Aridity influences root versus shoot contributions to steppe grassland soil carbon stock and its stability

•SOC were primarily contributed by roots in dry grasslands, but more by shoots in wetter areas.•Soil microbial C was an important C pathway to SOC.•Soil heavy fraction increased with aridity, suggesting SOC in drier sites are more stable.•Soil heavy fraction was more contributed by roots in dry grasslands but by shoots in wetter areas.