Roles of soil organic carbon and iron oxides on aggregate formation and stability in two paddy soils

• Soil organic carbon improved aggregate stability against mechanical disruption. • Fe (DCB) oxides increased soil aggregate stability when SOC contents were low. • SOC was the major binding agent for >2 mm and 2–0.25 mm aggregates. • Fe oxide was a crucial binding agent of micro-aggregate (0.25–0.053 mm). • Organo-mineral complexes promoted the aggregation of high SOC–low iron oxides soil. Knowledges of aggregate stability and formation depending on soil organic carbon (SOC) and iron oxides are essential to the conservation of soil structure. In this work, the effects of SOC and different forms of iron oxides on the stability of aggregates of paddy soils were determined by applying the Le Bissonnais (LB) and extractions method by water, oxalate (OA), dithionite–citrate–bicarbonate (DCB), or hydrogen peroxide (H 2 O 2 ). Two topsoils (0–20 cm) were selected based on two levels of SOC and iron oxides, high SOC (22.1 g kg −1 )–low iron oxides (S1) and low SOC (8.50 g kg −1 )–high iron oxides (S2) paddy soils from subtropical China. For the fast-wetting (FW) test, the percentage of 2–0.25 mm (34.0%) and >2 mm (42.2%) aggregate fractions were highest in S1 and S2, respectively. Both the soils had a >2 mm fraction that was above 60% under the slow-wetting (SW) and stirring after prewetting (WS) test. The free Fe oxide (Fe (DCB) ) could increase in aggregate stability when SOC contents were low in S2. All three chemical extractions considerably decreased the >2 mm fractions (35.5–87.5%), and consequently increased the <0.053 mm fractions. The S1 soil presented the lower mean weight diameter (MWD) after DCB extractions, whereas it was no significantly different after OA or DCB extraction in S2 soil, indicating that Fe (DCB) had a greater effect on the stability of aggregates than amorphous Fe oxide (Fe (OA) ) in the S1 soil, but both had the same effect for S2. The H 2 O 2 treatment destructed>2 mm and 2–0.25 mm aggregates intensively, indicating that SOC was the major binding agent for aggregates of these sizes. The SOC was a key agent influencing soil aggregation not only in macro-aggregates (>0.25 mm) but also in micro-aggregates (0.25–0.053 mm) in the S1. The formation of aggregates in S1 was attributed to its higher content of organo-mineral complexes because the contents of SOC (3.30–11.2 g kg −1 ) were higher in >0.053 mm aggregates compared with S2 after H 2 O 2 extractions. Fe oxides were the key agents for <0.25 mm aggregates formation. The MWD of the FW treatment was found to have a significant positive correlations with Fe (DCB) , and MWD of the WS treatment proved significant positive correlations with SOC and Fe (OA) oxide, indicated that SOC and Fe oxides improved aggregate stability against external force collapse. This implies that the differential roles of SOC and Fe oxides in mechanisms of aggregate formation and stability in two paddy soils.