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

Environmental context Soil carbon sequestration plays an important role in achieving the goal of carbon neutrality. We studied the characteristics of organic carbon distribution and sequestration by stable carbon isotopes in nine types of soils in China and found that macro-aggregates possessed more organic carbon with a low degree of decomposition, while the overall direction of organic carbon transfer between aggregates was from macro-aggregates through micro-aggregates to the grain-size fractions of chalky clay. These results provide a foundation for understanding soil carbon sequestration in China’s cultivated lands. 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 13C 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 0.053–0.02 mm aggregates and higher in <0.02 mm aggregates. The ?13C 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 ?13C values, suggest that cultivated soils are more susceptible to soil type than wasteland soils.