Abstract: Shrub expansion is a widespread phenomenon in global grasslands, yet its impact on soil organic carbon (SOC) content remains inconsistent. Investigating litter input and decomposition rates can clarify the dynamics and mechanisms driving SOC formation during grassland shrubification. This study investigated the impacts of shrub-induced changes in litter input and decomposition on SOC and its composition within the 0–20 and 20–40 cm soil layers on the Tibetan Plateau. We found that shrubs produced higher aboveground and root litter inputs compared to grasses. However, shrub shoot litter decomposed much more slowly than grass shoots and roots due to its high lignin and low nitrogen (N) contents. Shrub expansion did not increase the total SOC content in either soil layer. Instead, it fundamentally altered SOC composition: particulate organic carbon (POC) under shrubs was 27.9 % and 28.6 % higher in the 0–20 cm and 20–40 cm layers, respectively, compared to grasses. This increase in POC was not significantly correlated with root litter input in either soil layer; rather, it was primarily driven by the lower decomposition rates of shrub shoot litter. Concurrently, while shrub expansion increased dissolved organic carbon (DOC) across both soil depths and stimulated surface phospholipid fatty acids (PLFA), subsoil mineral-associated organic carbon (MAOC) significantly decreased. We concluded that shrub expansion increases the labile POC without altering the total SOC content on the Tibetan Plateau. This compositional shift might lead to SOC instability, making it more susceptible to mineralization.

Key words: Shrub expansion, Litter production, Litter decomposition, C sequestration, Tibetan Plateau