Role of chemical protection by binding to oxyhydrates in SOC sequestration in three typical paddy soils under long-term agro-ecosystem experiments from South China

Physical protection in aggregates has been commonly accepted as a fundamental mechanism for soil organic carbon (SOC) sequestration for China's paddy soils. However, the role of chemical protection of physically protected SOC has not yet been evaluated. Herewith the authors report a cross-site study on SOC binding forms of both bulk soils and different particle size fractions (PSFs) obtained by low-energy ultrasonic dispersion procedure of three typical paddy soils, which were under long-term agro-ecosystem experiments from across South China with different tillage and fertilization treatments since the 1980s. The studied paddy soils included a purple paddy soil (Soil PP), a red earth-derived paddy soil (Soil RP), and a Tai Lake lucustrine-derived paddy soil (Soil TP). Results showed that calcium-bound SOC (Ca-OC) was the predominant in the coarse PSF of 2000–200 µm of all the three soils and turned higher in Soil PP and lower in Soil RP. In contrast, iron/aluminum-bound SOC (Fe/Al-OC) was dominant in the PSF of < 2 µm and tended higher in Soil RP and lower in Soil PP. Changes in content of Fe/Al-OC were found well correlated with the content of Fe/Al oxyhydrates of different PSFs. The ratio of LOC to SOC for different PSFs was significantly positively correlated with that of Ca-OC to SOC, and significantly negatively with that of Fe/Al-OC to SOC. Furthermore, the wide variation of contents of Ca-OC and Fe/Al-OC in the PSF of 2000–200 µm with different treatments suggested the highly sensitive response of C binding in this coarse soil particle fraction to the long-term management practices compared to the PSFs in other size ranges. However, the accumulated SOC under good management practices was found only proportional to the increased content of Fe/Al-OC compared to under control treatments. Thus, the chemical protection of physically protected SOC in the coarse PSF by binding to Fe/Al oxyhydrates with micro-aggregate turnover was supposed to be a prevailing mechanism of SOC sequestration in these paddy soils of China.