Redox interface-associated organo-mineral interactions: A mechanism for C sequestration under a rice-wheat cropping system

Paddy soils constitute the largest anthropogenic wetlands on earth. However, the biogeochemical cycles of iron (Fe) oxides and organic compounds at the redox interfaces in paddy soils are poorly understood. Here, we propose a conceptual mechanism based on our data set, which links seasonal turnover of soil aggregates and the organo-mineral associations at the aggregate interfaces in the rice-wheat cropping system. Our results showed that the application of organic amendments significantly (P < 0.05) increased the aggregate mean weight diameter (MWD) compared to chemical fertilization alone. The aggregate MWD was always higher after the wheat harvest than after the rice harvest from 2013 to 2015, indicating a seasonal dynamic management-induced process of aggregation, disaggregation and re-aggregation. The oxalate-extractable Fe oxides (Feo) may preferentially retain aromatic organic compounds as indicated by the significant positive correlation between Feo and the percentage of aromatic carbon (C). Furthermore, the specific surface area of the ferrihydrite significantly increased after the microbial reduction, showing the potential of adsorbing more aromatic organic compounds. Moreover, there were more aromatic organic compounds at the aggregate surfaces than in the entire entities, suggesting that aromatic organic compounds were selectively preserved at the aggregate interfaces possibly by forming aromatic-Fe complexes. Therefore, soil disaggregation coupled with the anaerobic conditions exposed more fresh surfaces for C accumulation by forming aromatic-Fe complexes, which were subsequently protected by soil re-aggregation in the aerobic conditions. Organic fertilization improved soil aggregation and promoted the organo-mineral interactions, thus contributing to C sequestration in the rice-wheat cropping system.