Soil sampling depth matters in assessing the impact of shrubification on soil organic carbon storage in grazed alpine meadows

Abstract

The distribution of shrubs is expanding in grasslands of the Qinghai–Tibetan Plateau, but research is rare on the effects of this expansion on the storage of soil organic carbon (SOC). We explored the spatial heterogeneity of SOC by grid sampling in the upper 1.2 m soil profile in a grazed alpine shrubby meadow, in response to patchy distribution of shrub species (leguminous Caragana brevifolia and non-leguminous Potentilla fruticose, Spiraea alpina and Salix oritrepha). The SOC content in the top 0.4 m increased with the size (height and crown diameter) of shrubs, but the soil bulk density decreased with the size of the shrubs. As a result, the SOC stock in the top 0.4 m soil was generally similar among shrub and grass patches across the meadow. The increase in the SOC stock in the entire 1.2 m profile by 35 % under non-leguminous shrubs P. fruticosa, S. alpina and S. oritrepha (39.8–40.5 kg m−2) compared with grasses (29.7 kg m−2) was mainly due to the SOC accumulation at the lower 0.4–1.2 m. The leguminous shrub C. brevifolia did not affect the SOC stock in the 1.2 m soil (32.3 kg m−2) compared with the grasses. Consistent patterns of changes in the natural δ13C signature between plants and SOC across different types of vegetation patches provided robust evidence that the heterogeneity of SOC across the field was a result of the patchy distribution of vegetation. We emphasize the importance of soil sampling depth in assessing the impact of shrubification on SOC storage. The influence depth of shrubification on SOC storage is at least 1.2 m in the profile in grazed Qinghai–Tibetan grasslands.