC was preserved in the loess and paleosol after 375 ka since their formation. StOC could be preserved at least 762 ka. The preservation mechanism were mainly physical adsorption and chemisorption affacted by ligand exchange of kaolinite, and Fep mainly affected by complexation on SOC preservation. In dark loessial soil, the preservation mechanism were mainly physical adsorption of illite, and Fed mainly affected by chemisorption on SOC preservation, and preservation effects mainly in 0.5～0.25 mm,0.25～0.05 mm aggregates. |
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