Stellera chamaejasme encroachment enhances plant carbon density but drives vertical soil organic carbon redistribution in degraded typical steppe during the early growing season

Abstract Background The expansion of poisonous weed Stellera chamaejasme resulting from unreasonable grazing practices is altering plant composition and soil properties in degraded grasslands in Inner Mongolia. However, the impacts of S. chamaejasme encroachment on plant carbon density and the vertical changes in soil organic carbon (SOC) in grasslands are poorly understood, which hinders the accurate estimation of C stocks in degraded grasslands. Methodology This study compared three vegetation degradation types (degraded grassland with S. chamaejasme encroachment (SS), degraded grassland without S. chamaejasme encroachment (NS), and non-degraded natural grassland (CK) in Inner Mongolia typical steppe during the early growing season. We aimed to elucidate the differences in plant carbon stocks, the vertical patterns of SOC fractions across 0–50 cm layers, and their underlying driving mechanisms. Results During the early growing season of typical steppe, SS maintained plant carbon density comparable to CK via higher plant aboveground biomass and plant belowground carbon content, whereas total SOC density (0–50 cm) was similarly reduced in both degraded types. Moreover, SS induced a distinct vertical SOC redistribution. Compared to NS, SS showed significantly elevated SOC in the topsoil (0–10 cm), similar levels at 10–20 cm, and significantly lower contents among 20–50 cm. In particular, the SS plot exhibited significantly higher particulate organic carbon (POC) in the first two soil layer (0–20 cm). Furthermore, partial least squares path modeling indicated that the mechanisms controlling SOC also shifted with depth, moving from plant-driven POC inputs in the surface (0–20 cm) to mineral-associated organic carbon (MAOC) stabilization below 30 cm governed by soil properties. Conclusions Our findings highlight that S. chamaejasme encroachment represents a unique degradation pathway that significantly alters carbon storage patterns during the early growing season of typical steppe in Inner Mongolia. Increases in plant and topsoil carbon density due to S. chamaejasme encroachment must be interpreted with caution, given the potential trade-offs with deeper soil carbon pools and long-term ecosystem stability.