Assessing the contributions of sesquioxides and soil organic matter to aggregation in an Ultisol under long-term fertilization

• Assessing biotic and abiotic contributions to aggregation in a typical Ultisol.
• Fe/Al oxides are primary binding agents of aggregates in this Ultisol.
• SOM is only in 0.25–2.0 mm aggregates the major binding agent.
• Aggregates are mainly formed either by SOM or by Fe/Al oxides, but not by both together.

In tropical and subtropical soils, sesquioxides and soil organic matter (SOM) are major binding agents for aggregates. However, the biotic and abiotic contributions to aggregation are often difficult to distinguish. In this study, we attempted to assess their contributions to aggregation separately, as indicated by aggregate size distribution and specific surface area (SSA). Our objectives were (i) to determine aggregate size distribution and SSA before and after removal of sesquioxides and SOM, and (ii) to assess the contributions of sesquioxides and SOM to soil aggregation. An oxide-rich Ultisol under long-term fertilization was extracted by water as a control, oxalate, dithionite–citrate–bicarbonate (DCB), or by H2O2 in the absence of any physical disturbance. The aggregate size distribution, Fe/Al oxides, soil organic C (SOC), and SSA of the soil before and after extraction were determined. Our results showed that the DCB and oxalate solutions broke down the sand-sized aggregates most intensively, whereas the H2O2 treatment disrupted 0.25–2.0 mm aggregates intensively, indicating that SOM is the major binding agent for aggregates of this size. A slight change either in SOC stock after removal of Fe/Al oxides by DCB and oxalate or in Fe/Al oxides after removal of SOC by H2O2 indicated that organo-mineral complexes are a minor binding mechanism of aggregation in the soil studied. The SSA was reduced by 72–84% in the soil extracted by DCB, followed by 32.0–35.9% after the oxalate extraction, whereas the removal of SOM increased SSA by 3.8–12.6%. Our results showed that Fe/Al oxides played a major role in aggregation in the Ultisols studied. The difference in the major binding agent for different aggregate size classes is another reason to explain why the hierarchy aggregate concept is not applicable to oxide-rich soils. This study, however, could not assess their contributions to soil aggregation precisely, because of the difficulty in tracing aggregate dynamics. To better understand the mechanisms of soil aggregation we need more works in the future.