ObjectiveSoil organic matter(SOM) content and its stability are important indexes to evaluate soil quality. Thermogravimetric analysis has gained attention due to its good performance in reflecting SOM content and SOM stability. The objective of this paper is to explore the vertical variation patterns of SOM thermal stability in black soils under different pedogenetic modes. The causes for these variations are also explored. This study will provide a theoretical reference for the protection of black soil resources and carbon sequestration.MethodBlack soils of two major pedogenetic modes, i.e., naturally developed mode and depositional mode were selected for comparison. The former one included three typical black soils under stable topographic conditions (flat terrain) and the latter one included two black soils affected by erosional-depositional processes under unstable topographic conditions (with surface erosion and sedimentation) in the typical black soil area of Northeast China. Thermogravimetric analysis was used as a tool to evaluate SOM thermal stability. Two SOM fractions were recognized based on mass loss in responding to different temperature intervals, with Exo1(mass loss during 200~350℃) representing thermally labile SOM and Exo2 (mass loss during 350~550℃) representing thermally stable SOM. In addition, derivative thermogravimetry curves and two thermogravimetric parameters including Exo1/Exo2 and TG-50 were adopted to characterize the variations of SOM thermal stability in different soil profiles. Fourier transform infrared spectroscopy was used to assess SOM chemical stability.ResultFor naturally developed black soils from the stable land surface, the content of thermally labile SOM (Exo1) decreased at a higher rate with depth than that of thermally stable SOM (Exo2). With the increase of depth, Exo1/Exo2 decreased, TG-T50 increased, and aliphatic C/aromatic C decreased, indicating that the SOM of naturally developed black soils tended to be thermally stable with depth. Under unstable geomorphic settings, on the contrary, the thermal stability of SOM did not show a regular decrease trend with depth, and the content of SOM and Exo1/Exo2 in deeper layers could be higher than those in the surface layer. This was mainly due to the different land surface histories that caused variations in sources of soil parent material and SOM.ConclusionThis study confirmed the usefulness of thermogravimetric analysis in reflecting SOM stability. Spatial variations in SOM stability in black soils were largely conditioned by a pedogenetic mode which was largely related to geomorphic stability. We found that depositional landscape positions tended to stack a large amount of labile SOM, which was carried by erosional processes. This occurs both at hillslope and watershed scales. These thermally unstable SOMs could be preserved in the deep soil for a long time, due to the blocking effect of burial. Once eroded, however, these blocked labile SOMs are easily decomposed, which may also lead to the release of a large amount of buried 'old carbon' and become the 'hot spot' of carbon emissions in the black soil area.