Characterisation of organic carbon distribution and turnover by stable carbon isotopes in major types of soils in China

Rationale. Soil is the largest carbon reservoir of terrestrial ecosystems. Studying carbon turnover in cultivated soils is vital for China to achieve its carbon neutrality goal. Methodology. In this research, we investigated the distribution characteristics of soil organic carbon and its turnover among aggregates in nine major soil types in China by C-13 stable isotopes. Results. Since the second national soil survey in China 1979, the surface soil organic carbon (SOC) content of cultivated lands in boggy soils, black soils and dark brown forest soils in northeastern China has decreased (129.06, 2.70 and 7.78 g kg(-1), respectively) while increased in cinnamon soils, brown soils, red soils, latosols, paddy soils and chestnut soils (0.85-7.79 g kg(-1)). Overall, the SOC content was highest in 1-0.25 mm aggregates, lowest in 0.053-0.02mm aggregates and higher in <0.02 mm aggregates. The Delta C-13 value showed that the silt-clay size class fraction > micro-aggregates > macro-aggregates. Discussion. The overall turnover direction of SOC among aggregates starts from macro-aggregates to micro-aggregates and ends with the silt-clay size class fraction. The result directly confirms both the 'hierarchical development model' and 'embryonic development model' of soil aggregate formation. The SOC flow direction and mobility potential between aggregates, as indicated by Delta C-13 values, suggest that cultivated soils are more susceptible to soil type than wasteland soils.