Effects of organic carbon and iron oxides on soil aggregate stability under different tillage systems in a rice–rape cropping system

Studies of soil organic carbon (SOC) and iron oxides are crucial for understanding soil aggregate formation and stability in paddy–upland rotation systems. Many of these soils contain large amounts of SOC and iron oxides, which may be lost due to changes in the hydrothermal conditions. This study primarily investigated the dynamics of aggregates and soil binders (SOC and iron oxides) after rice and rape harvests under four treatments: conventional tillage without straw returning (T1), conventional tillage with straw returning (T2), no-tillage without straw returning (T3), and no-tillage with straw returning (T4) in paddy soils. The percentage content of >5 mm aggregates was higher than that of other aggregates in all treatments, except T1 and T2 after the rape harvest. T4 significantly (P < 0.05) facilitated the formation and stability of soil aggregates (>2 mm). Relative to T1, the SOC content was increased under T2, T3, and T4 by 1.64–24.96% after the rape harvest and 3.23–34.74% after the rice harvest, and the highest SOC content was found under T4. Alkanes-C, aromatic-C, and alkenes-C were primarily distributed in >0.25 mm aggregates, whereas polysaccharides-C was mainly distributed in 0.25–0.053 mm aggregates. There were higher alkanes-C and alkenes-C concentrations in the no-tillage treatments (T3 and T4) compared to conventional tillage (T1 and T2). The relative phenolics-C concentration in all aggregates and polysaccharides-C concentration in >0.053 mm aggregates increased in the T2 treatment. The concentrations of amorphous and complex iron oxides significantly (P < 0.05) increased in the T2 and T4 treatment by 1.60–14.40% and 12.70–41.10%, respectively. Iron oxides were distributed disproportionally among the different size fractions of soil aggregates. The soil aggregate stability, and SOC and iron oxide concentrations were higher after the rice harvest than the rape harvest. Soil aggregate stability was strongly correlated with SOC (P < 0.01) and iron oxides (amorphous and complex iron oxides) (P < 0.05). Alkanes-C, aromatic-C, alkenes-C, and amorphous iron oxides were the most important binding agents in the >0.25 mm aggregates, while polysaccharides-C and amorphous and complex iron oxides were the main binding agents in the 0.25–0.053 mm aggregates. It was concluded that the high levels of SOC and amorphous and complex iron oxides in soil under T4 were responsible for the aggregate stability.