The quantity and stability of soil organic carbon following vegetation degradation in a salt-affected region of Northeastern China

• Vegetation type and soil depth significantly influence the total SOC and its oxidizable fractions. • Content of oxidizable resistant carbon in the LEY community is higher than that in the SUA community. • The recalcitrant carbon fraction can be affected by land degradation. • Influence of vegetation degradation on the stability of SOC is not significant. Understanding the influence of vegetation conversion induced by land degradation on soil organic carbon (SOC) dynamics provides a basis for the sustainable use of grasslands. While labile carbon fractions often vary with the type of vegetation, little is known about whether and how vegetation degradation affects the recalcitrant carbon fraction that is highly resistant to oxidation. Here, we studied the influence of vegetation degradation on changes in SOC and four oxidizable fractions with different stabilities in Northeastern China. The soil was collected from a vegetation degradation sequence, including Leymus chinensis (LEY, native grassland) , Puccinellia tenuiflora (PUC, light degradation), Chloris virgata (CHL, moderate degradation), and Suaeda heteroptera (SUA, severe degradation). Contents of total SOC and four oxidizable SOC fractions extracted under a gradient of oxidation (F1, very labile; F2, labile; F3, less labile; F4, oxidizable resistant) were measured using a modified Walkley-Black method. Results showed that the contents of total SOC and oxidizable SOC fractions were significantly different under different vegetation types and soil depths. Under PUC, CHL, and SUA, contents of total SOC (29.0%, 34.3%, and 55.4%, respectively), F1 (22.5%, 20.8%, and 40.8%, respectively), F2 (28.2%, 53.8%, and 72.1%, respectively), F3 (52.0%, 40.6%, and 66.2%, respectively), and F4 (15.0%, 21.9%, and 42.8%, respectively) were lower than those under LEY. Vegetation degradation had a significant effect on the F4 fraction as well as the F1, F2, and F3 fractions. Regression coefficients of the relationship between oxidizable SOC fractions as a function of total SOC showed that these fractions, as a proportion of total SOC, were 22% for F1 and F2, 26% for F3, and 31% for F4. There were no significant differences in the percentages of active carbon fractions (F1 + F2) in total SOC and the lability of total SOC among the four vegetation types. These findings suggest that the recalcitrant carbon fraction can be affected by land degradation as well as the labile carbon fraction, and that shifts in land use affect the contents of total SOC and its fractions, but have no effect on the stability of SOC in semi-arid areas.