The significant role of inorganic matters in preservation and stability of soil organic carbon in the Baoji and Luochuan loess/paleosol profiles, Central China

The preservation and stability mechanisms of soil organic carbon (SOC) are the important factors to evaluate the capacity of soil carbon pool and the potential of sustainable utilization. To understand the preservation time and mechanisms of SOC under burial conditions, in the present study, the distributions of total organic carbon (TOC) and stable organic carbon (StOC), and their correlations with the contents of clays and clay minerals and different forms of iron oxides were investigated in the Baoji and Luochuan loess–paleosol profiles. Four facts were observed as the followings. (1) The labile SOC almost was decomposed and the mostly stable SOC was preserved in the loess and paleosol after 375 kyr since their formation. StOC could be preserved at least 762 kyr in both loess and paleosol under burial condition. (2) The TOC was positively correlated with clay contents, with correlation coefficients of 0.72 (Baoji) and 0.63 (Luochuan). (3) The TOC, StOC, mineral-protected organic carbon (MOC), and recalcitrant organic carbon (ROC) were positively correlated with kaolinite, with correlation coefficients of 0.93, 0.72, 0.52, 0.81 (Baoji) and 0.78, 0.58, 0.50, 0.49 (Luochuan), respectively, both with neither illite nor vermiculite. (4) The TOC was highly correlated with complex iron (Fep) with correlation coefficients of 0.90 (Baoji) and 0.82 (Luochuan), so with amorphous oxides of iron (Feo) as well. Among them, Feo mainly affected by sorption and Fep by complexation on SOC preservation, whereas kaolinite had both chemical and physical effects. The values of coefficients further highlight that the contributions of inorganic matters to the fixation of organic carbon were ranked to an order of kaolinite > Fep > Feo.