Shrub encroachment increases soil carbon and nitrogen stocks in alpine grassland ecosystems of the central Tibetan Plateau

•Shrub encroachment promoted C and N accumulation in alpine meadow and alpine steppe.•Shrub encroachment increased herbaceous plant richness and aboveground biomass.•Microbe-derived soil carbon and nitrogen were also enhanced by shrub encroachment.•Mechanisms leading to SOCS and STNS accumulation varied between alpine meadow and alpine steppe.